Question

How is it possible that C# attributes have “Attribute” in their name (e.g. DataMemberAttribute) but are initialized without this suffix? e.g.:

[DataMember]
private int i;

Answer 1

According to the C# Language Specification,

By convention, attribute classes are named with a suffix of Attribute. An attribute-name of the form type-name may either include or omit this suffix.

This is a shortcut provided by the C# compiler and by no means a CLR feature. Another example of special treatment of attributes by the compiler is an ObsoleteAttribute attribute: this one forces a compiler to issue a warning/error, but it has no special meaning for the CLR.

As for how attributes are resolved, see the link above. To sum it up:

If an attribute class is found both with and without this suffix, an ambiguity is present, and a compile-time error results. If the attribute-name is spelled such that its right-most identifier is a verbatim identifier, then only an attribute without a suffix is matched, thus enabling such an ambiguity to be resolved.

A “verbatim identifier” is an identifier with an @ prefix.

Continuing with MSDN:

using System;

[AttributeUsage(AttributeTargets.All)]
public class X: Attribute
{}

[AttributeUsage(AttributeTargets.All)]
public class XAttribute: Attribute
{}

[X]                  // Error: ambiguity
class Class1 {}

[XAttribute]         // Refers to XAttribute
class Class2 {}

[@X]                  // Refers to X
class Class3 {}

[@XAttribute]         // Refers to XAttribute
class Class4 {}

The attribute [X] is ambiguous, since it could refer to either X or XAttribute. Using a verbatim identifier allows the exact intent to be specified in such rare cases. The attribute [XAttribute] is not ambiguous (although it would be if there was an attribute class named XAttributeAttribute!). If the declaration for class X is removed, then both attributes refer to the attribute class named XAttribute, as follows:

using System;
[AttributeUsage(AttributeTargets.All)]
public class XAttribute: Attribute
{}

[X]                  // Refers to XAttribute
class Class1 {}

[XAttribute]         // Refers to XAttribute
class Class2 {}

[@X]                  // Error: no attribute named "X"
class Class3 {}