C# TypeSymbol.ContainsTypeParameter方法代码示例

        private static bool ReportAsOperatorConversionDiagnostics(
            CSharpSyntaxNode node,
            DiagnosticBag diagnostics,
            Compilation compilation,
            TypeSymbol operandType,
            TypeSymbol targetType,
            ConversionKind conversionKind,
            ConstantValue operandConstantValue)
        {
            // SPEC:    In an operation of the form E as T, E must be an expression and T must be a reference type,
            // SPEC:    a type parameter known to be a reference type, or a nullable type.
            // SPEC:    Furthermore, at least one of the following must be true, or otherwise a compile-time error occurs:
            // SPEC:    •	An identity (§6.1.1), implicit nullable (§6.1.4), implicit reference (§6.1.6), boxing (§6.1.7), 
            // SPEC:        explicit nullable (§6.2.3), explicit reference (§6.2.4), or unboxing (§6.2.5) conversion exists
            // SPEC:        from E to T.
            // SPEC:    •	The type of E or T is an open type.
            // SPEC:    •	E is the null literal.

            // SPEC VIOLATION:  The specification contains an error in the list of legal conversions above.
            // SPEC VIOLATION:  If we have "class C<T, U> where T : U where U : class" then there is
            // SPEC VIOLATION:  an implicit conversion from T to U, but it is not an identity, reference or
            // SPEC VIOLATION:  boxing conversion. It will be one of those at runtime, but at compile time
            // SPEC VIOLATION:  we do not know which, and therefore cannot classify it as any of those.
            // SPEC VIOLATION:  See Microsoft.CodeAnalysis.CSharp.UnitTests.SyntaxBinderTests.TestAsOperator_SpecErrorCase() test for an example.

            // SPEC VIOLATION:  The specification also unintentionally allows the case where requirement 2 above:
            // SPEC VIOLATION:  "The type of E or T is an open type" is true, but type of E is void type, i.e. T is an open type.
            // SPEC VIOLATION:  Dev10 compiler correctly generates an error for this case and we will maintain compatibility.

            bool hasErrors = false;
            switch (conversionKind)
            {
                case ConversionKind.ImplicitReference:
                case ConversionKind.Boxing:
                case ConversionKind.ImplicitNullable:
                case ConversionKind.Identity:
                case ConversionKind.ExplicitNullable:
                case ConversionKind.ExplicitReference:
                case ConversionKind.Unboxing:
                    break;

                default:
                    // Generate an error if there is no possible legal conversion and both the operandType
                    // and the targetType are closed types OR operandType is void type, otherwise we need a runtime check
                    if (!operandType.ContainsTypeParameter() && !targetType.ContainsTypeParameter() ||
                        operandType.SpecialType == SpecialType.System_Void)
                    {
                        SymbolDistinguisher distinguisher = new SymbolDistinguisher(compilation, operandType, targetType);
                        Error(diagnostics, ErrorCode.ERR_NoExplicitBuiltinConv, node, distinguisher.First, distinguisher.Second);
                        hasErrors = true;
                    }

                    break;
            }

            if (!hasErrors)
            {
                ReportAsOperatorConstantWarnings(node, diagnostics, operandType, targetType, conversionKind, operandConstantValue);
            }

            return hasErrors;
        }

        internal static ConstantValue GetIsOperatorConstantResult(TypeSymbol operandType, TypeSymbol targetType, ConversionKind conversionKind, ConstantValue operandConstantValue)
        {
            Debug.Assert((object)targetType != null);

            // SPEC:    The result of the operation depends on D and T as follows:
            // SPEC:    1)	If T is a reference type, the result is true if D and T are the same type, if D is a reference type and
            // SPEC:        an implicit reference conversion from D to T exists, or if D is a value type and a boxing conversion from D to T exists.
            // SPEC:    2)	If T is a nullable type, the result is true if D is the underlying type of T.
            // SPEC:    3)	If T is a non-nullable value type, the result is true if D and T are the same type.
            // SPEC:    4)	Otherwise, the result is false.

