400 Bad Request while trying to add a user to a group using ShP 2013 REST api

Question

I am having such a bad time trying to figure this out. I'm trying to add a user to a group by posting to:

_api/web/sitegroups(8)/users  

The posted data looks like this:

{ '__metadata': { 'type': 'SP.User' }, 'LoginName' : 'i:0#.w|domain\\user' }

I get Bad Request 400 and I have no idea what is wrong.

public void AddUserToGroup()
            {
                string requestPath = "/_api/web/sitegroups(8)/users";
                string userPostBody = "{ '__metadata': { 'type': 'SP.User' }, 'LoginName':'i:0#.w|domain\\user' }";
                ExecuteAuthorizedPostRequest(requestPath, userPostBody);
            }

        private HttpWebResponse ExecuteAuthorizedPostRequest(string requestPath, string postBody)
        { 
            string formDigestValue = GetFormDigestValue();
            HttpWebRequest request = PreparePostRequest(requestPath, formDigestValue, postBody);

            HttpWebResponse response = (HttpWebResponse)request.GetResponse();

            return response;
        }

        private HttpWebRequest PreparePostRequest(string requestPath, string formDigestValue, string postBody)
        {
            HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(sharepointUrl.ToString() + requestPath);
            request.Method = "POST";
            request.ContentType = "application/json;odata=verbose";
            request.Accept = "application/json;odata=verbose";
            request.Headers.Add("X-RequestDigest", formDigestValue);
            request.UseDefaultCredentials = true;
            request.ContentLength = postBody.Length;

            byte[] postData = Encoding.ASCII.GetBytes(postBody);

            Stream requestStream = request.GetRequestStream();
            requestStream.Write(postData, 0, postData.Length);
            requestStream.Close();

            return request;
        }

Answer 1

Fiddler is your friend. If you watch the traffic (response) you see the real error message:

Invalid JSON. An unrecognized escape sequence '\u' was found in a JSON string value.

(where 'u' is the first character of the user name)

You should double-escape the backslash \, once for C# and once for JSON. That means i:0#.w|domain\\\\user (four backslashes!) instead of i:0#.w|domain\\user (two backslashes). The same is true for JavaScript.

The posted data should be (two backslashes!):

{ '__metadata': { 'type': 'SP.User' }, 'LoginName' : 'i:0#.w|domain\\user' }

Answer 2

I'll just add this as an alternative:
If you serialize the JSON, e.g., with JSON.NET, you would not have stumbled upon the problem @pholpar pointed out to you.

Like this:

string loginName = ...;
var postBody = JsonConvert.SerializeObject(new
{
    __metadata = new
    {
        type = "SP.User"
    },
    LoginName = loginName
});

Answer 3

Just for the sake of completeness, according to Using WCF Data Service With Restricted Characters as Keys:

the symbols %,&,*,:,<,>,+,#, /, ?,\ are considered as "special" in WCF Data Service

In order to avoid such kind of errors, the serialization of POCO is commonly used.

Examples

var loginName = "i:0#.w|domain\\user";

var userPayload = new
{
      __metadata = new
      {
         type = "SP.User"
      },
      LoginName = loginName
};

Using JSON.NET

var postData = JsonConvert.SerializeObject(userPayload);

Using JavaScriptSerializer

var serializer = new JavaScriptSerializer();
var postData = serializer.Serialize(userPayload);