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

This article covers the following compiler errors:

  • 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.
  • CS0582: The Conditional attribute is not valid on interface members.
  • 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.
  • 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.
  • 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.
  • CS0685: Conditional member 'member' cannot have an out parameter.
  • 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.
  • CS9331: Attribute cannot be applied manually.

Attribute arguments and parameters

The following errors occur when you use attribute arguments or parameters incorrectly:

  • CS0416: 'type parameter': an attribute argument cannot use type parameters.
  • CS0447: Attribute cannot be used with type arguments.
  • 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.

To correct these errors, follow these rules.

  • Use compile-time constant values for attribute arguments instead of type parameters (CS0416). Attribute arguments must be evaluated at compile time.
  • Don't use type arguments with attributes (CS0447). Type arguments aren't allowed in attribute usage.
  • Provide all required attribute parameters when you apply the attribute (CS7046, CS7047). Check the attribute's constructor to see which parameters are mandatory.
  • Specify default values for optional constructor parameters when you define custom attributes (CS7067). Use the syntax parameterType parameterName = defaultValue in the attribute constructor.

For more information, see Attributes and Generics.

Note

CS0447 is no longer used in Roslyn.

Attribute class requirements

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

  • CS0404: Attribute is not valid on this declaration type.
  • CS0641: This attribute is only valid on classes derived from System.Attribute`.
  • CS0653: Cannot apply attribute class 'class' because it is abstract.

To correct these errors, follow these rules.

  • Apply attributes only to valid declaration types (CS0404). Check the attribute's AttributeUsage to see which targets are allowed.
  • Remove the abstract modifier from attribute classes, or derive from a non-abstract attribute class (CS0653). Attributes must be instantiable.
  • Apply AttributeUsageAttribute only to classes that derive from Attribute (CS0641). This attribute controls how other attributes are used.

For more information, see Create custom attributes.

Attribute location context

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

  • 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.
  • CS7014: Attributes are not valid in this context.

To correct these errors, follow these rules. For more information, see Attribute Targets.

  • Use valid attribute target specifiers for the declaration (CS0657). Check the error message to see which targets are valid for the specific declaration.
  • Remove invalid attribute target specifiers (CS0658). Valid specifiers include assembly:, module:, type:, method:, property:, field:, event:, param:, and return:.
  • Move attributes to valid contexts (CS7014). Attributes can only be applied to program elements that support them, as defined by AttributeUsageAttribute.
  • Apply attributes at the appropriate scope level. Use assembly: or module: prefixes for assembly-level or module-level attributes.

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.
  • CS9331: Attribute cannot be applied manually.

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

  • Remove IndexerNameAttribute from explicit interface implementations (CS0415). Apply it only to public indexers.
  • Remove IndexerName from indexers marked with override (CS0609). Override indexers inherit the name from the base class.
  • Add FieldOffsetAttribute to all instance fields in types with StructLayoutAttribute set to LayoutKind.Explicit (CS0625). Explicit layout requires explicit field positions.
  • Apply FieldOffset only to types with StructLayout(LayoutKind.Explicit) (CS0636). Add the StructLayout attribute to the type declaration.
  • Remove FieldOffset from static or const fields (CS0637). Explicit layout applies only to instance fields.
  • Remove DefaultMemberAttribute from types that contain indexers (CS0646). Indexers automatically define the default member.
  • Replace compiler-generated attributes with the equivalent C# syntax (CS9331). Use language keywords instead of manually applying reserved attributes.

Conditional attribute usage

The following errors occur 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.

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

  • Remove the Conditional attribute from override methods, or remove the override keyword (CS0243). The compiler binds to the base method, not the override.
  • Don't apply Conditional to constructors, finalizers, operators, lambda expressions, or explicit interface implementations (CS0577). These member types don't support conditional compilation.
  • Change the method's return type to void, or remove the Conditional attribute (CS0578). Conditional methods must return void because calls may be compiled out.
  • Remove the Conditional attribute from interface member declarations (CS0582). Interface members can't be conditional.
  • Remove the Conditional attribute from interface member implementations (CS0629). Methods implementing interface members can't be conditional.
  • Remove out parameters from conditional methods, or remove the Conditional attribute (CS0685). The out variable value would be undefined when the method call is compiled out.
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