Resolve errors and warnings related to attribute declarations or attribute use in your code

This article covers the following compiler errors:

• CS0181: Attribute constructor parameter has a type, which is not a valid attribute parameter type
• CS0243: The Conditional attribute is not valid on 'method' because it is an override method.
• CS0404: Attribute is not valid on this declaration type.
• CS0415: This attribute is valid only on an indexer that is not an explicit interface member declaration.
• CS0416: 'type parameter': an attribute argument cannot use type parameters.
• CS0447: Attribute cannot be used with type arguments.
• CS0577: The Conditional attribute is not valid because it is a constructor, destructor, operator, lambda expression, or explicit interface implementation.
• CS0578: The Conditional attribute is not valid on 'function' because its return type is not void.
• CS0579: Duplicate attribute
• CS0582: The Conditional attribute is not valid on interface members.
• CS0592: Attribute is not valid on this declaration type. It is only valid on specific declarations.
• CS0609: Cannot set the attribute on an indexer marked override.
• CS0616: Type is not an attribute class
• CS0625: Instance field in types marked with StructLayout(LayoutKind.Explicit) must have a FieldOffset attribute.
• CS0629: Conditional member 'member' cannot implement interface member 'base class member' in type 'Type Name'.
• CS0636: The FieldOffset attribute can only be placed on members of types marked with the StructLayout(LayoutKind.Explicit).
• CS0637: The FieldOffset attribute is not allowed on static or const fields.
• CS0641: This attribute is only valid on classes derived from System.Attribute`.
• CS0646: Cannot specify the DefaultMember attribute on a type containing an indexer.
• CS0647: Error emitting attribute
• CS0653: Cannot apply attribute class 'class' because it is abstract.
• CS0657: Location is not a valid attribute location for this declaration. Valid attribute locations for this declaration are listed. All attributes in this block will be ignored.
• CS0658: Location is not a recognized attribute location. Valid attribute locations for this declaration are listed. All attributes in this block will be ignored.
• CS0668: Two indexers have different names; the IndexerName attribute must be used with the same name on every indexer within a type
• CS0685: Conditional member 'member' cannot have an out parameter.
• CS0735: Invalid type specified as an argument for TypeForwardedTo attribute
• CS0739: Duplicate TypeForwardedToAttribute
• CS1608: The RequiredAttribute attribute is not permitted on C# types
• CS1614: Attribute name is ambiguous. Either use '@name' or explicitly include the 'Attribute' suffix.
• CS1618: Cannot create delegate with method because it or a method it overrides has a Conditional attribute
• CS1667: Attribute is not valid on property or event accessors. It is only valid on specific declarations.
• CS1689: Attribute is only valid on methods or attribute classes
• CS7014: Attributes are not valid in this context.
• CS7046: Attribute parameter must be specified.
• CS7047: Attribute parameter 'parameter1' or 'parameter2' must be specified.
• CS7067: Attribute constructor parameter is optional, but no default parameter value was specified.
• CS8959: CallerArgumentExpressionAttribute cannot be applied because there are no standard conversions from type1 to type2
• CS8960: The CallerArgumentExpressionAttribute applied to parameter will have no effect. It is overridden by the CallerLineNumberAttribute.
• CS8961: The CallerArgumentExpressionAttribute applied to parameter will have no effect. It is overridden by the CallerFilePathAttribute.
• CS8962: The CallerArgumentExpressionAttribute applied to parameter will have no effect. It is overridden by the CallerMemberNameAttribute.
• CS8963: The CallerArgumentExpressionAttribute applied to parameter will have no effect. It is applied with an invalid parameter name.
• CS8968: An attribute type argument cannot use type parameters
• CS8970: Type cannot be used in this context because it cannot be represented in metadata.
• CS9331: Attribute cannot be applied manually.

