drag and drop file into textbox

I want to drag and drop a file so that the textbox shows the full file path. I have used the drag enter and drag drop events but I find that they are not entering the events.

private void sslCertField_DragDrop(object sender, DragEventArgs e)
{
    if (e.Data.GetDataPresent(DataFormats.FileDrop, false) == true)
    {
        e.Effect = DragDropEffects.All;
    } 
}

private void sslCertField_DragEnter(object sender, DragEventArgs e)
{
    string file = (string)e.Data.GetData(DataFormats.FileDrop);
    serverURLField.Text = file;
}

Can anyone point out what I am doing wrong?

UPDATE: Does not work if program is set to run with elevated permissions (vista/win 7)