ASP.NET Core 使用 WebClient 从 URL 下载

本文使用 ASP .NET Core 3.1，但它在.NET 5、 .NET 6和.NET 8上也同样适用。如果使用较旧的.NET Framework，请参阅本文，不过，变化不大。

如果想要从 URL 下载任何数据类型，请参阅本文：HttpClient

使用WebClient.DownloadFile()是一种非常有效的方法来下载文件并将其保存在某处以进行处理。请注意，cookie 正在发送到服务器。

using (System.Net.WebClient wc = new System.Net.WebClient())
{
   wc.Headers.Add("Cookie: Authentication=user"); // NOTE: add a cookie header to the request
   try
   {
       string filename = System.Guid.NewGuid().ToString("N") + ".html"; // NOTE: use globally unique identifier for file name to avoid naming conflicts
       wc.DownloadFile("http://www.prowaretech.com/", filename); // NOTE: could add a file extension here
       // NOTE: do something with file
   }
   catch (System.Exception ex)
   {
       // NOTE: check exception object for the error
   }
}

WebClient.DownloadString()对服务器内存的使用效率较低，但对于下载JSON来说效果很好。

using (System.Net.WebClient wc = new System.Net.WebClient())
{
   try
   {
       wc.Headers["Accept"] = "application/json"; // NOTE: accept JSON code (optional)
       string strJson = wc.DownloadString("http://www.prowaretech.com/json/some-file.json");
       // NOTE: do something with "strJson"
   }
   catch (System.Exception ex)
   {
       // NOTE: check exception object for the error
   }
}

WebClient.UploadString()用于提交 POST 请求并下载结果（在本例中为 JSON）。

using (System.Net.WebClient wc = new System.Net.WebClient())
{
   try
   {
       wc.Headers["Accept"] = "application/json"; // NOTE: accept JSON code (optional)
       wc.Headers["User-Agent"] = "Custom User-Agent"; // NOTE: set the user-agent header (optional)
       string postBody = "field1=data1&field2=data2&field3=data3"; // NOTE: special characters in the data must be escaped
       string strResult = wc.UploadString("http://www.prowaretech.com/some/endpoint", postBody);
       // NOTE: do something with "strResult"
   }
   catch (System.Exception ex)
   {
       // NOTE: check exception object for the error
   }
}

WebClient.DownloadData()是另一种效率较低的服务器内存使用方式。它将文件/端点/url 下载到字节数组中。

using (System.Net.WebClient wc = new System.Net.WebClient())
{
   try
   {
       byte[] data = wc.DownloadData("http://www.prowaretech.com/file.html");
       if (wc.ResponseHeaders != null)
       {
           string? contentType = wc.ResponseHeaders["Content-Type"];
           // NOTE: do something with data and contentType (Content-Type should BEGIN with "text/html" in this case)
       }
       else
       {
           // NOTE: do something with data
       }
   }
   catch (System.Exception ex)
   {
       // NOTE: check exception object for the error
   }
}

WebClient.OpenRead()如果将整个流读入内存，则服务器内存的使用效率会降低。StreamReader.Read()在服务器上处理大型文件时，应谨慎使用内存。

using (System.Net.WebClient wc = new System.Net.WebClient())
{
   try
   {
       using (System.IO.Stream stream = wc.OpenRead("http://www.prowaretech.com/"))
       {
           using (System.IO.StreamReader reader = new System.IO.StreamReader(stream))
           {
               string str = reader.ReadToEnd(); // NOTE: could read one character at a time with reader.Read() which is more efficient with regard to memory usage
               // NOTE: do something with str
           }
       }
   }
   catch (System.Exception ex)
   {
       // NOTE: check exception object for the error
   }
}