Someone

public void Show(IWin32Window owner)
{
    if (owner == this)
    {
        throw new InvalidOperationException(SR.GetString("OwnsSelfOrOwner", new object[] { "Show" }));
    }
    if (base.Visible)
    {
        throw new InvalidOperationException(SR.GetString("ShowDialogOnVisible", new object[] { "Show" }));
    }
    if (!base.Enabled)
    {
        throw new InvalidOperationException(SR.GetString("ShowDialogOnDisabled", new object[] { "Show" }));
    }
    if (!this.TopLevel)
    {
        throw new InvalidOperationException(SR.GetString("ShowDialogOnNonTopLevel", new object[] { "Show" }));
    }
    if (!SystemInformation.UserInteractive)
    {
        throw new InvalidOperationException(SR.GetString("CantShowModalOnNonInteractive"));
    }
    if (((owner != null) && ((((int) UnsafeNativeMethods.GetWindowLong(new HandleRef(owner, Control.GetSafeHandle(owner)), -20)) & 8) == 0)) && (owner is Control))
    {
        owner = ((Control) owner).TopLevelControlInternal;
    }
    IntPtr activeWindow = UnsafeNativeMethods.GetActiveWindow();
    IntPtr handle = (owner == null) ? activeWindow : Control.GetSafeHandle(owner);
    base.Properties.SetObject(PropDialogOwner, owner);
    Form ownerInternal = this.OwnerInternal;
    if ((owner is Form) && (owner != ownerInternal))
    {
        this.Owner = (Form) owner;
    }
    if ((handle != IntPtr.Zero) && (handle != base.Handle))
    {
        if (UnsafeNativeMethods.GetWindowLong(new HandleRef(owner, handle), -8) == base.Handle)
        {
            throw new ArgumentException(SR.GetString("OwnsSelfOrOwner", new object[] { "show" }), "owner");
        }
        UnsafeNativeMethods.GetWindowLong(new HandleRef(this, base.Handle), -8);
        UnsafeNativeMethods.SetWindowLong(new HandleRef(this, base.Handle), -8, new HandleRef(owner, handle));
    }
    base.Visible = true;
}