.CS File Format – Attributes

Using attributes

[Serializable]
public class Order { }

[Obsolete("Use Add instead", error: true)]
public void Insert() { }

[Conditional("DEBUG"), Description("Logs")]    // two attributes in one section
public void Log() { }

[return: NotNull]
public string Name() => "";

An attribute section is written in square brackets before a declaration, and may list several attributes. The Attribute suffix may be left out of the name, so [Serializable] means SerializableAttribute. If both forms exist and are both attributes, the short name is ambiguous; a leading @ selects the exact name, as in [@Serializable]. Arguments are in parentheses: positional first, then named.

[assembly: AssemblyVersion("1.0.0.0")]

public record Person([property: JsonPropertyName("n")] string Name);

[field: NonSerialized]
public int Cached { get; set; }

Declaring an attribute

An attribute is a class derived from System.Attribute (clause 23.2).

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = true, Inherited = false)]
public sealed class AuthorAttribute : Attribute
{
    public AuthorAttribute(string name) => Name = name;  // positional parameter
    public string Name { get; }
    public int Version { get; set; }                     // named parameter
}

[Author("Ann", Version = 2)]
class Report { }

• Constructor parameters are positional parameters. Public read-write fields and properties are named parameters.
• Argument values must be constant expressions, typeof expressions or single-dimensional arrays of these (clause 23.2.4).
• The usable parameter types are bool, byte, char, double, float, int, long, sbyte, short, string, uint, ulong, ushort, object, System.Type, enums, and one-dimensional arrays of these.
• Since C# 11 an attribute class may be generic, used as [Author<int>]. The type arguments must be fully known at compile time.

The default AttributeUsage allows any target, once, and is inherited. At run time, GetCustomAttributes and related reflection methods read them.

Attributes the compiler recognises

Interoperation attributes such as DllImport, StructLayout and FieldOffset are described in clause 23.6.