How do you get the value from a field of a nested class in another assembly?

I have a nested class for constant values:

public static class Constants
{
    public static readonly string None = string.Empty;

    public static class FieldOperators
    {
        public static readonly string A01 = "Foo";
        public static readonly string A02 = "Bar";
    }
}

I realize that this is not ideal, that Enums makes more sense, but I can't change this legacy code. I have to work with what I have.

I am trying to write a helper function that gets me the values based on the full name of the field:

public static string GetFieldValue(string fullname)
{
    return ???;
}

For example:

• "Constants.None" would return "" (empty string).
• "Constants.FieldOperators.A01" would return "Foo".
• "Constants.NonExistant.Blah" would return null since that does not exist.