C# CType.IsTypeParameterType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.........

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

        private bool UpperBoundTypeParameterInference(CType pSource, CType pDest)
        {
            // SPEC:  If V is one of the unfixed Xi then U is added to the set of upper bounds
            // SPEC:   for Xi.
            if (pDest.IsTypeParameterType())
            {
                TypeParameterType pTPType = pDest.AsTypeParameterType();
                if (pTPType.IsMethodTypeParameter() && IsUnfixed(pTPType))
                {
                    AddUpperBound(pTPType, pSource);
                    return true;
                }
            }
            return false;
        }

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

        private bool LowerBoundInterfaceInference(CType pSource, AggregateType pDest)
        {
            if (!pDest.isInterfaceType())
            {
                return false;
            }

            // SPEC:  Otherwise, if V is an interface CType C<V1...Vk> and U is a class CType
            // SPEC:   or struct CType and there is a unique set U1...Uk such that U directly 
            // SPEC:   or indirectly implements C<U1...Uk> then an
            // SPEC:   exact, upper-bound, or lower-bound inference ...
            // SPEC:  ... and U is an interface CType ...
            // SPEC:  ... and U is a CType parameter ...

            //TypeArray pInterfaces = null;

            if (!pSource.isStructType() && !pSource.isClassType() &&
                !pSource.isInterfaceType() && !pSource.IsTypeParameterType())
            {
                return false;
            }

            var interfaces = pSource.AllPossibleInterfaces();
            AggregateType pInterface = null;
            foreach (AggregateType pCurrent in interfaces)
            {
                if (pCurrent.GetOwningAggregate() == pDest.GetOwningAggregate())
                {
                    if (pInterface == null)
                    {
                        pInterface = pCurrent;
                    }
                    else if (pInterface != pCurrent)
                    {
                        // Not unique. Bail out.
                        return false;
                    }
                }
            }
            if (pInterface == null)
            {
                return false;
            }
            LowerBoundTypeArgumentInference(pInterface, pDest);
            return true;
        }

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

        private bool LowerBoundClassInference(CType pSource, AggregateType pDest)
        {
            if (!pDest.isClassType())
            {
                return false;
            }

            // SPEC:  Otherwise, if V is a class CType C<V1...Vk> and U is a class CType which
            // SPEC:   inherits directly or indirectly from C<U1...Uk> 
            // SPEC:   then an exact inference is made from each Ui to the corresponding Vi.
            // SPEC:  Otherwise, if V is a class CType C<V1...Vk> and U is a CType parameter
            // SPEC:   with effective base class C<U1...Uk> 
            // SPEC:   then an exact inference is made from each Ui to the corresponding Vi.
            // SPEC:  Otherwise, if V is a class CType C<V1...Vk> and U is a CType parameter
            // SPEC:   with an effective base class which inherits directly or indirectly from
            // SPEC:   C<U1...Uk> then an exact inference is made
            // SPEC:   from each Ui to the corresponding Vi.

            AggregateType pSourceBase = null;

            if (pSource.isClassType())
            {
                pSourceBase = pSource.AsAggregateType().GetBaseClass();
            }
            else if (pSource.IsTypeParameterType())
            {
                pSourceBase = pSource.AsTypeParameterType().GetEffectiveBaseClass();
            }

            while (pSourceBase != null)
            {
                if (pSourceBase.GetOwningAggregate() == pDest.GetOwningAggregate())
                {
                    ExactTypeArgumentInference(pSourceBase, pDest);
                    return true;
                }
                pSourceBase = pSourceBase.GetBaseClass();
            }
            return false;
        }

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

        private bool LowerBoundArrayInference(CType pSource, CType pDest)
        {
            // SPEC:  Otherwise, if U is an array CType Ue[...] and V is either an array
            // SPEC:   CType Ve[...] of the same rank, or if U is a one-dimensional array
            // SPEC:   CType Ue[] and V is one of IEnumerable<Ve>, ICollection<Ve>, 
            // SPEC:   IList<Ve>, IReadOnlyCollection<Ve> or IReadOnlyList<Ve> then
            // SPEC:    if Ue is known to be a reference CType then a lower-bound inference
            // SPEC:     from Ue to Ve is made.
            // SPEC:    otherwise an exact inference from Ue to Ve is made.

