C# CType.IsNullableType方法代码示例

        public virtual ACCESSERROR CheckAccess2(Symbol symCheck, AggregateType atsCheck, Symbol symWhere, CType typeThru)
        {
            Debug.Assert(symCheck != null);
            Debug.Assert(atsCheck == null || symCheck.parent == atsCheck.getAggregate());
            Debug.Assert(typeThru == null ||
                   typeThru.IsAggregateType() ||
                   typeThru.IsTypeParameterType() ||
                   typeThru.IsArrayType() ||
                   typeThru.IsNullableType() ||
                   typeThru.IsErrorType());

#if DEBUG

            switch (symCheck.getKind())
            {
                default:
                    break;
                case SYMKIND.SK_MethodSymbol:
                case SYMKIND.SK_PropertySymbol:
                case SYMKIND.SK_FieldSymbol:
                case SYMKIND.SK_EventSymbol:
                    Debug.Assert(atsCheck != null);
                    break;
            }

#endif // DEBUG

            ACCESSERROR error = CheckAccessCore(symCheck, atsCheck, symWhere, typeThru);
            if (ACCESSERROR.ACCESSERROR_NOERROR != error)
            {
                return error;
            }

            // Check the accessibility of the return CType.
            CType CType = symCheck.getType();
            if (CType == null)
            {
                return ACCESSERROR.ACCESSERROR_NOERROR;
            }

            // For members of AGGSYMs, atsCheck should always be specified!
            Debug.Assert(atsCheck != null);

            if (atsCheck.getAggregate().IsSource())
            {
                // We already check the "at least as accessible as" rules.
                // Does this always work for generics?
                // Could we get a bad CType argument in typeThru?
                // Maybe call CheckTypeAccess on typeThru?
                return ACCESSERROR.ACCESSERROR_NOERROR;
            }

            // Substitute on the CType.
            if (atsCheck.GetTypeArgsAll().size > 0)
            {
                CType = SymbolLoader.GetTypeManager().SubstType(CType, atsCheck);
            }

            return CheckTypeAccess(CType, symWhere) ? ACCESSERROR.ACCESSERROR_NOERROR : ACCESSERROR.ACCESSERROR_NOACCESS;
        }

        //
        // SymbolLoader forwarders (end)
        /////////////////////////////////////////////////////////////////////////////////

        //
        // Utility methods
        //
        protected ACCESSERROR CheckAccessCore(Symbol symCheck, AggregateType atsCheck, Symbol symWhere, CType typeThru)
        {
            Debug.Assert(symCheck != null);
            Debug.Assert(atsCheck == null || symCheck.parent == atsCheck.getAggregate());
            Debug.Assert(typeThru == null ||
                   typeThru.IsAggregateType() ||
                   typeThru.IsTypeParameterType() ||
                   typeThru.IsArrayType() ||
                   typeThru.IsNullableType() ||
                   typeThru.IsErrorType());

            switch (symCheck.GetAccess())
            {
                default:
                    throw Error.InternalCompilerError();
                //return ACCESSERROR.ACCESSERROR_NOACCESS;

                case ACCESS.ACC_UNKNOWN:
                    return ACCESSERROR.ACCESSERROR_NOACCESS;

                case ACCESS.ACC_PUBLIC:
                    return ACCESSERROR.ACCESSERROR_NOERROR;

                case ACCESS.ACC_PRIVATE:
                case ACCESS.ACC_PROTECTED:
                    if (symWhere == null)
                    {
                        return ACCESSERROR.ACCESSERROR_NOACCESS;
                    }
                    break;

                case ACCESS.ACC_INTERNAL:
                case ACCESS.ACC_INTERNALPROTECTED:   // Check internal, then protected.

                    if (symWhere == null)
                    {
                        return ACCESSERROR.ACCESSERROR_NOACCESS;
                    }
                    if (symWhere.SameAssemOrFriend(symCheck))
                    {
                        return ACCESSERROR.ACCESSERROR_NOERROR;
                    }
                    if (symCheck.GetAccess() == ACCESS.ACC_INTERNAL)
                    {
                        return ACCESSERROR.ACCESSERROR_NOACCESS;
                    }
                    break;
            }

