How can I generate a random set of alpha only characters in coded ui test?

I have used the following code to generate a random set of characters for customer last names. Today, the developers restricted last names to alpha only and now I can't figure out how to get only alpha. Any suggestions would be greatly appreciated.

`            var alph = Guid.NewGuid().ToString().Substring(0, 8);
             var LastName = TestContext.DataRow["LastName"].ToString() + add + alph;`