Attribute arguments and parameters

You see the following errors when you use attribute arguments or parameters incorrectly:

• CS0181: Attribute constructor parameter has a type, which is not a valid attribute parameter type
• CS0416: 'type parameter': an attribute argument cannot use type parameters.
• CS0447: Attribute cannot be used with type arguments.
• CS0647: Error emitting attribute
• CS7046: Attribute parameter must be specified.
• CS7047: Attribute parameter 'parameter1' or 'parameter2' must be specified.
• CS7067: Attribute constructor parameter is optional, but no default parameter value was specified.
• CS8968: An attribute type argument cannot use type parameters
• CS8970: Type cannot be used in this context because it cannot be represented in metadata.

To correct these errors, follow these rules:

• Attribute constructor parameters must use valid attribute parameter types (CS0181). The C# language specification restricts attribute parameter types to primitive types (bool, byte, char, double, float, int, long, short, and string), object, System.Type, enum types, and single-dimensional arrays of these types. Function pointers and other types that can't be represented in metadata aren't valid attribute parameter types.
• Attribute arguments must be compile-time constant values, so you can't use type parameters as attribute arguments (CS0416). The compiler must resolve attribute arguments at compile time, and type parameters aren't known until the generic type is constructed.
• You can't apply type arguments to a non-generic attribute (CS0447). If an attribute class isn't generic, its usage can't include type arguments in angle brackets.
• All values passed to an attribute constructor must be correctly formatted and within the valid range for each parameter (CS0647). For example, a GuidAttribute requires a valid GUID format string.
• You must provide all required attribute parameters when you apply the attribute (CS7046, CS7047). Check the attribute's constructor signature to determine which parameters are mandatory, and supply valid arguments for each parameter.
• When you define a custom attribute with optional constructor parameters, specify default values for those parameters (CS7067). Use the syntax parameterType parameterName = defaultValue in the attribute constructor so callers can omit those arguments.
• Generic attribute type arguments must be concrete types, not type parameters (CS8968). The compiler must fully determine generic attribute type arguments at compile time, so open type parameters aren't permitted.
• Types used as attribute arguments must be representable in metadata (CS8970). Some constructed types, such as those involving dynamic or certain tuple element names, can't be encoded in metadata and aren't permitted as attribute type arguments.

For more information, see Attributes, Generics, and the C# language specification section on attributes.

Note

CS0447 and CS0647 are deprecated. The current compiler doesn't emit these errors.

Attribute class requirements

You see the following errors when you define attribute classes that don't meet the required constraints:

• CS0404: Attribute is not valid on this declaration type.
• CS0579: Duplicate attribute
• CS0616: Type is not an attribute class
• CS0641: This attribute is only valid on classes derived from System.Attribute`.
• CS0653: Cannot apply attribute class 'class' because it is abstract.
• CS1614: Attribute name is ambiguous. Either use '@name' or explicitly include the 'Attribute' suffix.

To correct these errors, follow these rules:

• Apply attributes only to declaration types that the attribute's AttributeUsageAttribute allows (CS0404). Check the AttributeTargets value specified in the attribute's AttributeUsage to see which targets are valid.
• If you apply the same attribute more than once to a single target, either remove the duplicate or set AllowMultiple = true in the attribute's AttributeUsageAttribute (CS0579). By default, attributes can only appear once on each target.
• The type used in attribute syntax must inherit from System.Attribute (CS0616). Only classes derived from System.Attribute can be used as attributes. Other types cause this error even if they have a similar name.
• You can apply AttributeUsageAttribute only to classes that derive from Attribute (CS0641). The AttributeUsage attribute controls how other attributes are used, and is itself restricted to attribute classes.
• Attribute classes can't be abstract because the compiler must instantiate them (CS0653). Remove the abstract modifier from the attribute class, or derive a concrete class from the abstract base.
• When both Example and ExampleAttribute attribute classes exist, the compiler can't determine which one [Example] refers to (CS1614). Disambiguate by using [@Example] for the shorter name, or [ExampleAttribute] for the longer name.

For more information, see Create custom attributes and the C# language specification section on attributes.