            // Should always have atsCheck for private and protected access check.
            // We currently don't need it since access doesn't respect instantiation.
            // We just use symWhere.parent.AsAggregateSymbol() instead.
            AggregateSymbol aggCheck = symCheck.parent.AsAggregateSymbol();

            // Find the inner-most enclosing AggregateSymbol.
            AggregateSymbol aggWhere = null;

            for (Symbol symT = symWhere; symT != null; symT = symT.parent)
            {
                if (symT.IsAggregateSymbol())
                {
                    aggWhere = symT.AsAggregateSymbol();
                    break;
                }
                if (symT.IsAggregateDeclaration())
                {
                    aggWhere = symT.AsAggregateDeclaration().Agg();
                    break;
                }
            }

            if (aggWhere == null)
            {
                return ACCESSERROR.ACCESSERROR_NOACCESS;
            }

            // First check for private access.
            for (AggregateSymbol agg = aggWhere; agg != null; agg = agg.GetOuterAgg())
            {
                if (agg == aggCheck)
                {
                    return ACCESSERROR.ACCESSERROR_NOERROR;
                }
            }

            if (symCheck.GetAccess() == ACCESS.ACC_PRIVATE)
            {
                return ACCESSERROR.ACCESSERROR_NOACCESS;
            }

            // Handle the protected case - which is the only real complicated one.
            Debug.Assert(symCheck.GetAccess() == ACCESS.ACC_PROTECTED || symCheck.GetAccess() == ACCESS.ACC_INTERNALPROTECTED);

//.........这里部分代码省略.........

        // !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
        // RUNTIME BINDER ONLY CHANGE
        // !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

        internal bool GetBestAccessibleType(CSemanticChecker semanticChecker, BindingContext bindingContext, CType typeSrc, out CType typeDst)
        {
            // This method implements the "best accessible type" algorithm for determining the type
            // of untyped arguments in the runtime binder. It is also used in method type inference
            // to fix type arguments to types that are accessible.

            // The new type is returned in an out parameter. The result will be true (and the out param
            // non-null) only when the algorithm could find a suitable accessible type.

            Debug.Assert(semanticChecker != null);
            Debug.Assert(bindingContext != null);
            Debug.Assert(typeSrc != null);

            typeDst = null;

            if (semanticChecker.CheckTypeAccess(typeSrc, bindingContext.ContextForMemberLookup()))
            {
                // If we already have an accessible type, then use it. This is the terminal point of the recursion.
                typeDst = typeSrc;
                return true;
            }

            // These guys have no accessibility concerns.
            Debug.Assert(!typeSrc.IsVoidType() && !typeSrc.IsErrorType() && !typeSrc.IsTypeParameterType());

            if (typeSrc.IsParameterModifierType() || typeSrc.IsPointerType())
            {
                // We cannot vary these.
                return false;
            }

            CType intermediateType;
            if ((typeSrc.isInterfaceType() || typeSrc.isDelegateType()) && TryVarianceAdjustmentToGetAccessibleType(semanticChecker, bindingContext, typeSrc.AsAggregateType(), out intermediateType))
            {
                // If we have an interface or delegate type, then it can potentially be varied by its type arguments
                // to produce an accessible type, and if that's the case, then return that.
                // Example: IEnumerable<PrivateConcreteFoo> --> IEnumerable<PublicAbstractFoo>
                typeDst = intermediateType;

                Debug.Assert(semanticChecker.CheckTypeAccess(typeDst, bindingContext.ContextForMemberLookup()));
                return true;
            }

            if (typeSrc.IsArrayType() && TryArrayVarianceAdjustmentToGetAccessibleType(semanticChecker, bindingContext, typeSrc.AsArrayType(), out intermediateType))
            {
                // Similarly to the interface and delegate case, arrays are covariant in their element type and
                // so we can potentially produce an array type that is accessible.
                // Example: PrivateConcreteFoo[] --> PublicAbstractFoo[]
                typeDst = intermediateType;

