Question

Today I was creating a default parameter value in a constructor.

public SomeClass (String something = String.Empty)
{
   // ...
}

The compiler complained.

Default parameter value for “something” must be a compile-time
constant.

I was under the impression that Empty on the String class was a compile-time constant.

.field public static initonly string Empty

Am I missunderstanding the meaning of compile-time constant, or is it just more wackyness that I need to accept?

Answer 1

The accepted answer to this SO Question Why isn’t String.Empty a constant? is:

The reason that static readonly is used instead of const is due to use with unmanaged code, as indicated by Microsoft here in the Shared Source Common Language Infrastructure 2.0 Release. The file to look at is sscli20\clr\src\bcl\system\string.cs.

The Empty constant holds the empty string value. We need to call the String constructor so that the compiler doesn’t mark this as a literal.

Marking this as a literal would mean that it doesn’t show up as a field which we can access from native.

I found this information from this handy article at CodeProject.