OpenFileDialog

OpenFileDialog open = new OpenFileDialog();

//设置默认打开路径
open.InitialDirectory = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);

//筛选文件的类型
open.Filter = "图片(*.jpg)|*.jpg|图片(*.gif)|*.gif|图片(*.png)|*.png|图片(*.bmp)|*.bmp|所有文件（*）|*.*";
          
//设置文件类型框默认显示的类型，FilterIndex是从1开始的
open.FilterIndex = 5;
          
//设置对话框的标题
open.Title = "打开图片";
          
//打开
open.ShowDialog();

//判断是否点击打开按钮
if (open.ShowDialog() == DialogResult.OK)
{
  this.txtDir.Text = open.FileName;
}