                Debug.Assert(semanticChecker.CheckTypeAccess(typeDst, bindingContext.ContextForMemberLookup()));
                return true;
            }

            if (typeSrc.IsNullableType())
            {
                // We have an inaccessible nullable type, which means that the best we can do is System.ValueType.
                typeDst = this.GetOptPredefAgg(PredefinedType.PT_VALUE).getThisType();

                Debug.Assert(semanticChecker.CheckTypeAccess(typeDst, bindingContext.ContextForMemberLookup()));
                return true;
            }

            if (typeSrc.IsArrayType())
            {
                // We have an inaccessible array type for which we could not earlier find a better array type
                // with a covariant conversion, so the best we can do is System.Array.
                typeDst = this.GetReqPredefAgg(PredefinedType.PT_ARRAY).getThisType();

                Debug.Assert(semanticChecker.CheckTypeAccess(typeDst, bindingContext.ContextForMemberLookup()));
                return true;
            }

            Debug.Assert(typeSrc.IsAggregateType());

            if (typeSrc.IsAggregateType())
            {
                // We have an AggregateType, so recurse on its base class.
                AggregateType aggType = typeSrc.AsAggregateType();
                AggregateType baseType = aggType.GetBaseClass();

                if (baseType == null)
                {
                    // This happens with interfaces, for instance. But in that case, the
                    // conversion to object does exist, is an implicit reference conversion,
                    // and so we will use it.
                    baseType = this.GetReqPredefAgg(PredefinedType.PT_OBJECT).getThisType();
                }

                return GetBestAccessibleType(semanticChecker, bindingContext, baseType, out typeDst);
            }

            return false;
        }

        ////////////////////////////////////////////////////////////////////////////////

        private bool ExactNullableInference(CType pSource, CType pDest)
        {
            // SPEC:  Otherwise, if U is the CType U1? and V is the CType V1? 
            // SPEC:   then an exact inference is made from U to V.
            if (!pSource.IsNullableType() || !pDest.IsNullableType())
            {
                return false;
            }
            ExactInference(pSource.AsNullableType().GetUnderlyingType(),
                pDest.AsNullableType().GetUnderlyingType());
            return true;
        }

            /////////////////////////////////////////////////////////////////////////////////

            private static EXPR GenerateOptionalArgument(
                    SymbolLoader symbolLoader,
                    ExprFactory exprFactory,
                    MethodOrPropertySymbol methprop,
                    CType type,
                    int index)
            {
                CType pParamType = type;
                CType pRawParamType = type.IsNullableType() ? type.AsNullableType().GetUnderlyingType() : type;

                EXPR optionalArgument = null;
                if (methprop.HasDefaultParameterValue(index))
                {
                    CType pConstValType = methprop.GetDefaultParameterValueConstValType(index);
                    CONSTVAL cv = methprop.GetDefaultParameterValue(index);

                    if (pConstValType.isPredefType(PredefinedType.PT_DATETIME) &&
                        (pRawParamType.isPredefType(PredefinedType.PT_DATETIME) || pRawParamType.isPredefType(PredefinedType.PT_OBJECT) || pRawParamType.isPredefType(PredefinedType.PT_VALUE)))
                    {
                        // This is the specific case where we want to create a DateTime
                        // but the constval that stores it is a long.

                        AggregateType dateTimeType = symbolLoader.GetReqPredefType(PredefinedType.PT_DATETIME);
                        optionalArgument = exprFactory.CreateConstant(dateTimeType, new CONSTVAL(DateTime.FromBinary(cv.longVal)));
                    }
                    else if (pConstValType.isSimpleOrEnumOrString())
                    {
                        // In this case, the constval is a simple type (all the numerics, including
                        // decimal), or an enum or a string. This covers all the substantial values,
                        // and everything else that can be encoded is just null or default(something).

