Disambiguating Attribute Targets (C# Programming Guide) 

In certain situations, the target of an attribute, that is, the entity to which the attribute applies, appears to be ambiguous. For example, in the following method declaration, the SomeAttr attribute could apply to the method or to the method's return value:

public class SomeAttr : System.Attribute { }

[SomeAttr]
int Method()
{
    return 0;
}

This sort of situation arises frequently when marshaling. To resolve the ambiguity, C# has a set of default targets for each kind of declaration, which can be overridden by explicitly specifying attribute targets.

// default: applies to method
[SomeAttr]
int Method1() { return 0; } 

// applies to method
[method: SomeAttr]
int Method2() { return 0; } 

// applies to return value
[return: SomeAttr]
int Method3() { return 0; } 

Note that this is independent of the targets on which SomeAttr is defined to be valid; that is, even if SomeAttr were defined to apply only to return values, the return target would still have to be specified. In other words, the compiler will not use AttributeUsage information to resolve ambiguous attribute targets. For more information, see AttributeUsage (C# Programming Guide).

The syntax for attribute targets is as follows:

[target : attribute-list]

Parameters

• target
  One of the following: assembly, field, event, method, module, param, property, return, type.

• attribute-list
  A list of applicable attributes.

The table below lists all declarations where attributes are allowed; for each declaration, the possible targets for attributes on the declaration are listed in the second column. Targets in bold are the defaults.

Declaration	Possible targets
assembly	assembly
module	module
class	type
struct	type
interface	type
enum	type
delegate	type, return
method	method, return
parameter	param
Field	field
property — indexer	property
property — get accessor	method, return
property — set accessor	method, param, return
event — field	event, field, method
event — property	event, property
event — add	method, param
event — remove	method, param

Assembly and module-level attributes have no default target. For more information, see Global Attributes.

Example

using System.Runtime.InteropServices;

[Guid("12345678-1234-1234-1234-123456789abc"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
interface ISampleInterface
{
    [DispId(17)]  // set the DISPID of the method
    [return: MarshalAs(UnmanagedType.Interface)]  // set the marshaling on the return type
    object DoWork();
}

See Also

Reference

Using Attributes (C# Programming Guide)
Creating Custom Attributes (C# Programming Guide)
Accessing Attributes With Reflection (C# Programming Guide)
System.Reflection
Attribute

Concepts

C# Programming Guide
Reflection (C# Programming Guide)
Attributes (C# Programming Guide)