如何验证文本框中输入的每个新字符

Question

So I've been working on this Temperature Converter from Fahrenheit to Celsius and vice-versa, and basically what it does is that when the user inputs a number in a respective textbox it automatically shows the converted result in a label under it, I'm trying to not make possible for someone to write a letter in the textbox and I've succeeded when the person types the first character as a letter, but when the user inputs a number and then a letter the program tries to calculate and it crashes, I don't know how to make the if function verify every new character input BEFORE calculating it and displaying the result

I've tried to use Val(txtFahrenheit.Text) but it only verifies the first character and also I've tried Char.isDigit(txtFahrenheit.Text) = False and it also only verifies the first character and I get the same output.

' This section is executed after the respective textbox is changed
Private Sub txtFahrenheit_TextChanged(ByVal sender As Object, ByVal e As EventArgs) Handles txtFahrenheit.TextChanged

  ' This section sets a pop-up when the user's input equals to eight characters
  If txtFahrenheit.TextLength = 8 Then
    MessageBox.Show("You have exceeded the max number of input characters")
  End If

  ' This section validates the user's input, enables the Clear Button, calculates the reuslt and displays it
  If Char.isDigit(txtFahrenheit.Text) = True Then
    btnClear.Enabled = True
    Dim fahrenCel As Integer = (txtFahrenheit.Text - 32) / 1.8
    lblResultC.Text = fahrenCel.ToString + " °C"
  End If

  ' This section verifies if the user inputs a letter first and removes it also displaying a pop-up
  If Val(txtFahrenheit.Text) = False And (txtFahrenheit.Text = "0") = False And (txtFahrenheit.Text = String.Empty) = False Then
    txtFahrenheit.Text = String.Empty
    lblResultC.Text = String.Empty
    MessageBox.Show("The input is not valid. Please use only numbers")
    Return
  End If

  ' This section clears the label result when the text box is empty
  If txtFahrenheit.Text = "" Then
    lblResultC.Text = String.Empty
    Return
  End If

End Sub

Answer 1

The simplest (and in my humble opinion the best) answer is to let WinForms handle the data validation for you by using a NumericUpDown control. You would handle the ValueChanged event to do you conversion and get the Value of the control.

It is also worth mentioning that you should probably move your conversion into a separate function.

Answer 2

You can try using try catch like this:

try
{
    double d = Convert.ToDouble(txtFahrenheit.Text);//
    // Calculate the equivalent Celsius and show it
}
catch (Exception)
{
}