                        // For enum parameters, we create a constant of the enum type. For everything
                        // else, we create the appropriate constant.

                        if (pRawParamType.isEnumType() && pConstValType == pRawParamType.underlyingType())
                        {
                            optionalArgument = exprFactory.CreateConstant(pRawParamType, cv);
                        }
                        else
                        {
                            optionalArgument = exprFactory.CreateConstant(pConstValType, cv);
                        }
                    }
                    else if ((pParamType.IsRefType() || pParamType.IsNullableType()) && cv.IsNullRef())
                    {
                        // We have an "= null" default value with a reference type or a nullable type.

                        optionalArgument = exprFactory.CreateNull();
                    }
                    else
                    {
                        // We have a default value that is encoded as a nullref, and that nullref is
                        // interpreted as default(something). For instance, the pParamType could be
                        // a type parameter type or a non-simple value type.

                        optionalArgument = exprFactory.CreateZeroInit(pParamType);
                    }
                }
                else
                {
                    // There was no default parameter specified, so generally use default(T),
                    // except for some cases when the parameter type in metatdata is object.

                    if (pParamType.isPredefType(PredefinedType.PT_OBJECT))
                    {
                        if (methprop.MarshalAsObject(index))
                        {
                            // For [opt] parameters of type object, if we have marshal(iunknown),
                            // marshal(idispatch), or marshal(interface), then we emit a null.

                            optionalArgument = exprFactory.CreateNull();
                        }
                        else
                        {
                            // Otherwise, we generate Type.Missing

                            AggregateSymbol agg = symbolLoader.GetOptPredefAgg(PredefinedType.PT_MISSING);
                            Name name = symbolLoader.GetNameManager().GetPredefinedName(PredefinedName.PN_CAP_VALUE);
                            FieldSymbol field = symbolLoader.LookupAggMember(name, agg, symbmask_t.MASK_FieldSymbol).AsFieldSymbol();
                            FieldWithType fwt = new FieldWithType(field, agg.getThisType());
                            EXPRFIELD exprField = exprFactory.CreateField(0, agg.getThisType(), null, 0, fwt, null);

                            if (agg.getThisType() != type)
                            {
                                optionalArgument = exprFactory.CreateCast(0, type, exprField);
                            }
                            else
                            {
                                optionalArgument = exprField;
                            }
                        }
                    }
                    else
                    {
                        // Every type aside from object that doesn't have a default value gets
                        // its default value.

                        optionalArgument = exprFactory.CreateZeroInit(pParamType);
//.........这里部分代码省略.........

 /***************************************************************************************************
     Determines whether there is a wrapping conversion from typeSrc to typeDst
     
 13.7 Conversions involving nullable types
 
 The following terms are used in the subsequent sections:
 *   The term wrapping denotes the process of packaging a value, of type T, in an instance of type T?. 
     A value x of type T is wrapped to type T? by evaluating the expression new T?(x).
 ***************************************************************************************************/
 public static bool FWrappingConv(CType typeSrc, CType typeDst)
 {
     return typeDst.IsNullableType() && typeSrc == typeDst.AsNullableType().GetUnderlyingType();
 }

        private static bool CheckSingleConstraint(CSemanticChecker checker, ErrorHandling errHandling, Symbol symErr, TypeParameterType var, CType arg, TypeArray typeArgsCls, TypeArray typeArgsMeth, CheckConstraintsFlags flags)
        {
            bool fReportErrors = 0 == (flags & CheckConstraintsFlags.NoErrors);

            if (arg.IsOpenTypePlaceholderType())
            {
                return true;
            }

            if (arg.IsErrorType())
            {
                // Error should have been reported previously.
                return false;
            }

            if (checker.CheckBogus(arg))
            {
                if (fReportErrors)
                {
                    errHandling.ErrorRef(ErrorCode.ERR_BogusType, arg);
                }

                return false;
            }

            if (arg.IsPointerType() || arg.isSpecialByRefType())
            {
                if (fReportErrors)
                {
                    errHandling.Error(ErrorCode.ERR_BadTypeArgument, arg);
                }

