struct ImgInfo

{

    public string url;

    public string path;

};

ImgInfo img1;  //定义一个结构体，用于向线程中传递下载的参数

img1.url = s1;

img1.path = path;

Thread thread1 = new Thread(new ParameterizedThreadStart(DownImg));

thread1.Start(img1);

 private void DownImg(object img)

 {

    ImgInfo img1 = (ImgInfo)img;

    WebClient wc1 = new WebClient();

    if (L1.InvokeRequired)

    {

       this.BeginInvoke(new Action<string>((x) => { L1.Items.Add(x.ToString()); }), "正在下载-->   "+img1.url);

    }

    wc1.DownloadFileAsync(new Uri(img1.url), img1.path);

 }