How to make calculator that allows user to use only 1 decimal point and not more

Im making a calculator as a school project and I would like to know how to make it so that only 1 decimal is allowed in the textbox. The current code that need's changing is (And I know its basic but im just learning it):

Code:

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Private Sub btnNumber_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) _
        Handles btn0.Click, btn1.Click, btn2.Click, btn3.Click, btn4.Click,
        btn5.Click, btn6.Click, btn7.Click, btn8.Click, btn9.Click, btnDot.Click

                Dim btnNumberClicked As Button = sender 'Contains properties of clicked button

                If blnNewOp Then
                        Me.txtDisplay.Text = btnNumberClicked.Tag
                        blnNewOp = False
                Else
                        Me.txtDisplay.Text = Me.txtDisplay.Text & btnNumberClicked.Tag
                End If

        End Sub

---

Moved to .Net area as this is not VB6 code.

One simple method would be to use the .IndexOf method to see if there is a decimal point in your text and then if there is call it again with a new starting point and see if there is another.

You should be able to find examples for using IndexOf fairly easy

Another option is when they click the decimal button check and see if there is already a decimal in the string and disallow that entry.

Or yet another disable the decimal button once they click it once within any given number.

If they can type into the field though button trapping will not catch that, same if they paste into it so you may need to use more than one method to insure you cover all the bases