                return false;
            }

            if (arg.isStaticClass())
            {
                if (fReportErrors)
                {
                    checker.ReportStaticClassError(null, arg, ErrorCode.ERR_GenericArgIsStaticClass);
                }

                return false;
            }

            bool fError = false;
            if (var.HasRefConstraint() && !arg.IsRefType())
            {
                if (fReportErrors)
                {
                    errHandling.ErrorRef(ErrorCode.ERR_RefConstraintNotSatisfied, symErr, new ErrArgNoRef(var), arg);
                }

                fError = true;
            }

            TypeArray bnds = checker.GetSymbolLoader().GetTypeManager().SubstTypeArray(var.GetBounds(), typeArgsCls, typeArgsMeth);
            int itypeMin = 0;

            if (var.HasValConstraint())
            {
                // If we have a type variable that is constrained to a value type, then we
                // want to check if its a nullable type, so that we can report the 
                // constraint error below. In order to do this however, we need to check 
                // that either the type arg is not a value type, or it is a nullable type.
                //
                // To check whether or not its a nullable type, we need to get the resolved
                // bound from the type argument and check against that.

                bool bIsValueType = arg.IsValType();
                bool bIsNullable = arg.IsNullableType();
                if (bIsValueType && arg.IsTypeParameterType())
                {
                    TypeArray pArgBnds = arg.AsTypeParameterType().GetBounds();
                    if (pArgBnds.size > 0)
                    {
                        bIsNullable = pArgBnds.Item(0).IsNullableType();
                    }
                }

                if (!bIsValueType || bIsNullable)
                {
                    if (fReportErrors)
                    {
                        errHandling.ErrorRef(ErrorCode.ERR_ValConstraintNotSatisfied, symErr, new ErrArgNoRef(var), arg);
                    }

                    fError = true;
                }

                // Since FValCon() is set it is redundant to check System.ValueType as well.
                if (bnds.size != 0 && bnds.Item(0).isPredefType(PredefinedType.PT_VALUE))
                {
                    itypeMin = 1;
                }
            }

            for (int j = itypeMin; j < bnds.size; j++)
            {
                CType typeBnd = bnds.Item(j);
//.........这里部分代码省略.........

        public bool IsBaseClass(CType pDerived, CType pBase)
        {
            Debug.Assert(pDerived != null);
            Debug.Assert(pBase != null);
            // A base class has got to be a class. The derived type might be a struct.

            if (!pBase.isClassType())
            {
                return false;
            }
            if (pDerived.IsNullableType())
            {
                pDerived = pDerived.AsNullableType().GetAts(ErrorContext);
                if (pDerived == null)
                {
                    return false;
                }
            }

            if (!pDerived.IsAggregateType())
            {
                return false;
            }

            AggregateType atsDer = pDerived.AsAggregateType();
            AggregateType atsBase = pBase.AsAggregateType();
            AggregateType atsCur = atsDer.GetBaseClass();
            while (atsCur != null)
            {
                if (atsCur == atsBase)
                {
                    return true;
                }
                atsCur = atsCur.GetBaseClass();
            }
            return false;
        }

        /*
            Same as CanConvertArg1 but with the indices interchanged!
        */
        private bool CanConvertArg2(BinOpArgInfo info, CType typeDst, out LiftFlags pgrflt,
                                      out CType ptypeSig1, out CType ptypeSig2)
        {
            Debug.Assert(!typeDst.IsNullableType());
            ptypeSig1 = null;
            ptypeSig2 = null;

            if (canConvert(info.arg2, typeDst))
                pgrflt = LiftFlags.None;
            else
            {
                pgrflt = LiftFlags.None;
                if (!GetSymbolLoader().FCanLift())
                    return false;
                typeDst = GetSymbolLoader().GetTypeManager().GetNullable(typeDst);
                if (!canConvert(info.arg2, typeDst))
                    return false;
                pgrflt = LiftFlags.Convert2;
            }
            ptypeSig2 = typeDst;