Attribute location context

The following errors occur when you apply attributes in invalid locations or use incorrect target specifiers:

• CS0592: Attribute is not valid on this declaration type. It is only valid on specific declarations.
• CS0657: Location is not a valid attribute location for this declaration. Valid attribute locations for this declaration are listed. All attributes in this block will be ignored.
• CS0658: Location is not a recognized attribute location. Valid attribute locations for this declaration are listed. All attributes in this block will be ignored.
• CS1667: Attribute is not valid on property or event accessors. It is only valid on specific declarations.
• CS7014: Attributes are not valid in this context.

To correct these errors, follow these rules. For more information, see Attribute Targets and the C# language specification section on attribute specification.

• Each attribute's AttributeUsageAttribute specifies which declaration types it targets. You must apply the attribute only to those types (CS0592). For example, you can't apply an attribute defined with AttributeTargets.Interface to a class.
• When you use an attribute target specifier such as method: or property:, the specifier must be valid for the declaration where it appears (CS0657). Check the error message to see which target specifiers are allowed for the specific declaration.
• The attribute target specifier you used isn't a recognized specifier (CS0658). Valid specifiers include assembly:, module:, type:, method:, property:, field:, event:, param:, and return:.
• Some attributes, such as ObsoleteAttribute and CLSCompliantAttribute, aren't valid on property or event accessors (CS1667). Move the attribute from the accessor to the property or event declaration itself.
• Attributes can only appear on program elements that support them (CS7014). If you're applying assembly-level or module-level attributes, use the assembly: or module: target specifiers and place them at the top of the file.

Predefined attributes

The following errors occur when you use specific predefined .NET attributes incorrectly:

• CS0415: This attribute is valid only on an indexer that is not an explicit interface member declaration.
• CS0609: Cannot set the attribute on an indexer marked override.
• CS0625: Instance field in types marked with StructLayout(LayoutKind.Explicit) must have a FieldOffset attribute.
• CS0636: The FieldOffset attribute can only be placed on members of types marked with the StructLayout(LayoutKind.Explicit).
• CS0637: The FieldOffset attribute is not allowed on static or const fields.
• CS0646: Cannot specify the DefaultMember attribute on a type containing an indexer.
• CS0668: Two indexers have different names; the IndexerName attribute must be used with the same name on every indexer within a type
• CS0735: Invalid type specified as an argument for TypeForwardedTo attribute
• CS0739: Duplicate TypeForwardedToAttribute
• CS1608: The RequiredAttribute attribute is not permitted on C# types
• CS9331: Attribute cannot be applied manually.

To correct these errors, follow these rules. For more information, see Indexers, Structure types, TypeForwardedToAttribute, and Platform Invoke (P/Invoke).

• The IndexerNameAttribute can only be applied to indexers that aren't explicit interface member declarations (CS0415). Remove the attribute from explicit interface indexers, because the interface already defines the indexer name.
• You can't apply IndexerName to indexers marked with override because override indexers inherit their name from the base class (CS0609). Remove the IndexerName attribute from the override indexer.
• Every instance field in a type marked with StructLayout(LayoutKind.Explicit) must have a FieldOffsetAttribute (CS0625). Explicit layout requires that you specify the byte offset for each instance field.
• The FieldOffsetAttribute can only be placed on members of types that have StructLayoutAttribute set to LayoutKind.Explicit (CS0636). Add the StructLayout attribute to the containing type declaration.
• The FieldOffset attribute isn't allowed on static or const fields because explicit layout applies only to instance fields (CS0637). Remove the FieldOffset attribute from the static or const field.
• You can't apply DefaultMemberAttribute to a type that already contains an indexer because the compiler automatically defines the default member for types with indexers (CS0646). Remove the DefaultMember attribute.
• All IndexerNameAttribute attributes within a type must specify the same name (CS0668). Change the names to match, because the runtime uses a single name for all indexers on a type.
• The type specified as an argument for TypeForwardedToAttribute must be a non-generic, non-nested, non-pointer, non-array type (CS0735). Only top-level named types are valid forwarding targets.
• An assembly can have only one TypeForwardedToAttribute for each external type (CS0739). Locate and remove the duplicate TypeForwardedTo declaration.
• The RequiredAttributeAttribute isn't permitted on types defined in C# (CS1608). This attribute is reserved for other languages that need to force compilers to require a particular feature.
• Some attributes are reserved for the compiler and can't be applied manually in source code (CS9331). Replace the attribute with the equivalent C# language syntax that causes the compiler to generate it.

