WCF dynamic json to Dictionary

As you want to receive raw data into your method param, you have to implement your method in a way:

[OperationContract]
[WebInvoke(Method = "POST", UriTemplate = "form/data",
    RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json, BodyStyle = WebMessageBodyStyle.WrappedRequest)]
public String StoreFormData(Stream fileContents)
{
    using (StreamReader reader = new StreamReader(fileContents))
    {
        JavaScriptSerializer jsSerializer = new JavaScriptSerializer();

        //here is your json, received from a client
        string jsonData = reader.ReadToEnd();

        // I think for that you case it's better to use Newtonsoft.Json library. It will allow you to parse more complex data
        //JObject data = JObject.Parse(jsonData); 

        //Dictionary<string, string> formValues = jss.Deserialize<Dictionary<string, string>>(formData);
    }

    return "aaaaa";
}

In such a way you will be able to receive plain json data in the way it was formed on a client side. Then you can parse them in a way you need/want.

EDIT 1:

P.S. Don't forget to change UriTemplate = "form/data" to whatever you need.

EDIT 2:

I think that for your case it's better to use Newtonsoft.Json library. It will allow you to parse more complex data:

JObject data = JObject.Parse(jsonData); 

There were 2 problems:
1. I used dynamic as @Legart mentioned.
2. The parameter was a json object.

There are a working solution:

function storeDataInSession(formData) {
    var data = {};
    data["formData"] = JSON.stringify(formData);

    $.ajax({
        url: "MYURL/StoreFormData",
        type: "post",
        data: JSON.stringify(data),
        contentType: 'application/json',
        dataType: 'json',
        success: function (data, textStatus, xhr) {
            console.log(data);
            console.log("success");
        },
        error: function (xhr, textStatus, errorThrown) {
            console.log("failure");
        }
    });
}

The server side:

[OperationContract]
    [WebInvoke(Method = "POST", RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json, BodyStyle = WebMessageBodyStyle.WrappedRequest)]
    public String StoreFormData(string formData) 
    {        
        JavaScriptSerializer jss = new JavaScriptSerializer();
        Dictionary<string, string> formValues = jss.Deserialize<Dictionary<string, string>>(formData);

        string test = "test:  ";
        foreach (KeyValuePair<string, string> item in formValues) 
        {
            test += String.Format(" key: {0}  value: {1} ", item.Key, item.Value);
        }



        return formData;
    }