            if (info.type1.IsNullableType())
            {
                pgrflt = pgrflt | LiftFlags.Lift1;
                ptypeSig1 = GetSymbolLoader().GetTypeManager().GetNullable(info.typeRaw1);
            }
            else
                ptypeSig1 = info.typeRaw1;

            return true;
        }

        ////////////////////////////////////////////////////////////////////////////////
        // For a base call we need to remap from the virtual to the specific override 
        // to invoke.  This is also used to map a virtual on pObject (like ToString) to 
        // the specific override when the pObject is a simple type (int, bool, char, 
        // etc). In these cases it is safe to assume that any override won't later be 
        // removed.... We start searching from "typeObj" up the superclass hierarchy 
        // until we find a method with an exact signature match.

        public static void RemapToOverride(SymbolLoader symbolLoader, SymWithType pswt, CType typeObj)
        {
            // For a property/indexer we remap the accessors, not the property/indexer.
            // Since every event has both accessors we remap the event instead of the accessors.
            Debug.Assert(pswt && (pswt.Sym.IsMethodSymbol() || pswt.Sym.IsEventSymbol() || pswt.Sym.IsMethodOrPropertySymbol()));
            Debug.Assert(typeObj != null);

            // Don't remap static or interface methods.
            if (typeObj.IsNullableType())
            {
                typeObj = typeObj.AsNullableType().GetAts(symbolLoader.GetErrorContext());
                if (typeObj == null)
                {
                    VSFAIL("Why did GetAts return null?");
                    return;
                }
            }

            // Don't remap non-virtual members
            if (!typeObj.IsAggregateType() || typeObj.isInterfaceType() || !pswt.Sym.IsVirtual())
            {
                return;
            }

            symbmask_t mask = pswt.Sym.mask();

            AggregateType atsObj = typeObj.AsAggregateType();

            // Search for an override version of the method.
            while (atsObj != null && atsObj.getAggregate() != pswt.Sym.parent)
            {
                for (Symbol symT = symbolLoader.LookupAggMember(pswt.Sym.name, atsObj.getAggregate(), mask);
                     symT != null;
                     symT = symbolLoader.LookupNextSym(symT, atsObj.getAggregate(), mask))
                {
                    if (symT.IsOverride() && (symT.SymBaseVirtual() == pswt.Sym || symT.SymBaseVirtual() == pswt.Sym.SymBaseVirtual()))
                    {
                        pswt.Set(symT, atsObj);
                        return;
                    }
                }
                atsObj = atsObj.GetBaseClass();
            }
        }

        /////////////////////////////////////////////////////////////////////////////////

        private void LiftArgument(EXPR pArgument, CType pParameterType, bool bConvertBeforeLift,
                                            out EXPR ppLiftedArgument, out EXPR ppNonLiftedArgument)
        {
            EXPR pLiftedArgument = mustConvert(pArgument, pParameterType);
            if (pLiftedArgument != pArgument)
            {
                MarkAsIntermediateConversion(pLiftedArgument);
            }

            EXPR pNonLiftedArgument = pArgument;
            if (pParameterType.IsNullableType())
            {
                if (pNonLiftedArgument.isNull())
                {
                    pNonLiftedArgument = mustCast(pNonLiftedArgument, pParameterType);
                }
                pNonLiftedArgument = mustCast(pNonLiftedArgument, pParameterType.AsNullableType().GetUnderlyingType());
                if (bConvertBeforeLift)
                {
                    MarkAsIntermediateConversion(pNonLiftedArgument);
                }
            }
            else
            {
                pNonLiftedArgument = pLiftedArgument;
            }
            ppLiftedArgument = pLiftedArgument;
            ppNonLiftedArgument = pNonLiftedArgument;
        }

        public bool HasImplicitBoxingConversion(CType pSource, CType pDest)
        {
            Debug.Assert(pSource != null);
            Debug.Assert(pDest != null);

            // Certain type parameter conversions are classified as boxing conversions.

