Why KeyDown event handler not working properly after MessageBox.Show() in C#? Then how can I alert the User?

Question

This code works fine only if I comment out the MessageBox.Show().

private void textBox1_KeyDown( object sender, KeyEventArgs e ) {
    if( textBox1.Text.Contains('.') && ( e.KeyCode == Keys.Decimal || e.KeyCode == Keys.OemPeriod ) ) {
        MessageBox.Show("More than one decimal point!");
        e.SuppressKeyPress = true;
    }
}

What is the reason? and How can I alert the User?

EDIT

Then How can I alert for the wrong Key Press?

Answer 1

You should try this

private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
    if(textBox1.Text.Contains('.') && e.KeyChar == '.')
    {
        e.Handled = true;
        MessageBox.Show("More than one decimal point!");
    }
}

It would better work on KeyPress Instead of KeyDown

While discovering I got a wonderful page saying about the difference between the KeyDown and KeyPress events in .net

Answer 2

When you show message box, it leaves the focus from TextBox, and your code e.SuppressKeyPress = true doesn't execute at that time.

You should place your MessageBox after handling the input.

e.SuppressKeyPress = true;
MessageBox.Show("More than one decimal point!");