Sum Of TextBox

Option Explicit
Private Sub TextBox1_Change()
    addtextboxes
End Sub
Private Sub TextBox2_Change()
    addtextboxes
End Sub


Sub addtextboxes()
    Me.TextBox3 = CLng(IIf(TextBox1.Value = vbNullString, 0, TextBox1.Value)) + CLng(IIf(TextBox2.Value = vbNullString, 0, TextBox2.Value))
End Sub