private void txt_PreviewTextInput(object sender, TextCompositionEventArgs e)
{
e.Handled = !ValidNumeric(e.Text);
base.OnPreviewTextInput(e);
}
 
bool ValidNumeric(string str)
{
bool ret = true;
 
int l = str.Length;
for (int i = 0; i < l; i++)
{
char ch = str[i];
ret &= Char.IsDigit(ch);
}
 
return ret;
}