Write A Program That Will Compare Two Strings In Different Text Boxes?

I am trying to write a program that will compare two strings in different text boxes. Then it must add 1 to a third text box for every time a letter appears in both text boxes. So for example if textbox1 said ABCDF and textbox 2 said ABTR it should return the number 2 because A and B appear in both.
This is what I have so far

Public Class Form1 Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles string1.TextChanged
End Sub

Private Sub Compare_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Compare.Click

Dim string1 As String
Dim string2 As String
Dim Result As Integer
End Sub

Function stringcompare(ByVal string1 As String, ByVal string2 As String, ByVal string3 As String) As Integer
stringcompare(string1 As string, string2 As string) End Function End Class