TextBox Allow Decimal and minus sign

Question

I have one textbox in c# window forms that must allow only decimal numbers and minus sign. I need to write the code in textbox_textchanged event Regards Angela

Vulpes · Answer

If you're going to do this in the Text_Changed event then the invalid characters will already have been inserted and so you'll need to remove them. The following should do that. As well as removing invalid characters it should also ensure that: 1. There's no more than one decimal point (only the last one entered is retained). 2. If there is a minus sign it's at the start of the string. 3. There's always at least one zero before a decimal point. private void textBox1_TextChanged(object sender EventArgs e) { textBox1.Text = textBox1.Text.Trim(); if (textBox1.Text.Length == 0) return; StringBuilder sb = new StringBuilder(textBox1.Text); bool dotExists = false; for (int i = sb.Length - 1; i >= 0; i--) { if (!Char.IsDigit(sb[i]) && sb[i] != '.' && sb[i] != '-') { sb.Remove(i 1); } else if (sb[i] == '.') { if (dotExists) { sb.Remove(i 1); } else { dotExists = true; } } else if (sb[i] == '-' && i > 0) { sb.Remove(i 1); } } if (sb[0] == '.') sb.Insert(0 '0'); if (sb[0] == '-' && (sb.Length > 1 && sb[1] == '.')) sb.Insert(1 '0'); textBox1.Text = sb.ToString(); textBox1.SelectionStart = textBox1.Text.Length + 1; }