Custom Attributes in C#

// Built-in attributes
[Obsolete("Use NewMethod instead")]
public void OldMethod() { }

[Serializable]
public class MyClass { }

// Custom attribute
[AttributeUsage(AttributeTargets.Property)]
public class RequiredAttribute : Attribute { }

// Using custom attribute
public class User
{
    [Required]
    public string Name { get; set; }
}