Setting the start position for OpenFileDialog/SaveFileDialog

I suspect that the best you can do is make sure you use the overload of ShowDialog that accepts an IWin32Window to use as the parent. This might help it choose an appropriate location; most commonly:

using(var dlg = new OpenFileDialog()) {
    .... setup
    if(dlg.ShowDialog(this) == DialogResult.OK) {
        .... use
    }
}

Check out this article on CodeProject. Excerpt:

Here is when the handy .NET NativeWindow comes into the picture, a NativeWindow is a window wrapper where it processes the messages sent by the handle associated to it. It creates a NativeWindow and associates the OpenFileWindow handle to it. From this point, every message sent to OpenFileWindow will be redirected to our own WndProc method in the NativeWindow instead, and we can cancel, modify, or let them pass through.

In our WndProc, we process the message WM_WINDOWPOSCHANGING. If the open dialog is opening, then we will change the original horizontal or vertical size depending of the StartLocation set by the user. It will increment the size of the window to be created. This happens only once when the control is opened.

Also, we will process the message WM_SHOWWINDOW. Here, all controls inside the original OpenFileDialog are created, and we are going to append our control to the open file dialog. This is done by calling a Win32 API SetParent. This API lets you change the parent window. Then, basically what it does is attach our control to the original OpenFileDialog in the location it set, depending on the value of the StartLocation property.

The advantage of it is that we still have complete control over the controls attached to the OpenFileDialog window. This means we can receive events, call methods, and do whatever we want with those controls.

OpenFileDialog and SaveFileDialog position themselves in the upper-left corner of the client area of the most recently displayed window. So just create a new invisible window positioned where you want the the dialog to appear before creating and showing that dialog.

Window dialogPositioningWindow = new Window();
dialogPositioningWindow.Left = MainWindow.Left + <left position within main window>;
dialogPositioningWindow.Top  = MainWindow.Top  + <top  position within main window>;
dialogPositioningWindow.Width = 0; 
dialogPositioningWindow.Height = 0; 
dialogPositioningWindow.WindowStyle = WindowStyle.None;
dialogPositioningWindow.ResizeMode = ResizeMode.NoResize;
dialogPositioningWindow.Show();// OpenFileDialog is positioned in the upper-left corner
                               // of the last shown window (dialogPositioningWindow)
Microsoft.Win32.OpenFileDialog dialog = new Microsoft.Win32.OpenFileDialog();
...
if ((bool)dialog.ShowDialog()){
   ...
}
dialogPositioningWindow.Close();