Attributes

[Serializable]
public class MyClass
{
  private int myField;

  [NotSerialized]
  private string myBrand;
}

• Attributes are classes that derive from System.Attribute
• Attributes result in embedded metadata.
• Attributes are basically useless until another agent reflects upon them.
• Attributes are applied in C# using square brackets.

Constructor parameters may be supplied to an attribute, but the attribute is not actually instantiated until the parameters are reflected upon by another type or an external tool.

Custom Attributes

public sealed class MyCustomAttribute : System.Attribute
{
  public string Description { get; set; }

  public MyCustomAttribute(string description)
  {
    Description = description;
  }

  public MyCustomAttribute() { }
}

It is considered a .NET best practice to declare all custom attributes sealed.

Named Property Syntax

[MyCustomAttribute(Description = "Who's description?")]
public class MyClass { }

Attributes introduce a special "named property syntax" as shown in the example in the way the description is set. This is legal only if the attribute supplies a writable .NET property.

Restricting Attribute Usage

You can restrict what you can apply an attribute to with the AttributeUsage attribute.

The AttributeTargets enum defines all the possible attribute targets. Multiple of these can be specified using the, |, operator to string them together.

public enum AttributeTargets
{
  All, Assembly, Class, Constructor,
  Delegate, Enum, Event, Field,
  GenericParameter, Interface, Method, Module,
  Parameter, Property, ReturnValue, Struct
}

• You can set the named property AllowMultiple to allow the attribute to be used more than once on the same item (default is false).
• You can specify whether the attribute should be inherited by derived classes using the Inherited named property (default is true).

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct, Inherited = false)]
public sealed class MyOtherCustomAttribute { /* ... */ }

C# Attribute Shorthand

By convention, attribute classes are suffixed with 'Attribute'. For example, SerializableAttribute. However, C# allows you to leave out the word 'Attribute' when referring to it with the [...] syntax.

Also, if you're setting multiple attributes on an item you can use commas to separate the attributes.

[NotSerialized, XmlAttribute]
private int myInt;

[NotSerialized]
[XmlAttribute]
private int myOtherInt;

Assembly-Level Attributes

You can apply attributes to an entire assembly by specifying the [assembly:] tag in any class file and outside of the scope of any namespace.

A good practice is to define these after imports and before the namespace and class definitions.

Even better, Visual Studio projects typically define an AssemblyInfo.cs file, which can be viewed by expanding the Properties icon of the Solution Explorer. This file is the perfect place to put assembly-level attributes.

Some useful assembly-level attributes:

• [CLSCompliant(true)] - make sure only CLS-compliant language features are used.
• [AssemblyCompany]
• [AssemblyCopyright]
• [AssemblyCulture]
• [AssemblyDescription]
• [AssemblyKeyFile] - The name of the file containing the key pair used to sign the assembly (i.e., establish a strong name).
• [AssemblyProduct]
• [AssemblyTrademark]
• [AssemblyVersion] - In the format "<major.minor.build.revision>".