DirectoryInfo dir = new DirectoryInfo(AppContext.BaseDirectory);
var path = dir.FullName + @"tempFile\";
var filePath = path + Guid.NewGuid() + fileName;
if (!Directory.Exists(path))
Directory.CreateDirectory(path);
using (WebClient client = new WebClient())
{
// address:从中下载数据的 URI,www.xxxx.com/xx.jpg
// filePath:从中下载数据的 URI,C:\\tempFile\\xx.jpg
client.DownloadFile(address, filePath);
}