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There are several ways to validate a string. In the example below we have a string that should be a Social Security Number.
It should have a length of 9 and consist entirely of digits.

Build the form as shown:

1. There is a label lblInstructions with the words "Enter your social security number as 9 digits:"
2. There is a text box named txtInput.
3. There is a button, btnOK.
4. There is a label named lblOutput.
5. Set the accept button for the form to btnOK.
   

Test various different values. Try enough variations so that you are sure it works.

private void btnOK_Click(object sender, EventArgs e)
{
   // Check if the input is a valid Social Security number, ie. 9 digits.
   String s = txtInput.Text;
   // Assume the string is valid until you find it to be invalid.
   Boolean ok = true;
   if (s.Length != 9) ok = false;
   else
   {
      // Convert the string to an array of characters.
      char[] chars = s.ToCharArray();
      // Check that each character is a digit.
      for(int i=0;i<9;i++)
      {
        if (chars[i] < '0' || chars[i] > '9') ok = false;
      }
   }
   // Display the results.
   if (ok) lblOutput.Text = "That is a valid Social Security Number";
   else lblOutput.Text = "That is NOT a valid Social Security Number";
 }
}

Experiment: How would you allow the 2 dashes: 314-15-9265?

How would you make it accept either with or without the dashes: 314159265 or 314-15-9265?

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