MDI and ShowDialog()

After considerable research I cannot find a way to open a form using
ShowDialog() within a MDI parent.

SomeForm SF = new SomeForm(parameters);
SF.MdiParent = MdiParent;
SF.ShowDialog(); - Blows up here with an exception thrown.
Do More Stuff here after form closes

All I have read says that this cannot be done, but I don't believe that.

Any help would be appreciated.

Thanks.