Difference between String.Empty and ""

Yesterday someone asked whether there is difference between String.Empty and "". I answered promptly "there is no difference".

What I thought at the time is that we can write a test:

Assert.AreEqual(String.Empty, "");

this will pass the test.

But today, when I think about this question more carefully. I will say there is difference between them.

String.Empty is intialed at run time.

so we can say:
const strA="";
static readonly strB=String.Empty;

but we cannot write const strA=String.Empty;Just take a quick notes.