Conditional attribute usage

You see the following errors when you apply the ConditionalAttribute in ways that violate its usage restrictions:

• CS0243: The Conditional attribute is not valid on 'method' because it is an override method.
• CS0577: The Conditional attribute is not valid because it is a constructor, destructor, operator, lambda expression, or explicit interface implementation.
• CS0578: The Conditional attribute is not valid on 'function' because its return type is not void.
• CS0582: The Conditional attribute is not valid on interface members.
• CS0629: Conditional member 'member' cannot implement interface member 'base class member' in type 'Type Name'.
• CS0685: Conditional member 'member' cannot have an out parameter.
• CS1618: Cannot create delegate with method because it or a method it overrides has a Conditional attribute
• CS1689: Attribute is only valid on methods or attribute classes

To correct these errors, follow these rules. For more information, see ConditionalAttribute, Conditional methods, and Attributes.

• The compiler binds calls to the base method declaration, not the override, so you can't apply the Conditional attribute to override methods (CS0243). Remove the Conditional attribute from the override method, or remove the override keyword.
• The Conditional attribute isn't valid on constructors, finalizers, operators, lambda expressions, or explicit interface implementations (CS0577). These member types can't be conditionally omitted because they have required roles in the type's lifecycle or contract.
• Conditional methods must return void because the compiler might omit the call entirely, and no return value would be available to the caller (CS0578). Change the method's return type to void, or remove the Conditional attribute.
• Interface members can't be conditional because all interface members must be implemented (CS0582). Remove the Conditional attribute from the interface member declaration.
• Methods that implement interface members can't be conditional because the interface contract requires them to be present in all builds (CS0629). Remove the Conditional attribute from the implementing method.
• Conditional methods can't have out parameters because the out variable value would be undefined when the compiler omits the method call (CS0685). Remove the out parameters from the method, or remove the Conditional attribute.
• You can't create a delegate that references a conditional method because the method might not exist in builds where the condition symbol isn't defined (CS1618). Remove the Conditional attribute from the method, or don't use it as a delegate target.
• The Conditional attribute is only valid on methods and attribute classes (CS1689). It isn't valid on other declaration types such as non-attribute classes, structs, or interfaces.

CallerArgumentExpression attribute usage

The following errors occur when you apply CallerArgumentExpressionAttribute incorrectly or in conflict with other caller info attributes:

• CS8959: CallerArgumentExpressionAttribute cannot be applied because there are no standard conversions from type1 to type2
• CS8960: The CallerArgumentExpressionAttribute applied to parameter will have no effect. It is overridden by the CallerLineNumberAttribute.
• CS8961: The CallerArgumentExpressionAttribute applied to parameter will have no effect. It is overridden by the CallerFilePathAttribute.
• CS8962: The CallerArgumentExpressionAttribute applied to parameter will have no effect. It is overridden by the CallerMemberNameAttribute.
• CS8963: The CallerArgumentExpressionAttribute applied to parameter will have no effect. It is applied with an invalid parameter name.

To correct these errors, follow these rules. For more information, see Caller information attributes and CallerArgumentExpressionAttribute.

• The parameter decorated with CallerArgumentExpression must have type string or a type with a standard conversion from string, because the attribute injects a string representation of the caller's argument expression (CS8959). Change the parameter type to string or a compatible type.
• The CallerArgumentExpression attribute has no effect on a parameter that also has CallerLineNumberAttribute, CallerFilePathAttribute, or CallerMemberNameAttribute (CS8960, CS8961, CS8962). Those caller info attributes take precedence, so remove the CallerArgumentExpression attribute from the parameter.
• The string argument passed to the CallerArgumentExpression constructor must match the name of another parameter in the same method signature (CS8963). If the parameter name is misspelled or refers to a nonexistent parameter, the attribute has no effect.