            if (pSource.IsTypeParameterType() &&
                HasImplicitBoxingTypeParameterConversion(pSource.AsTypeParameterType(), pDest))
            {
                return true;
            }

            // The rest of the boxing conversions only operate when going from a value type
            // to a reference type.

            if (!pSource.IsValType() || !pDest.IsRefType())
            {
                return false;
            }

            // A boxing conversion exists from a nullable type to a reference type
            // if and only if a boxing conversion exists from the underlying type.

            if (pSource.IsNullableType())
            {
                return HasImplicitBoxingConversion(pSource.AsNullableType().GetUnderlyingType(), pDest);
            }

            // A boxing conversion exists from any non-nullable value type to object,
            // to System.ValueType, and to any interface type implemented by the
            // non-nullable value type.  Furthermore, an enum type can be converted
            // to the type System.Enum.

            // We set the base class of the structs to System.ValueType, System.Enum, etc,
            // so we can just check here.

            if (IsBaseClass(pSource, pDest))
            {
                return true;
            }
            if (HasAnyBaseInterfaceConversion(pSource, pDest))
            {
                return true;
            }
            return false;
        }

//.........这里部分代码省略.........
            //    }

            // VARIANCE EXTENSIONS:
            // * From any class type S to any interface type T provided S implements an interface
            //   convertible to T.
            // * From any interface type S to any interface type T provided S implements an interface
            //   convertible to T.
            // * From any interface type S to any interface type T provided S is not T and S is 
            //   an interface convertible to T.

            if (pSource.isClassType() && pDest.isInterfaceType() && HasAnyBaseInterfaceConversion(pSource, pDest))
            {
                return true;
            }
            if (pSource.isInterfaceType() && pDest.isInterfaceType() && HasAnyBaseInterfaceConversion(pSource, pDest))
            {
                return true;
            }
            if (pSource.isInterfaceType() && pDest.isInterfaceType() && pSource != pDest &&
                HasInterfaceConversion(pSource.AsAggregateType(), pDest.AsAggregateType()))
            {
                return true;
            }

            // * From an array type S with an element type SE to an array type T with element type TE
            //   provided that all of the following are true:
            //   * S and T differ only in element type. In other words, S and T have the same number of dimensions.
            //   * Both SE and TE are reference types.
            //   * An implicit reference conversion exists from SE to TE.
            if (pSource.IsArrayType() && pDest.IsArrayType() &&
                HasCovariantArrayConversion(pSource.AsArrayType(), pDest.AsArrayType()))
            {
                return true;
            }
            // * From any array type to System.Array or any interface implemented by System.Array.
            if (pSource.IsArrayType() && (pDest.isPredefType(PredefinedType.PT_ARRAY) ||
                IsBaseInterface(GetReqPredefType(PredefinedType.PT_ARRAY, false), pDest)))
            {
                return true;
            }
            // * From a single-dimensional array type S[] to IList<T> and its base
            //   interfaces, provided that there is an implicit identity or reference
            //   conversion from S to T.
            if (pSource.IsArrayType() && HasArrayConversionToInterface(pSource.AsArrayType(), pDest))
            {
                return true;
            }

            // * From any delegate type to System.Delegate
            // 
            // SPEC OMISSION:
            // 
            // The spec should actually say
            //
            // * From any delegate type to System.Delegate 
            // * From any delegate type to System.MulticastDelegate
            // * From any delegate type to any interface implemented by System.MulticastDelegate
            if (pSource.isDelegateType() &&
                (pDest.isPredefType(PredefinedType.PT_MULTIDEL) ||
                pDest.isPredefType(PredefinedType.PT_DELEGATE) ||
                IsBaseInterface(GetReqPredefType(PredefinedType.PT_MULTIDEL, false), pDest)))
            {
                return true;
            }

            // VARIANCE EXTENSION:
            // * From any delegate type S to a delegate type T provided S is not T and
            //   S is a delegate convertible to T

            if (pSource.isDelegateType() && pDest.isDelegateType() &&
                HasDelegateConversion(pSource.AsAggregateType(), pDest.AsAggregateType()))
            {
                return true;
            }

