.Net WebClientBase, WebClientWrapper and WebClientFake for a testable System.Net.WebClient

WebClientBaseAndWrapper.cs

public abstract class WebClientBase : IDisposable
{
    WebHeaderCollection headers = new WebHeaderCollection();

    public abstract void UploadValuesAsync(Uri address, string method, NameValueCollection data);
    public abstract void UploadValuesAsync(Uri address, string method, NameValueCollection data, object userToken);
    public abstract byte[] UploadData(Uri address, string method, byte[] data);
    public abstract byte[] UploadFile(Uri address, string method, string fileName);
    public abstract byte[] UploadValues(Uri address, string method, NameValueCollection data);

    public abstract string UploadString(Uri address, string method, string data);

    public static implicit operator WebClientBase(WebClient webclient)
    {
        return new WebClientWrapper(webclient);
    }

    public abstract void Dispose();

    public virtual WebHeaderCollection Headers
    {
        get { return headers; }
        set { headers = value; }
    }

    public abstract Task<byte[]> UploadValuesTaskAsync(Uri address, string method, NameValueCollection data);
}

public class WebClientWrapper : WebClientBase
{
    readonly WebClient webClient;

    public WebClientWrapper(WebClient webClient)
    {
        this.webClient = webClient;
    }

    public override void Dispose(){ webClient.Dispose(); }

    public override WebHeaderCollection Headers
    {
        get { return webClient.Headers;}
        set { webClient.Headers = value; }
    }

    public override async Task<byte[]> UploadValuesTaskAsync(Uri address, string method, NameValueCollection data)
    {
        return await webClient.UploadValuesTaskAsync(address, method, data);
    }

    public override void UploadValuesAsync(Uri address, string method, NameValueCollection data)
    {
        webClient.UploadValuesAsync(address, method, data);
    }

    public override void UploadValuesAsync(Uri address, string method, NameValueCollection data, object userToken)
    {
        webClient.UploadValuesAsync(address, method, data, userToken);
    }

    public override byte[] UploadData(Uri address, string method, byte[] data)
    {
        return webClient.UploadData(address, method, data);
    }

    public override byte[] UploadFile(Uri address, string method, string fileName)
    {
        return webClient.UploadFile(address, method, fileName);
    }

    public override byte[] UploadValues(Uri address, string method, NameValueCollection data)
    {
        return webClient.UploadValues(address, method, data);
    }

    public override string UploadString(Uri address, string method, string data)
    {
        return webClient.UploadString(address, method, data);
    }
}

WebClientFake.cs

public class WebClientFake : WebClientBase
{
    public byte[] FakeResponse = Encoding.UTF8.GetBytes(JsonConvert.SerializeObject(new {id = 1}));

    public override void UploadValuesAsync(Uri address, string method, NameValueCollection data)
    {
        UploadedValues.Add(Tuple.Create(address, method, data));
    }

    public override void UploadValuesAsync(Uri address, string method, NameValueCollection data, object userToken)
    {
        UploadedValues.Add(Tuple.Create(address, method, data));
    }

    public override byte[] UploadData(Uri address, string method, byte[] data)
    {
        UploadedStrings.Add(Tuple.Create(address, method, Encoding.Unicode.GetString(data)));
        return FakeResponse;
    }

    public override byte[] UploadFile(Uri address, string method, string fileName)
    {
        UploadedFileNames.Add(Tuple.Create(address, method, fileName));
        return FakeResponse;
    }

    public override byte[] UploadValues(Uri address, string method, NameValueCollection data)
    {
        UploadedValues.Add(Tuple.Create(address, method, data));
        return FakeResponse;
    }

    public override string UploadString(Uri address, string method, string data)
    {
        UploadedStrings.Add(Tuple.Create(address, method, data));
        return Encoding.Unicode.GetString( FakeResponse);
    }


    public override void Dispose(){}

    public override async Task<byte[]> UploadValuesTaskAsync(Uri address, string method, NameValueCollection data)
    {
        return await Task.FromResult(UploadValues(address, method, data));
    }

    public readonly List<Tuple<Uri, string, NameValueCollection>> UploadedValues = new List<Tuple<Uri, string, NameValueCollection>>();
    public readonly List<Tuple<Uri,string,string>> UploadedStrings = new List<Tuple<Uri, string, string>>();
    public readonly List<Tuple<Uri,string,string>> UploadedFileNames = new List<Tuple<Uri, string, string>>();


    public WebClientFake(){Headers= new WebHeaderCollection();}
}

Note these are uncomplete. I've only used UploadValuesTaskAsync, UploadValues so far, so you will need to carry on adding methods, as and when you need them.