.NET WebClient.DownloadData获取文件类型？

Question

In order to handle cases of downloading data from a url that has no file extension, I need to know what the file type is.

for example, how can the WebClient.DownloadData method reveal that it downloaded a png [edit: jpeg] image using the url below?

https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcTw4P3HxyHR8wumE3lY3TOlGworijj2U2DawhY9wnmcPKnbmGHg

I did not find anything in the documentation that describes how to do this.

Answer 1

If you trust the header information, this is possible to do using WebClient —you don't need to use HttpClient :

var webClient = new WebClient();
var result = webClient.DownloadData(url);
var contentType = webClient.ResponseHeaders["Content-Type"];

if (contentType != null && 
    contentType.StartsWith("image", StringComparison.OrdinalIgnoreCase))
{
    // it's probably an image
}

Answer 2

It can't, directly.

If you trust the headers the web server sends back, you could use a different HTTP client (eg WebRequest or HttpClient ) to make the entire response available rather than just the body. You can then look at the Content-Type header.

Other than that, you'll need to look at the content itself. Various file types have "magic numbers" which you could use to identify the file - they're typically at the start of the file, and if you only have a limited set of file types to look for, this may well be a viable approach. It won't be able to identify all file types though.

As an example, the first four bytes of the image you've linked to are ff d8 ff e0. That reveals that actually it's not a jpeg image. As it happens, the server response also included a header of content-type: image/jpeg .

Answer 3

You may use HttpClient for doing this GET request.

Sample code:

            HttpClient client = new HttpClient();
            var response = await client.GetAsync("https://encrypted-tbn2.gstatic.com/images?q=tbn%3aANd9GcTw4P3HxyHR8wumE3lY3TOlGworijj2U2DawhY9wnmcPKnbmGHg");
            var filetype = response.Content.Headers.ContentType.MediaType;
            var imageArray = await response.Content.ReadAsByteArrayAsync();

On the above code, filetype variable has the file type and also extension as image/JPEG or image/PNG etc.

Answer 4

You can try using FindMimeFromData API. Here is the snippet. It may help you.

WebClient webClient = new WebClient();
var result = webClient.DownloadData(new Uri("url"));
IntPtr mimeout;
int result2 = FindMimeFromData(IntPtr.Zero, "sample", result, 4096, null, 0, out mimeout, 0);
if (result2 != 0)
    throw Marshal.GetExceptionForHR(result2);
string mime = Marshal.PtrToStringUni(mimeout);
Marshal.FreeCoTaskMem(mimeout);
Console.WriteLine(mime);

And here is the API declaration. (Copied from here )

[DllImport("urlmon.dll", CharSet = CharSet.Unicode, ExactSpelling = true, SetLastError = false)]
static extern int FindMimeFromData(IntPtr pBC, [MarshalAs(UnmanagedType.LPWStr)] string pwzUrl, [MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.I1, SizeParamIndex = 3)] 
byte[] pBuffer, int cbSize, [MarshalAs(UnmanagedType.LPWStr)]  string pwzMimeProposed, int dwMimeFlags, out IntPtr ppwzMimeOut, int dwReserved);