            // * From the null literal to any reference type
            // NOTE: We extend the specification here. The C# 3.0 spec does not describe
            // a "null type". Rather, it says that the null literal is typeless, and is
            // convertible to any reference or nullable type. However, the C# 2.0 and 3.0
            // implementations have a "null type" which some expressions other than the
            // null literal may have. (For example, (null??null), which is also an
            // extension to the specification.)
            if (pSource.IsNullType() && pDest.IsRefType())
            {
                return true;
            }
            if (pSource.IsNullType() && pDest.IsNullableType())
            {
                return true;
            }

            // * Implicit conversions involving type parameters that are known to be reference types.
            if (pSource.IsTypeParameterType() &&
                HasImplicitReferenceTypeParameterConversion(pSource.AsTypeParameterType(), pDest))
            {
                return true;
            }

            return false;
        }

 protected bool IsNullableValueType(CType pType)
 {
     if (pType.IsNullableType())
     {
         CType pStrippedType = pType.StripNubs();
         return pStrippedType.IsAggregateType() && pStrippedType.AsAggregateType().getAggregate().IsValueType();
     }
     return false;
 }

        // Check the constraints of any type arguments in the given Type.
        public static bool CheckConstraints(CSemanticChecker checker, ErrorHandling errHandling, CType type, CheckConstraintsFlags flags)
        {
            type = type.GetNakedType(false);

            if (type.IsNullableType())
            {
                CType typeT = type.AsNullableType().GetAts(checker.GetErrorContext());
                if (typeT != null)
                    type = typeT;
                else
                    type = type.GetNakedType(true);
            }

            if (!type.IsAggregateType())
                return true;

            AggregateType ats = type.AsAggregateType();

            if (ats.GetTypeArgsAll().size == 0)
            {
                // Common case: there are no type vars, so there are no constraints.
                ats.fConstraintsChecked = true;
                ats.fConstraintError = false;
                return true;
            }

            if (ats.fConstraintsChecked)
            {
                // Already checked.
                if (!ats.fConstraintError || (flags & CheckConstraintsFlags.NoDupErrors) != 0)
                {
                    // No errors or no need to report errors again.
                    return !ats.fConstraintError;
                }
            }

            TypeArray typeVars = ats.getAggregate().GetTypeVars();
            TypeArray typeArgsThis = ats.GetTypeArgsThis();
            TypeArray typeArgsAll = ats.GetTypeArgsAll();

            Debug.Assert(typeVars.size == typeArgsThis.size);

            if (!ats.fConstraintsChecked)
            {
                ats.fConstraintsChecked = true;
                ats.fConstraintError = false;
            }

            // Check the outer type first. If CheckConstraintsFlags.Outer is not specified and the
            // outer type has already been checked then don't bother checking it.
            if (ats.outerType != null && ((flags & CheckConstraintsFlags.Outer) != 0 || !ats.outerType.fConstraintsChecked))
            {
                CheckConstraints(checker, errHandling, ats.outerType, flags);
                ats.fConstraintError |= ats.outerType.fConstraintError;
            }

            if (typeVars.size > 0)
                ats.fConstraintError |= !CheckConstraintsCore(checker, errHandling, ats.getAggregate(), typeVars, typeArgsThis, typeArgsAll, null, (flags & CheckConstraintsFlags.NoErrors));

            // Now check type args themselves.
            for (int i = 0; i < typeArgsThis.size; i++)
            {
                CType arg = typeArgsThis.Item(i).GetNakedType(true);
                if (arg.IsAggregateType() && !arg.AsAggregateType().fConstraintsChecked)
                {
                    CheckConstraints(checker, errHandling, arg.AsAggregateType(), flags | CheckConstraintsFlags.Outer);
                    if (arg.AsAggregateType().fConstraintError)
                        ats.fConstraintError = true;
                }
            }
            return !ats.fConstraintError;
        }