Attribute

In the C# programming language, attributes are metadata attached to a field or a block of code like assemblies, members and types, and are equivalent to annotations in Java. Attributes are accessible to both the compiler and programmatically through reflection. In contrast, properties, in C# terminology, are members of a class which syntactically are used like instance (or class) variables, but are implemented as a pair of getter/setter functions. (In the absence of a setter, properties are read-only.)

[Obsolete("Use class C1 instead", IsError = true)]  // causes compiler message saying
public class C // that C is obsolete
{...}

// class name ends with "Attribute"
// but can be used as "Obsolete"
public class ObsoleteAttribute : Attribute 
{
    public string Message { get; }
    public bool IsError { get; set; }
    public ObsoleteAttribute() {...}
    public ObsoleteAttribute(string msg) {...}
    public ObsoleteAttribute(string msg, bool error) {...}
}

[Obsolete]
[Obsolete("This is obsolete")]
[Obsolete("This is obsolete", false)]
[Obsolete("This is obsolete", IsError = false)]