How to place a messagebox and a filedialogbox inside my mainform

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C#7.0

Hello
How to place a MessageBox and a FileDialogBox inside my mainForm

I have tried the code below with the properies

TopMost = true,
StartPosition = FormStartPosition.CenterParent

but without success.
Thanks
Bernard

What I have tried:

DialogResult res = new DialogResult();
            res = MessageBox.Show(new Form()
            {
                TopMost = true,
                StartPosition = FormStartPosition.CenterParent
            },
                "Choisissez le fichier GeoDB.",
                "Choix du fichier source",
                MessageBoxButtons.YesNoCancel,
                MessageBoxIcon.Question);

            if (res == DialogResult.Cancel || res == DialogResult.No)
                return;

            OpenFileDialog OFD = new OpenFileDialog();
            OFD.Filter = "Fichier Source GeoDB (*.gdb) | *.gdb";
            OFD.Title = "Choix d'un fichier source GeoDB";

            if (OFD.ShowDialog(new Form()
            {
                TopMost = true,
                StartPosition = FormStartPosition.CenterParent
            }) == DialogResult.OK)
            {
                nomFichierGeoDB = OFD.FileName;
                GeoDBConString = GetGeoDBConString();
            }