Restricting use of characters in a textbox

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Hey!

I'm currently having a little problem. I have a winforms project and want to restrict the use of the characters in certain textboxes to letters and backspaces only.

Using the code example below, it does detect the numbers and correctly empties the field but there are two problems I'm running into:

1. The MessageBox shows twice, once when it detects the number and once when it empties the textbox

2. Backspaces are not allowed, even if I add \b

What I have tried:

private void textBox_Ansprechpartner_Name_TextChanged(object sender, EventArgs e)
{
    if (!System.Text.RegularExpressions.Regex.IsMatch(textBox_Ansprechpartner_Name.Text, "^[a-zA-Z ]"))
    {
        MessageBox.Show("This textbox may only contain letters.");
        textBox_Ansprechpartner_Name.Text = string.Empty;
    }            
}

Posted 17-Apr-18 23:33pm

Member 13777741

Updated 17-Apr-18 23:44pm

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2 solutions

Solution 1

1. The MessageBox is shown twice because TextChanged is triggered when you assign string.Empty. You can temporarily unassign the TextChanged handler before clearing and then restore it.

2. In your example, you can press the backspace key, which will remove the text to the left of the cursor and then trigger TextChanged again. If it's not empty it must've been valid before, and by removing a character it should still be.
Could you clarify the problem maybe?

Posted 17-Apr-18 23:44pm

User 7429338

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OriginalGriff · Accepted Answer · 2018-04-17T23:45:00

Firstly, that's a nasty bit of UI - clearing out everything because he made a minor slip is rather unpleasant, and second because it won't work as is.
If I type "A123345!"£$%^&*(" is will accept it, because it starts with a valid character.

You can fix it so it works and only accepts letters and spaces very easily:

C#

if (!System.Text.RegularExpressions.Regex.IsMatch(textBox_Ansprechpartner_Name.Text, "^[a-zA-Z ]*$"))

But I'd look at saving the previous version:

private string lastEntry = "";
private void textBox_Ansprechpartner_Name_TextChanged(object sender, EventArgs e)
    {
    if (!System.Text.RegularExpressions.Regex.IsMatch(textBox_Ansprechpartner_Name.Text, "^[a-zA-Z ]*$"))
        {
        int loc = textBox_Ansprechpartner_Name.SelectionStart;
        MessageBox.Show("This textbox may only contain letters.");
        textBox_Ansprechpartner_Name.Text = lastEntry;
        textBox_Ansprechpartner_Name.SelectionStart = loc;
        }
    lastEntry = textBox_Ansprechpartner_Name.Text;
    }