Where is the class definition usually stored in programming?

In C#, a class definition is usually stored in a separate file with the extension .cs. By convention, the filename matches the class name, which makes navigation and maintenance easier. For example, a class User will be defined in a file User.cs.

// User.cs
public class User
{
    public string Name { get; set; }
    public int Age { get; set; }
}