How to get the address (memory) of private field using reflection in c#?

I want to get the address of a private field in c#.

Here is my code:

        var theString = "Text";
        var firstCharRef = theString.GetType().GetField("m_firstChar", BindingFlags.NonPublic | BindingFlags.Instance);
        var val = (char)firstCharRef.GetValue(theString);

        unsafe
        {
            char* pointer = &val;//of course, this gives me val address

        }

I want to know the address of "m_firstChar".

How to accomplish that?