Trouble posting CREATERAWTRANSACTION to Bitcoin Core via JSON-RPC

I'm trying to post to a local bitcoin full node via json-rpc but I'm getting an error from the server.

Following the documentation here: https://bitcoincore.org/en/doc/0.17.0/rpc/rawtransactions/createrawtransaction/

I can see the following example structure for a createrawtransaction request:

{"jsonrpc": "1.0", "id":"curltest", "method": "createrawtransaction", "params": ["[{\"txid\":\"myid\",\"vout\":0}]", "[{\"address\":0.01}]"] }

My code creates the following structure, which seems to match the structure of the example from bitcoincore.org:

{"jsonrpc":"1.0","id":"1","method":"createrawtransaction","params":["[{\"txid\":\"1a43a1f27c5837d5319a45217aa948a4d39c1d89faf497ce59de5bd570a64a26\",\"vout\":1}]","[{\"2NAZpRsvj9BstxxCDkKoe1FVjmPPxdmvqKj\":0.01}]"]}

But it gives an error:

System.Net.WebException
  HResult=0x80131509
  Message=The remote server returned an error: (500) Internal Server Error.
  Source=RawTransactions
  StackTrace:
   at RawTransactions.Form1.RequestServer(String methodName, List`1 parameters) in C:\Users\userthree\Documents\Visual Studio 2017\Projects\RawTransactions\RawTransactions\Form1.cs:line 132
   at RawTransactions.Form1.button1_Click(Object sender, EventArgs e) in C:\Users\userthree\Documents\Visual Studio 2017\Projects\RawTransactions\RawTransactions\Form1.cs:line 77
   at System.Windows.Forms.Control.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.ButtonBase.WndProc(Message& m)
   at System.Windows.Forms.Button.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
   at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
   at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.Run(Form mainForm)
   at RawTransactions.Program.Main() in C:\Users\userthree\Documents\Visual Studio 2017\Projects\RawTransactions\RawTransactions\Program.cs:line 19

Below is the method I am using to make the RPC request, which I got from the API reference here:

https://en.bitcoin.it/wiki/API_reference_(JSON-RPC)#.NET_.28C.23.29

public static string RequestServer(string methodName, List<string> parameters)
{
    string ServerIp = "http://localhost:18332";
    string UserName = "USERNAME";
    string Password = "PASSWORD";

    HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(ServerIp);
    webRequest.Credentials = new NetworkCredential(UserName, Password);

    webRequest.ContentType = "application/json-rpc";
    webRequest.Method = "POST";

    string respVal = string.Empty;

    JObject joe = new JObject();
    joe.Add(new JProperty("jsonrpc", "1.0"));
    joe.Add(new JProperty("id", "1"));
    joe.Add(new JProperty("method", methodName));

    JArray props = new JArray();
    foreach (var parameter in parameters)
    {
        props.Add(parameter);
    }

    joe.Add(new JProperty("params", props));

    // serialize json for the request
    string s = JsonConvert.SerializeObject(joe);
    byte[] byteArray = Encoding.UTF8.GetBytes(s);
    webRequest.ContentLength = byteArray.Length;

    Stream dataStream = webRequest.GetRequestStream();
    dataStream.Write(byteArray, 0, byteArray.Length);
    dataStream.Close();

    StreamReader streamReader = null;
    try
    {
        WebResponse webResponse = webRequest.GetResponse();

        streamReader = new StreamReader(webResponse.GetResponseStream(), true);

        respVal = streamReader.ReadToEnd();
        var data = JsonConvert.DeserializeObject(respVal).ToString();
        return data;
    }
    catch (Exception exp)
    {
        throw (exp);
    }
    finally
    {
        if (streamReader != null)
        {
            streamReader.Close();
        }
    }

    return string.Empty;
}

Here is my attempt to use the above method:

private void button1_Click(object sender, EventArgs e)
{
    StringBuilder sb1 = new StringBuilder();

    sb1.Append("[{\"");
    sb1.Append("txid");
    sb1.Append("\":\"");
    sb1.Append(Convert.ToString(data["result"][Convert.ToInt32(txtFromJSON.Text)]["txid"]));
    sb1.Append("\",\"");
    sb1.Append("vout");
    sb1.Append("\":");
    sb1.Append(Convert.ToString(data["result"][Convert.ToInt32(txtFromJSON.Text)]["vout"]));
    sb1.Append("}]");

    StringBuilder sb2 = new StringBuilder();

    sb2.Append("[{\"");
    sb2.Append(Convert.ToString(data["result"][Convert.ToInt32(txtToJSON.Text)]["address"]));
    sb2.Append("\":");
    sb2.Append(txtAmountToSpend.Text);
    sb2.Append("}]");

    // {"jsonrpc":"1.0","id":"1","method":"createrawtransaction","params":["[{\"txid\":\"1a43a1f27c5837d5319a45217aa948a4d39c1d89faf497ce59de5bd570a64a26\",\"vout\":1}]","[{\"2NAZpRsvj9BstxxCDkKoe1FVjmPPxdmvqKj\":0.01}]"]}
    data = JObject.Parse(RequestServer("createrawtransaction", new List<string>() { Convert.ToString(sb1), Convert.ToString(sb2) }));

    MessageBox.Show(Convert.ToString(data));
}

Other commands such these as do work:

// {"jsonrpc":"1.0","id":"1","method":"sendtoaddress","params":["2N8hwP1WmJrFF5QWABn38y63uYLhnJYJYTF","0.1"]}
data = JObject.Parse(RequestServer("sendtoaddress", new List<string>() { "2N8hwP1WmJrFF5QWABn38y63uYLhnJYJYTF", Convert.ToString(0.1) } ));

This also works:

// {"jsonrpc":"1.0","id":"1","method":"listunspent","params":[]}
data = JObject.Parse(RequestServer("listunspent", new List<String>() { }));

My Question:

What have I done wrong with createrawtransaction?

