OpenFileDialog dlg = new OpenFileDialog();
dlg.Filter = "All File(*.*)|*.*";
if (dlg.ShowDialog() == DialogResult.OK)
{
       txtPath.Text = dlg.FileName;
}     
//Filter是经常容易忘记的选项，再举一个例子
dlg.Filter = "Xml file (*.xml)|*.xml|All Files|*.*";
//csharp/193