            // Consider the following:
            //
            // abstract class B<T> { public abstract M<U>(U u) : where U : T; }
            // class D : B<int[]> {
            //   static void M<X>(X[] x) { }
            //   public override M<U>(U u) { M(u); } // should infer M<int>
            // }

            if (pSource.IsTypeParameterType())
            {
                pSource = pSource.AsTypeParameterType().GetEffectiveBaseClass();
            }

            if (!pSource.IsArrayType())
            {
                return false;
            }
            ArrayType pArraySource = pSource.AsArrayType();
            CType pElementSource = pArraySource.GetElementType();
            CType pElementDest = null;

            if (pDest.IsArrayType())
            {
                ArrayType pArrayDest = pDest.AsArrayType();
                if (pArrayDest.rank != pArraySource.rank)
                {
                    return false;
                }
                pElementDest = pArrayDest.GetElementType();
            }
            else if (pDest.isPredefType(PredefinedType.PT_G_IENUMERABLE) ||
                pDest.isPredefType(PredefinedType.PT_G_ICOLLECTION) ||
                pDest.isPredefType(PredefinedType.PT_G_ILIST) ||
                pDest.isPredefType(PredefinedType.PT_G_IREADONLYCOLLECTION) ||
                pDest.isPredefType(PredefinedType.PT_G_IREADONLYLIST))
            {
                if (pArraySource.rank != 1)
                {
                    return false;
                }
                AggregateType pAggregateDest = pDest.AsAggregateType();
                pElementDest = pAggregateDest.GetTypeArgsThis().Item(0);
            }
            else
            {
                return false;
            }

            if (pElementSource.IsRefType())
            {
                LowerBoundInference(pElementSource, pElementDest);
            }
            else
            {
                ExactInference(pElementSource, pElementDest);
            }
            return true;
        }

        // !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
        // 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;
        }

        // It would be nice to make this a virtual method on typeSym.
        public AggregateType GetAggTypeSym(CType typeSym)
        {
            Debug.Assert(typeSym != null);
            Debug.Assert(typeSym.IsAggregateType() ||
                   typeSym.IsTypeParameterType() ||
                   typeSym.IsArrayType() ||
                   typeSym.IsNullableType());

            switch (typeSym.GetTypeKind())
            {
                case TypeKind.TK_AggregateType:
                    return typeSym.AsAggregateType();
                case TypeKind.TK_ArrayType:
                    return GetReqPredefType(PredefinedType.PT_ARRAY);
                case TypeKind.TK_TypeParameterType:
                    return typeSym.AsTypeParameterType().GetEffectiveBaseClass();
                case TypeKind.TK_NullableType:
                    return typeSym.AsNullableType().GetAts(ErrorContext);
            }
            Debug.Assert(false, "Bad typeSym!");
            return null;
        }

        ////////////////////////////////////////////////////////////////////////////////
        // Determine whether the arg type satisfies the typeBnd constraint. Note that 
        // typeBnd could be just about any type (since we added naked type parameter
        // constraints).

        private static bool SatisfiesBound(CSemanticChecker checker, CType arg, CType typeBnd)
        {
            if (typeBnd == arg)
                return true;

            switch (typeBnd.GetTypeKind())
            {
                default:
                    Debug.Assert(false, "Unexpected type.");
                    return false;

                case TypeKind.TK_VoidType:
                case TypeKind.TK_PointerType:
                case TypeKind.TK_ErrorType:
                    return false;

                case TypeKind.TK_ArrayType:
                case TypeKind.TK_TypeParameterType:
                    break;

                case TypeKind.TK_NullableType:
                    typeBnd = typeBnd.AsNullableType().GetAts(checker.GetErrorContext());
                    if (null == typeBnd)
                        return true;
                    break;

                case TypeKind.TK_AggregateType:
                    break;
            }

            Debug.Assert(typeBnd.IsAggregateType() || typeBnd.IsTypeParameterType() || typeBnd.IsArrayType());

