Messagebox with a textbox.

Here's a very simple way to do this; it assumes:

1. You want to capture the co-ordinates of the MouseDown only when the user clicks on the Form with the Control-Alt keys held down.

2. When a valid click is received, then you want a TextBox to pop-up located where the user clicked on the Form, and this TextBox will be closed when the user hits the Escape Key.

At design-time: assume your main form is named 'form1

1. put a Textbox on the Form, size and style it in the way you want, set its 'Visible property to 'false. Name it 'tbCoordinates

2. add this code: link-up the Form's MouseDown EventHandler, and the TextBox's KeyDown and Leave EventHandlers to the code here:

private Point mouseDownPoint;

private void Form1_MouseDown(object sender, MouseEventArgs e)
{
    // if visible, this is an error: hide the TextBox
    if (tbCoordinates.Visible)
    {
        tbCoordinates.Hide();
        this.Cursor = Cursors.Default;
        return;
    }

    // test for Control/Alt down
    // note: 'HasFlag requires .NET 4.0
    if (!(ModifierKeys.HasFlag(Keys.Control) && ModifierKeys.HasFlag(Keys.Alt))) return;

    mouseDownPoint = e.Location;

    // show the TextBox, make it capture input
    tbCoordinates.Location = mouseDownPoint;
    tbCoordinates.Show();

    tbCoordinates.Focus();
    tbCoordinates.Capture = true;
    this.Cursor = Cursors.IBeam;
}

// exit on either Escape key, or Return/Enter key
private void tbCoordinates_KeyDown(object sender, KeyEventArgs e)
{
    if (e.KeyCode == Keys.Escape || e.KeyCode == Keys.Return)
    {
        tbCoordinates.Hide();
        this.Cursor = Cursors.Default;

        // validate the text ?
        // update your database with the
        // Point and text in tbCoordinates here
        // updateDataBase(mouseDownPoint, tbCoordinates.Text);
    }
}

// hide the control if it loses focus
private void tbCoordinates_Leave(object sender, EventArgs e)
{
    if (tbCoordinates.Visible)
    {
        tbCoordinates.Hide();
        this.Cursor = Cursors.Default;
        return;
    }
}

Notes:

1. You could, of course, display the TextBox for entering the name of the co-ordinate Points in a fixed position, or in a ToolStripMenu, or anywhere else you wished.

2. You could, if you wanted something "fancier," create a UserControl that displayed the co-ordinates clicked and any other data you wanted to; you could show it, and hide it, using the same techniques here, but you would have to manage transmitting the edited name to the Form for use with your database.