** Interface **
public interface IAPIHelper
{
    //RESTFul API
    [FaultContract(typeof(RequestValidationFault))]
    [WebInvoke(UriTemplate = "/ByteFromCaseData", Method = "POST", ResponseFormat = WebMessageFormat.Json, RequestFormat = WebMessageFormat.Json)]
    [OperationContract]
    byte[] ByteFromCaseData(ByteFromCaseDataRequest caseData);
}

 *** Data contract ***

[DataContract]
public class ByteFromCaseDataRequest
{
    [DataMember]
    public Dictionary<string, string[]> HashMap;
}


  ***Implementation****

[ServiceBehavior]
[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
public class APIHelper : IAPIHelper
{
    public byte[] ByteFromCaseData(ByteFromCaseDataRequest caseData)
    {
        // Business Logic
    }
}

json request -
 {
   "HashMap" : {"key1":["1.1","1.2"],"key2":["2.1","2.2"],"key3":["3.1","3.2"]}
 }

{
   "HashMap" : [
                {"Key": "key1", "Value": ["1.1","1.2"]},
                {"Key": "key2", "Value": ["2.1","2.2"]}
               ]
 }