Winforms-How can I make MessageBox appear centered on MainForm?

It is possible with some servings of P/Invoke and the magic provided by Control.BeginInvoke(). Add a new class to your project and paste this code:

using System;
using System.Text;
using System.Drawing;
using System.Windows.Forms;
using System.Runtime.InteropServices;

class CenterWinDialog : IDisposable {
    private int mTries = 0;
    private Form mOwner;

    public CenterWinDialog(Form owner) {
        mOwner = owner;
        owner.BeginInvoke(new MethodInvoker(findDialog));
    }

    private void findDialog() {
        // Enumerate windows to find the message box
        if (mTries < 0) return;
        EnumThreadWndProc callback = new EnumThreadWndProc(checkWindow);
        if (EnumThreadWindows(GetCurrentThreadId(), callback, IntPtr.Zero)) {
            if (++mTries < 10) mOwner.BeginInvoke(new MethodInvoker(findDialog));
        }
    }
    private bool checkWindow(IntPtr hWnd, IntPtr lp) {
        // Checks if <hWnd> is a dialog
        StringBuilder sb = new StringBuilder(260);
        GetClassName(hWnd, sb, sb.Capacity);
        if (sb.ToString() != "#32770") return true;
        // Got it
        Rectangle frmRect = new Rectangle(mOwner.Location, mOwner.Size);
        RECT dlgRect;
        GetWindowRect(hWnd, out dlgRect);
        MoveWindow(hWnd,
            frmRect.Left + (frmRect.Width - dlgRect.Right + dlgRect.Left) / 2,
            frmRect.Top + (frmRect.Height - dlgRect.Bottom + dlgRect.Top) / 2,
            dlgRect.Right - dlgRect.Left,
            dlgRect.Bottom - dlgRect.Top, true);
        return false;
    }
    public void Dispose() {
        mTries = -1;
    }

    // P/Invoke declarations
    private delegate bool EnumThreadWndProc(IntPtr hWnd, IntPtr lp);
    [DllImport("user32.dll")]
    private static extern bool EnumThreadWindows(int tid, EnumThreadWndProc callback, IntPtr lp);
    [DllImport("kernel32.dll")]
    private static extern int GetCurrentThreadId();
    [DllImport("user32.dll")]
    private static extern int GetClassName(IntPtr hWnd, StringBuilder buffer, int buflen);
    [DllImport("user32.dll")]
    private static extern bool GetWindowRect(IntPtr hWnd, out RECT rc);
    [DllImport("user32.dll")]
    private static extern bool MoveWindow(IntPtr hWnd, int x, int y, int w, int h, bool repaint);
    private struct RECT { public int Left; public int Top; public int Right; public int Bottom; }
}

Sample usage:

    private void button1_Click(object sender, EventArgs e) {
        using (new CenterWinDialog(this)) {
            MessageBox.Show("Nobugz waz here");
        }
    }

Note that this code works for any of the Windows dialogs. MessageBox, OpenFormDialog, FolderBrowserDialog, PrintDialog, ColorDialog, FontDialog, PageSetupDialog, SaveFileDialog.