Why is String.Empty an invalid default parameter?

Question

If I type the following:

public Response GetArticles(string Filter = String.Empty)
{
    //Body
}

Visual Studio gives me this error:

Default parameter value for 'Filter' must be a compile-time constant

If I change the String.Empty to the classic "" it is fixed.

But I'm still curious about what is wrong with the String.Empty and its behavior.

Answer 1

Why is String.Empty an invalid default parameter?

Because "Default parameter value for 'Filter' must be a compile-time constant". And String.Empty is not a constant but only static readonly. String literals like "Foo" are constants as implementation detail(i haven't found documentation).

Further read: Why isn't String.Empty a constant?

Quote from 10.4 Constants of the C# language specification:

The readonly keyword is different from the const keyword. A const field can only be initialized at the declaration of the field. A readonly field can be initialized either at the declaration or in a constructor. Therefore, readonly fields can have different values depending on the constructor used. Also, while a const field is a compile-time constant, the readonly field can be used for runtime constants

Here is the MSDN quote according to optional parameters:

A default value must be one of the following types of expressions:

• a constant expression
• an expression of the form new ValType(), where ValType is a value type, such as an enum or a struct
• an expression of the form default(ValType), where ValType is a value type.

I'm suprised that you can use new ValType(), where ValType is a value type or struct. I didn't know that you can use the default constructor like new DateTime() but not new DateTime(2015,1,15). Learned something new from my own answer.