C# checkbox status not swithing back to unchecked state

I am writing a C# code for the checkbox here. I want to get the checkbox unchecked if the messsagebox replies with "No". But it still in checked status. I have tried several ways but not working yet. I think my logic is correct but the output is not. chkComplete.Checked = false; is not working.

    private void chkComplete_BeforeCheckStateChanged(object sender, System.ComponentModel.CancelEventArgs args)
{
    // ** Place Event Handling Code Here **
    if (!chkComplete.Checked) {

        DialogResult dr = MessageBox.Show("Are you sure you want to complete this project?", 
                              "Confirmation", MessageBoxButtons.YesNo);
        switch(dr)
        {
           case DialogResult.Yes:
                MessageBox.Show("Please make sure the project is saved!");
                epiShapeC1.Enabled = true;
                break;
           case DialogResult.No:
                MessageBox.Show("The project will stay incomplete!");
                chkComplete.Checked = false;
                epiShapeC1.Enabled = false;
                break;
        }
    } else {
         epiShapeC1.Enabled=false;
        }


}