How to close form with private function C#

for your code to work, 'frmclose needs to have a valid reference to the instance of 'frmDBVerify you wish to close:

private frmDBVerify frmclose;

private void Form1_Load(object sender, EventArgs e)
{
    frmclose = new FrmDBVerify();
    frmclose.Show();
}

private void Test() {

    if (frmclose == null) throw new NullReferenceException();

    this.frmclose.Close();
}

Note that calling 'Close on an instance of a Form that has been created using 'new, but not 'Shown, will not throw an error.