Is .Net attribute feature used at compile-time or run-time or both?

Question

In .Net, is the attribute feature used at compile-time or run-time or both? Can you give me some examples?

Answer 1

Most are used at runtime only. A very limited number are used by the compiler, including:

• [Conditional(...)] - omit method calls per build symbols
• [Obsolete(...)] - emit a warning/error as build output
• [Serializable] - gets written as a CLI flag
• [Extension] - used for extension methods
• [AttributeUsage] - affects how attributes are applied
-

There are a range of things like [AssemblyVersion], [AssemblyFileVersion] etc that are used by the compiler when creating the assembly file, and things like [InternalsVisibleTo] which affect accessibility.

Additionally, tools like PostSharp do extra post-compile steps based on attributes.

There are some other attributes that the compiler may add to generated types/methods (for anon-methods / types, iterator blocks, etc).