Prevent a dialog from closing by setting Form.DialogResult to None

winforms_dialogresult_none.cs

private void btnOK_Click(object sender, EventArgs e)  
{
    if (Validate())
    {
        // There's no need to explicitly hide the form
        // because the button's DialogResult property
        // has a value of DialogResult.OK

        // Details elided...
    }
    else
    {
        // Set the value of the form's DialogResult property
        // to DialogResult.None to prevent the form from closing.
        this.DialogResult = DialogResult.None;
    }
}