Attributes provide a powerful method of associating metadata, or declarative information, with code (assemblies, types, methods, properties, and so forth). After an attribute is associated with a program entity, the attribute can be queried at run time by using a technique called reflection. For more information, see Reflection (C#).
Attributes have the following properties:
- Attributes add metadata to your program. Metadata is information about the types defined in a program. All .NET assemblies contain a specified set of metadata that describes the types and type members defined in the assembly. You can add custom attributes to specify any additional information that is required. For more information, see, Creating Custom Attributes (C#).
- You can apply one or more attributes to entire assemblies, modules, or smaller program elements such as classes and properties.
- Attributes can accept arguments in the same way as methods and properties.
- Your program can examine its own metadata or the metadata in other programs by using reflection. For more information, see Accessing Attributes by Using Reflection (C#).
Attributes can be placed on most any declaration, though a specific attribute might restrict the types of declarations on which it is valid. In C#, you specify an attribute by placing the name of the attribute enclosed in square brackets () above the declaration of the entity to which it applies.
In this example, the xref:System.SerializableAttribute attribute is used to apply a specific characteristic to a class:
[!code-csharpUsing the serializable attribute]
A method with the attribute xref:System.Runtime.InteropServices.DllImportAttribute is declared like the following example:
[!code-csharpDeclaring a method to import from an external library]
More than one attribute can be placed on a declaration as the following example shows:
Some attributes can be specified more than once for a given entity. An example of such a multiuse attribute is xref:System.Diagnostics.ConditionalAttribute:
[!code-csharpUsing the conditional attribute]
[!NOTE] By convention, all attribute names end with the word "Attribute" to distinguish them from other items in the .NET libraries. However, you do not need to specify the attribute suffix when using attributes in code. For example,
[DllImport]is equivalent to
DllImportAttributeis the attribute's actual name in the .NET Framework Class Library.
Many attributes have parameters, which can be positional, unnamed, or named. Any positional parameters must be specified in a certain order and cannot be omitted. Named parameters are optional and can be specified in any order. Positional parameters are specified first. For example, these three attributes are equivalent:
[DllImport("user32.dll")] [DllImport("user32.dll", SetLastError=false, ExactSpelling=false)] [DllImport("user32.dll", ExactSpelling=false, SetLastError=false)]
The first parameter, the DLL name, is positional and always comes first; the others are named. In this case, both named parameters default to false, so they can be omitted. Positional parameters correspond to the parameters of the attribute constructor. Named or optional parameters correspond to either properties or fields of the attribute. Refer to the individual attribute's documentation for information on default parameter values.
The target of an attribute is the entity which the attribute applies to. For example, an attribute may apply to a class, a particular method, or an entire assembly. By default, an attribute applies to the element that it precedes. But you can also explicitly identify, for example, whether an attribute is applied to a method, or to its parameter, or to its return value.
To explicitly identify an attribute target, use the following syntax:
[target : attribute-list]
The list of possible
target values is shown in the following table.
|Target value||Applies to|
||Current assembly module|
||Field in a class or a struct|
||Method parameters or
||Return value of a method, property indexer, or
||Struct, class, interface, enum, or delegate|
You would specify the
field target value to apply an attribute to the backing field created for an auto-implemented property.
The following example shows how to apply attributes to assemblies and modules. For more information, see Common Attributes (C#).
using System; using System.Reflection; [assembly: AssemblyTitleAttribute("Production assembly 4")] [module: CLSCompliant(true)]
The following example shows how to apply attributes to methods, method parameters, and method return values in C#.
[!code-csharpApplying attributes to different code elements]
[!NOTE] Regardless of the targets on which
ValidatedContractis defined to be valid, the
returntarget has to be specified, even if
ValidatedContractwere defined to apply only to return values. In other words, the compiler will not use
AttributeUsageinformation to resolve ambiguous attribute targets. For more information, see AttributeUsage (C#).
Common uses for attributes
The following list includes a few of the common uses of attributes in code:
- Marking methods using the
WebMethodattribute in Web services to indicate that the method should be callable over the SOAP protocol. For more information, see xref:System.Web.Services.WebMethodAttribute.
- Describing how to marshal method parameters when interoperating with native code. For more information, see xref:System.Runtime.InteropServices.MarshalAsAttribute.
- Describing the COM properties for classes, methods, and interfaces.
- Calling unmanaged code using the xref:System.Runtime.InteropServices.DllImportAttribute class.
- Describing your assembly in terms of title, version, description, or trademark.
- Describing which members of a class to serialize for persistence.
- Describing how to map between class members and XML nodes for XML serialization.
- Describing the security requirements for methods.
- Specifying characteristics used to enforce security.
- Controlling optimizations by the just-in-time (JIT) compiler so the code remains easy to debug.
- Obtaining information about the caller to a method.
For more information, see:
- Creating Custom Attributes (C#)
- Accessing Attributes by Using Reflection (C#)
- How to: Create a C/C++ Union by Using Attributes (C#)
- Common Attributes (C#)
- Caller Information (C#)