            // NOTE:    The language specification talks about the runtime evaluation of the is operation.
            // NOTE:    However, we are interested in computing the compile time constant value for the expression.
            // NOTE:    Even though BoundIsOperator and BoundAsOperator will always have no ConstantValue
            // NOTE:    (they are non-constant expressions according to Section 7.19 of the specification),
            // NOTE:    we want to perform constant analysis of is/as expressions during binding to generate warnings (always true/false/null)
            // NOTE:    and during rewriting for optimized codegen.
            // NOTE: 
            // NOTE:    Because the heuristic presented here is used to change codegen, it must be conservative. It is acceptable
            // NOTE:    for us to fail to report a warning in cases where humans could logically deduce that the operator will
            // NOTE:    always return false. It is not acceptable to inaccurately warn that the operator will always return false
            // NOTE:    if there are cases where it might succeed.
            // 

            // To begin our heuristic: if the operand is literal null then we automatically return that the
            // result is false. You might think that we can simply check to see if the conversion is 
            // ConversionKind.NullConversion, but "null is T" for a type parameter T is actually classified
            // as an implicit reference conversion if T is constrained to reference types. Rather
            // than deal with all those special cases we can simply bail out here.

            if (operandConstantValue == ConstantValue.Null)
            {
                return ConstantValue.False;
            }

            Debug.Assert((object)operandType != null);

            switch (conversionKind)
            {
                case ConversionKind.NoConversion:
                    // Oddly enough, "x is T" can be true even if there is no conversion from x to T! 
                    //
                    // Scenario 1: Type parameter compared to System.Enum.
                    //
                    // bool M1<X>(X x) where X : struct { return x is Enum; }
                    //
                    // There is no conversion from X to Enum, not even an explicit conversion. But
                    // nevertheless, X could be constructed as an enumerated type.
                    // However, we can sometimes know that the result will be false.
                    //
                    // Scenario 2: Constrained type parameter compared to reference type.
                    //
                    // bool M2<X>(X x) where X : struct { return x is string; }
                    //
                    // We know that X, constrained to struct, will never be string.
                    //
                    // Scenario 3: Value type compared to type parameter.
                    //
                    // bool M3<T>(int x) { return x is T; }
                    //
                    // There is no conversion from int to T, but T could nevertheless be int.
                    //
                    // Scenario 4: Constructed type compared to open type
                    //
                    // bool M4<T>(C<int> x) { return x is C<T>; } 
                    //
                    // There is no conversion from C<int> to C<T>, but nevertheless, T might be int.
                    //
                    // Scenario 5: Open type compared to constructed type:
                    //
                    // bool M5<X>(C<X> x) { return x is C<int>);
                    //
                    // Again, X could be int.
                    // 
                    // We could then go on to get more complicated. For example, 
                    //
                    // bool M6<X>(C<X> x) where X : struct { return x is C<string>; }
                    //
                    // We know that C<X> is never convertible to C<string> no matter what
                    // X is. Or:
                    //
                    // bool M7<T>(Dictionary<int, int> x) { return x is List<T>; }
                    //
                    // We know that no matter what T is, the conversion will never succeed.
                    //
                    // As noted above, we must be conservative. We follow the lead of the native compiler,
                    // which uses the following algorithm:
                    //
                    // * If neither type is open and there is no conversion then the result is always false:

                    if (!operandType.ContainsTypeParameter() && !targetType.ContainsTypeParameter())
                    {
                        return ConstantValue.False;
                    }

                    // * Otherwise, at least one of them is of an open type. If the operand is of value type 
                    //   and the target is a class type other than System.Enum, then we are in scenario 2, 
                    //   not scenario 1, and can correctly deduce that the result is false.

                    if (operandType.IsValueType && targetType.IsClassType() && targetType.SpecialType != SpecialType.System_Enum)
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