Example
textBox.KeyPress += (sender, e) => e.Handled = !char.IsControl(e.KeyChar) && !char.IsDigit(e.KeyChar);
This will only permit the use of digits and control characters in the TextBox, other combinations are possible using the same approach of setting the Handle property to true to block the text.
The user can still copy/paste unwanted characters so an additional check should be on the TextChanged to cleanse the input:
textBox.TextChanged += (sender, e) => textBox.Text = Regex.Match(textBox.Text, @"\d+").Value
In this example a Regular expression is used to filter the text.
NumericUpDown should be preferred for numbers when possible.
