internal static bool ReportDefaultParameterErrors( Binder binder, Symbol owner, ParameterSyntax parameterSyntax, SourceParameterSymbol parameter, BoundExpression defaultExpression, BoundExpression convertedExpression, DiagnosticBag diagnostics) { bool hasErrors = false; // SPEC VIOLATION: The spec says that the conversion from the initializer to the // parameter type is required to be either an identity or a nullable conversion, but // that is not right: // // void M(short myShort = 10) {} // * not an identity or nullable conversion but should be legal // // void M(object obj = (dynamic)null) {} // * an identity conversion, but should be illegal // // void M(MyStruct? myStruct = default(MyStruct)) {} // * a nullable conversion, but must be illegal because we cannot generate metadata for it // // Even if the expression is thoroughly illegal, we still want to bind it and // stick it in the parameter because we want to be able to analyze it for // IntelliSense purposes. TypeSymbol parameterType = parameter.Type; HashSet <DiagnosticInfo> useSiteDiagnostics = null; Conversion conversion = binder.Conversions.ClassifyImplicitConversionFromExpression(defaultExpression, parameterType, ref useSiteDiagnostics); diagnostics.Add(defaultExpression.Syntax, useSiteDiagnostics); var refKind = GetModifiers(parameterSyntax.Modifiers, out SyntaxToken refnessKeyword, out SyntaxToken paramsKeyword, out SyntaxToken thisKeyword); // CONSIDER: We are inconsistent here regarding where the error is reported; is it // CONSIDER: reported on the parameter name, or on the value of the initializer? // CONSIDER: Consider making this consistent. if (refKind == RefKind.Ref || refKind == RefKind.Out) { // error CS1741: A ref or out parameter cannot have a default value diagnostics.Add(ErrorCode.ERR_RefOutDefaultValue, refnessKeyword.GetLocation()); hasErrors = true; } else if (paramsKeyword.Kind() == SyntaxKind.ParamsKeyword) { // error CS1751: Cannot specify a default value for a parameter array diagnostics.Add(ErrorCode.ERR_DefaultValueForParamsParameter, paramsKeyword.GetLocation()); hasErrors = true; } else if (thisKeyword.Kind() == SyntaxKind.ThisKeyword) { // Only need to report CS1743 for the first parameter. The caller will // have reported CS1100 if 'this' appeared on another parameter. if (parameter.Ordinal == 0) { // error CS1743: Cannot specify a default value for the 'this' parameter diagnostics.Add(ErrorCode.ERR_DefaultValueForExtensionParameter, thisKeyword.GetLocation()); hasErrors = true; } } else if (!defaultExpression.HasAnyErrors && !IsValidDefaultValue(defaultExpression.IsTypelessNew() ? convertedExpression : defaultExpression)) { // error CS1736: Default parameter value for '{0}' must be a compile-time constant diagnostics.Add(ErrorCode.ERR_DefaultValueMustBeConstant, parameterSyntax.Default.Value.Location, parameterSyntax.Identifier.ValueText); hasErrors = true; } else if (!conversion.Exists || conversion.IsUserDefined || conversion.IsIdentity && parameterType.SpecialType == SpecialType.System_Object && defaultExpression.Type.IsDynamic()) { // If we had no implicit conversion, or a user-defined conversion, report an error. // // Even though "object x = (dynamic)null" is a legal identity conversion, we do not allow it. // CONSIDER: We could. Doesn't hurt anything. // error CS1750: A value of type '{0}' cannot be used as a default parameter because there are no standard conversions to type '{1}' diagnostics.Add(ErrorCode.ERR_NoConversionForDefaultParam, parameterSyntax.Identifier.GetLocation(), defaultExpression.Display, parameterType); hasErrors = true; } else if (conversion.IsReference && (parameterType.SpecialType == SpecialType.System_Object || parameterType.Kind == SymbolKind.DynamicType) && (object)defaultExpression.Type != null && defaultExpression.Type.SpecialType == SpecialType.System_String || conversion.IsBoxing) { // We don't allow object x = "hello", object x = 123, dynamic x = "hello", etc. // error CS1763: '{0}' is of type '{1}'. A default parameter value of a reference type other than string can only be initialized with null diagnostics.Add(ErrorCode.ERR_NotNullRefDefaultParameter, parameterSyntax.Identifier.GetLocation(), parameterSyntax.Identifier.ValueText, parameterType); hasErrors = true; } else if (conversion.IsNullable && !defaultExpression.Type.IsNullableType() && !(parameterType.GetNullableUnderlyingType().IsEnumType() || parameterType.GetNullableUnderlyingType().IsIntrinsicType())) { // We can do: // M(int? x = default(int)) // M(int? x = default(int?)) // M(MyEnum? e = default(enum)) // M(MyEnum? e = default(enum?)) // M(MyStruct? s = default(MyStruct?)) // // but we cannot do: // // M(MyStruct? s = default(MyStruct)) // error CS1770: // A value of type '{0}' cannot be used as default parameter for nullable parameter '{1}' because '{0}' is not a simple type diagnostics.Add(ErrorCode.ERR_NoConversionForNubDefaultParam, parameterSyntax.Identifier.GetLocation(), defaultExpression.Type, parameterSyntax.Identifier.ValueText); hasErrors = true; } // Certain contexts allow default parameter values syntactically but they are ignored during // semantic analysis. They are: // 1. Explicitly implemented interface methods; since the method will always be called // via the interface, the defaults declared on the implementation will not // be seen at the call site. // // UNDONE: 2. The "actual" side of a partial method; the default values are taken from the // UNDONE: "declaring" side of the method. // // UNDONE: 3. An indexer with only one formal parameter; it is illegal to omit every argument // UNDONE: to an indexer. // // 4. A user-defined operator; it is syntactically impossible to omit the argument. if (owner.IsExplicitInterfaceImplementation() || owner.IsPartialImplementation() || owner.IsOperator()) { // CS1066: The default value specified for parameter '{0}' will have no effect because it applies to a // member that is used in contexts that do not allow optional arguments diagnostics.Add(ErrorCode.WRN_DefaultValueForUnconsumedLocation, parameterSyntax.Identifier.GetLocation(), parameterSyntax.Identifier.ValueText); } return(hasErrors); }