C# in a Nutshell, Second Edition

Compiler Support

In reality, custom attributes are simply types derived from System.Attribute with language constructs for specifying them on an element (see Section 4.7 in Chapter 4).

These language constructs are recognized by the compiler, which emits a small chunk of data into the metadata. This custom data includes a serialized call to the constructor of the custom attribute type (containing the values for the positional parameters), and a collection of property set operations (containing the values for the named parameters).

The compiler also recognizes a small number of pseudocustom attributes. These are special attributes that have direct representation in metadata and are stored natively (i.e., not as chunks of custom data). This is primarily a runtime performance optimization, although it has some implications for retrieving attributes via reflection, as discussed later.

To understand this, consider the following class with two specified attributes:

[Serializable, Obsolete]
class Foo {...}

When compiled in MSIL, the metadata for the class Foo looks like this:

.class private auto ansi serializable beforefieldinit Foo
       extends [mscorlib]System.Object
{
  .custom instance void 
    [mscorlib]System.ObsoleteAttribute::.ctor( ) = ( 01 00 00 00 )
    ...
  }
}

Compare the different treatment by the compiler of the Obsolete attribute, which is a custom attribute and is stored as a serialized constructor call to the System.ObsoleteAttribute type, to the treatment of the Serializable attribute, which ...