Slide 1

Attribute instances

An attribute instance is an instance that represents an attribute at run-time. An attribute is defined with an attribute class, positional arguments, and named arguments. An attribute instance is an instance of the attribute class that is initialized with the positional and named arguments.

Retrieval of an attribute instance involves both compile-time and run-time processing, as described in the following sections.

Compilation of an attribute

The compilation of an attribute with attribute class T, positional_argument_list P and named_argument_list N, consists of the following steps:

• Follow the compile-time processing steps for compiling an object_creation_expression of the form new T(P). These steps either result in a compile-time error, or determine an instance constructor C on T that can be invoked at run-time.
• If C does not have public accessibility, then a compile-time error occurs.
• For each named_argument Arg in N:
  • Let Name be the identifier of the named_argument Arg.
  • Name must identify a non-static read-write public field or property on T. If T has no such field or property, then a compile-time error occurs.
• Keep the following information for run-time instantiation of the attribute: the attribute class T, the instance constructor C on T, the positional_argument_list P and the named_argument_list N.

Slide 2

Run-time retrieval of an attribute instance

Compilation of an attribute yields an attribute class T, an instance constructor C on T, a positional_argument_list P, and a named_argument_list N. Given this information, an attribute instance can be retrieved at run-time using the following steps:

• Follow the run-time processing steps for executing an object_creation_expression of the form new T(P), using the instance constructor C as determined at compile-time. These steps either result in an exception, or produce an instance O of T.
• For each named_argument Arg in N, in order:
  • Let Name be the identifier of the named_argument Arg. If Name does not identify a non-static public read-write field or property on O, then an exception is thrown.
  • Let Value be the result of evaluating the attribute_argument_expression of Arg.
  • If Name identifies a field on O, then set this field to Value.
  • Otherwise, Name identifies a property on O. Set this property to Value.
  • The result is O, an instance of the attribute class T that has been initialized with the positional_argument_list P and the named_argument_list N.

Ref:- https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/attributes