First letter of the TextBox

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C#

Hi to all
I am facing the problem that is I want to give a message when the user types other than the mobile series (mobile series like 9, 7, 8) windows application C#.NET.
First number should be the 9 or 7 or 8 other I should give the message.
Here mobile1txt is the TextBox name.
Please check and correct it.

private void mobile1txt_KeyPress(object sender, KeyPressEventArgs e)
{
    if (mobile1txt.Text.StartsWith("9") || mobile1txt.Text.StartsWith("8") || mobile1txt.Text.StartsWith("7"))
    {
    }
    else
    {
        MessageBox.Show("Mobile number should start with 9, 8 or 7 only", "Information", MessageBoxButtons.OK, MessageBoxIcon.Warning);
        e.Handled = true;
        mobile1txt.Focus();
    }
}