Why arguments in square brackets before class fields?

Question

C++ / C#

Sometimes, before declaring a class field, or even the class itself, you can see the arguments in [square brackets]
example of serialization from meatint

[Required]
public string Id { get; set; }
[Required]
[StringLength(50, MinimumLength=3)]
public string Name { get; set; }
[Required]
[Range(1, 100)]
public int Age { get; set;

Or an example from the WFC documentation

[ServiceContract]
public interface IService
{
    [OperationContract]
    [WebGet]
    string EchoWithGet(string s);

    [OperationContract]
    [WebInvoke]
    string EchoWithPost(string s);
}

Logically, it is clear that this is some kind of wrapper that expands the functionality. But what is this design and where can I read about it?

Answer 1

Attributes (C#)