怎么样让通用对话框显示窗口最前面？

注：API函数GetForegroundWindow用来获得前台窗口的句柄，这里的“前台窗口”是指前台应用程序的活动窗口

类ForegroundWindow继承了框架提供的IWin32Window接口，并定义了一个静态的IWin32Window属性Instance，用来获得前台窗口的句柄。

public class ForegroundWindow : IWin32Window
{
private static ForegroundWindow _window = new ForegroundWindow();
private ForegroundWindow(){}

public static IWin32Window Instance
{
get { return _window; }
}

[DllImport("user32.dll")]
private static extern IntPtr GetForegroundWindow();

IntPtr IWin32Window.Handle
{
get
{
return GetForegroundWindow();
}
}
public class Example1

{

public void SaveFile(string str_filepath)

{

SaveFileDialog openfile=new SaveFileDialog();
openfile.Title="请输入RTF文件名";
openfile.Filter="word文档（*.rtf)|*.rtf";
openfile.FilterIndex=0;
openfile.ValidateNames=true;
openfile.InitialDirectory= str_filepath;
openfile.ShowDialog(ForegroundWindow.Instance);

}

}