public void OpenFolder()
        {
            FolderBrowserDialog fbd = new FolderBrowserDialog();
            fbd.Description = "请选择一个目录.";
            if (fbd.ShowDialog() == DialogResult.OK)
            {
                string[] PicPaht = Directory.GetFiles(fbd.SelectedPath, "*.jpg");
                //这里处理图片了.
            }
        }

------解决方案--------------------
引用 using System.IO;

------解决方案--------------------
有个控件叫FolderBrowserDialog，拖到form上，然后可以:
C# code

public void ChooseFolder()
{
    if (folderBrowserDialog1.ShowDialog() == DialogResult.OK) 
    {
        textBox1.Text = folderBrowserDialog1.SelectedPath;
    }
}