            switch (arg.GetTypeKind())
            {
                default:
                    return false;
                case TypeKind.TK_ErrorType:
                case TypeKind.TK_PointerType:
                    return false;
                case TypeKind.TK_NullableType:
                    arg = arg.AsNullableType().GetAts(checker.GetErrorContext());
                    if (null == arg)
                        return true;
                    // Fall through.
                    goto case TypeKind.TK_TypeParameterType;
                case TypeKind.TK_TypeParameterType:
                case TypeKind.TK_ArrayType:
                case TypeKind.TK_AggregateType:
                    return checker.GetSymbolLoader().HasBaseConversion(arg, typeBnd);
            }
        }

        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 HasBaseConversion(CType pSource, CType pDest)
        {
            // By a "base conversion" we mean:
            //
            // * an identity conversion
            // * an implicit reference conversion
            // * an implicit boxing conversion
            // * an implicit type parameter conversion
            //
            // In other words, these are conversions that can be made to a base
            // class, base interface or co/contravariant type without any change in
            // representation other than boxing.  A conversion from, say, int to double, 
            // is NOT a "base conversion", because representation is changed.  A conversion
            // from, say, lambda to expression tree is not a "base conversion" because 
            // do not have a type.
            //
            // The existence of a base conversion depends solely upon the source and
            // destination types, not the source expression.
            //
            // This notion is not found in the spec but it is useful in the implementation.

            if (pSource.IsAggregateType() && pDest.isPredefType(PredefinedType.PT_OBJECT))
            {
                // If we are going from any aggregate type (class, struct, interface, enum or delegate)
                // to object, we immediately return true. This may seem like a mere optimization --
                // after all, if we have an aggregate then we have some kind of implicit conversion
                // to object.
                //
                // However, it is not a mere optimization; this introduces a control flow change
                // in error reporting scenarios for unresolved type forwarders. If a type forwarder
                // cannot be resolved then the resulting type symbol will be an aggregate, but
                // we will not be able to classify it into class, struct, etc.
                //
                // We know that we will have an error in this case; we do not wish to compound
                // that error by giving a spurious "you cannot convert this thing to object"
                // error, which, after all, will go away when the type forwarding problem is
                // fixed.
                return true;
            }

            if (HasIdentityOrImplicitReferenceConversion(pSource, pDest))
            {
                return true;
            }
            if (HasImplicitBoxingConversion(pSource, pDest))
            {
                return true;
            }
            if (pSource.IsTypeParameterType() &&
                HasImplicitTypeParameterBaseConversion(pSource.AsTypeParameterType(), pDest))
            {
                return true;
            }
            return false;
        }

        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;
        }

        private bool HasImplicitTypeParameterBaseConversion(
            TypeParameterType pSource, CType pDest)
        {
            Debug.Assert(pSource != null);
            Debug.Assert(pDest != null);

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

        private bool HasImplicitReferenceTypeParameterConversion(
            TypeParameterType pSource, CType pDest)
        {
            Debug.Assert(pSource != null);
            Debug.Assert(pDest != null);

            if (!pSource.IsRefType())
            {
                // Not a reference conversion.
                return false;
            }

            // The following implicit conversions exist for a given type parameter T:
            //
            // * From T to its effective base class C.
            AggregateType pEBC = pSource.GetEffectiveBaseClass();
            if (pDest == pEBC)
            {
                return true;
            }
            // * From T to any base class of C.
            if (IsBaseClass(pEBC, pDest))
            {
                return true;
            }
            // * From T to any interface implemented by C.
            if (IsBaseInterface(pEBC, pDest))
            {
                return true;
            }
            // * From T to any interface type I in T's effective interface set, and
            //   from T to any base interface of I.
            TypeArray pInterfaces = pSource.GetInterfaceBounds();
            for (int i = 0; i < pInterfaces.Size; ++i)
            {
                if (pInterfaces.Item(i) == pDest)
                {
                    return true;
                }
            }
            // * From T to a type parameter U, provided T depends on U.
            if (pDest.IsTypeParameterType() && pSource.DependsOn(pDest.AsTypeParameterType()))
            {
                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;
        }