---

Update 1:

As suggested in the comments, I have changed the StringBuilder, and am now using objects and then serializing the objects using Newtonsoft.Json.

Here is my second attempt to use the API reference code from https://en.bitcoin.it/wiki/API_reference_(JSON-RPC)#.NET_.28C.23.29:

private void button1_Click(object sender, EventArgs e)
{
    JContainer jArray = new JArray();

    JObject jFromTx = new JObject
    {
        { "txid", data["result"][Convert.ToInt32(txtFromJSON.Text)]["txid"] },
        { "vout", data["result"][Convert.ToInt32(txtFromJSON.Text)]["vout"] }
    };

    jArray.Add(jFromTx);

    JObject jToTx = new JObject
    {
        { Convert.ToString(data["result"][Convert.ToInt32(txtToJSON.Text)]["address"]), Convert.ToDouble(txtAmountToSpend.Text) }
    };

    JContainer jArray2 = new JArray
    {
        jToTx
    };

    string strFrom = JsonConvert.SerializeObject(jArray);
    string strTo = JsonConvert.SerializeObject(jArray2);

    data = JObject.Parse(RequestServer("createrawtransaction", new List<string>() { strFrom, strTo }));
}

Here is the new serialized JSON:

{"jsonrpc":"1.0","id":"1","method":"createrawtransaction","params":["[{\"txid\":\"1a43a1f27c5837d5319a45217aa948a4d39c1d89faf497ce59de5bd570a64a26\",\"vout\":1}]","[{\"2NAZpRsvj9BstxxCDkKoe1FVjmPPxdmvqKj\":0.01}]"]}

Compared to the old StringBuilder JSON from my first attempt:

{"jsonrpc":"1.0","id":"1","method":"createrawtransaction","params":["[{\"txid\":\"1a43a1f27c5837d5319a45217aa948a4d39c1d89faf497ce59de5bd570a64a26\",\"vout\":1}]","[{\"2NAZpRsvj9BstxxCDkKoe1FVjmPPxdmvqKj\":0.01}]"]}

I am still getting the same error message as before (see above).

I think the problem is your params array is getting double-serialized, so the server doesn't know how to interpret the request. I do realize that your JSON looks the same as the example, so I could very well be wrong here; I'm definitely not an expert on using Bitcoin Core's API. However, I did look at a the source code for a third-party library which is supposed to be compatible with Bitcoin Core, and it does not appear to double-serialize the parameters for the createrawtransation request. This leads me to believe that the double-serialized parameters are the problem.

To fix, try the following:

1. Change the method signature for your existing RequestServer method from this:

   public static string RequestServer(string methodName, List<string> parameters)
   

   to this:

   public static string RequestServer(string methodName, List<JToken> parameters)
   

2. Create a new overload of the RequestServer method using the old signature which calls the existing one you just changed. This will allow your other methods which already work (e.g. sendtoaddress and listunspent) to keep working with no changes.

   public static string RequestServer(string methodName, List<string> parameters)
   {
       return RequestServer(methodName, parameters.Select(p => new JValue(p)).ToList<JToken>());
   }
   

3. Lastly, change the code in your button1_Click method so that it does not serialize jArray and jArray2, but instead passes them in a List<JToken> to RequestServer. In other words, change this code:

   string strFrom = JsonConvert.SerializeObject(jArray);
   string strTo = JsonConvert.SerializeObject(jArray2);
   
   data = JObject.Parse(RequestServer("createrawtransaction", new List<string>() { strFrom, strTo }));
   

   to this:

   data = JObject.Parse(RequestServer("createrawtransaction", new List<JToken>() { jArray, jArray2 }));
   

With these changes, the RPC JSON for createrawtransaction should end up looking like this:

{"jsonrpc":"1.0","id":"1","method":"createrawtransaction","params":[[{"txid":"1a43a1f27c5837d5319a45217aa948a4d39c1d89faf497ce59de5bd570a64a26","vout":1}],[{"2NAZpRsvj9BstxxCDkKoe1FVjmPPxdmvqKj":0.01}]]}

Notice that the extra quotes and backslashes in the params array are gone. Compare to what you had before:

{"jsonrpc":"1.0","id":"1","method":"createrawtransaction","params":["[{\"txid\":\"1a43a1f27c5837d5319a45217aa948a4d39c1d89faf497ce59de5bd570a64a26\",\"vout\":1}]","[{\"2NAZpRsvj9BstxxCDkKoe1FVjmPPxdmvqKj\":0.01}]"]}