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Create custom attributes

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You can create your own custom attributes by defining an attribute class, a class that derives directly or indirectly from Attribute, which makes identifying attribute definitions in metadata fast and easy. Suppose you want to tag types with the name of the programmer who wrote the type. You might define a custom Author attribute class:

C# 
[System.AttributeUsage(System.AttributeTargets.Class |
                       System.AttributeTargets.Struct)
]
public class AuthorAttribute : System.Attribute
{
    private string Name;
    public double Version;

    public AuthorAttribute(string name)
    {
        Name = name;
        Version = 1.0;
    }
}

The class name AuthorAttribute is the attribute's name, Author, plus the Attribute suffix. It's derived from System.Attribute, so it's a custom attribute class. The constructor's parameters are the custom attribute's positional parameters. In this example, name is a positional parameter. Any public read-write fields or properties are named parameters. In this case, version is the only named parameter. Note the use of the AttributeUsage attribute to make the Author attribute valid only on class and struct declarations.

You could use this new attribute as follows:

C# 
[Author("P. Ackerman", Version = 1.1)]
class SampleClass
{
    // P. Ackerman's code goes here...
}

AttributeUsage has a named parameter, AllowMultiple, with which you can make a custom attribute single-use or multiuse. In the following code example, a multiuse attribute is created.

C# 
[System.AttributeUsage(System.AttributeTargets.Class |
                       System.AttributeTargets.Struct,
                       AllowMultiple = true)  // Multiuse attribute.
]
public class AuthorAttribute : System.Attribute
{
    string Name;
    public double Version;

    public AuthorAttribute(string name)
    {
        Name = name;

        // Default value.
        Version = 1.0;
    }

    public string GetName() => Name;
}

In the following code example, multiple attributes of the same type are applied to a class.

C# 
[Author("P. Ackerman"), Author("R. Koch", Version = 2.0)]
public class ThirdClass
{
    // ...
}

See also