WebClient.UploadValues没有在c＃中发布帖子请求

I am trying to upload a single jpeg image to a PHP script. This my console app program.cs:

class Program
{
    static void Main(string[] args)
    {
        Stopwatch stopWatch = new Stopwatch();
        stopWatch.Start();

        string result = UploadHandler.Post(
            "http://localhost/upload_test",
            "frame.jpg"
        );

        stopWatch.Stop();
        TimeSpan ts = stopWatch.Elapsed;
        string elapsedTime = String.Format("{0:00}:{1:00}:{2:00}.{3:00}",
            ts.Hours, ts.Minutes, ts.Seconds,
            ts.Milliseconds / 10);

        Console.WriteLine("Result : {0}", result);
        Console.WriteLine("File uploaded in : {0}", elapsedTime);
        Console.ReadKey();
    }
}

This is my UploadHandler class:

class UploadHandler
{
    public static string Post(string serverUrl, string filePath)
    {
        string result = "";
        using (WebClient client = new WebClient())
        {
            byte[] response = client.UploadValues(serverUrl, new NameValueCollection()
            {
                { "frameData", ToBase64(filePath) }
            });
            result = Encoding.Default.GetString(response);
        }
        return result;
    }

    private static string ToBase64(string filePath)
    {
        return Convert.ToBase64String(
            File.ReadAllBytes(filePath)
        );
    }
}

and this is my php script that receives the upload:

<?php

if (count($_POST) && isset($_POST['frameData']))
{
    file_put_contents('frame.jpg', base64_decode($_POST['frameData']));
    exit("OK");
}
else
{
    print_r($_POST);
    exit("INVALID REQUEST");
}

And this is the response I get:

Any idea why this might be? It appears the C# app isn't making a HTTP POST request.

Try:

client.UploadValues(serverUrl, "POST", new NameValueCollection()
            {
                { "frameData", ToBase64(filePath) }
            });

EDIT: You should debug issues like this with Fiddler: http://www.telerik.com/fiddler

First make sure that your C# app sends the expected request by inspecting it in Fiddler, and then you can make sure that your PHP app is doing the right thing on the other end.

Right now you won't get very far, because it is not clear where the problem is.