C# how to sign multiple input using rsacryptoserviceprovider

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C#4.0

ASP.NET4

Hi All,

I am consuming a REST based service and I need to pass the signature to conume the service.My method takes 2 paramters.One is DateTime of type string and other is SubjectId of type string.I am signing the both these data using SignData method of RSACryptoServiceProvider as shown below.

How to sign both the input data one after the other to generate overall signature.

public static string GenerateSignature(string dateTime, string subjectId)
      {
          SHA1Managed sha1 = new SHA1Managed();
          UnicodeEncoding encoding = new UnicodeEncoding();

          string signature = string.Empty;
          X509Certificate2 objGetCertificate = null;
          RSACryptoServiceProvider privateKey = null;

          byte[] dateTimeBytes = Encoding.UTF8.GetBytes(dateTime);
          byte[] dateTimeByteshash = sha1.ComputeHash(dateTimeBytes);

          byte[] subjectIdBytes = Encoding.UTF8.GetBytes(subjectId);
          byte[] subjectIdByteshash = sha1.ComputeHash(subjectIdBytes);



          try
          {
              objGetCertificate = GetCertificate();
              privateKey = (RSACryptoServiceProvider)objGetCertificate.PrivateKey;

              privateKey.SignHash(dateTimeByteshash, CryptoConfig.MapNameToOID("SHA1"));
              byte[] signaturebytes=  privateKey.SignHash(subjectIdByteshash, CryptoConfig.MapNameToOID("SHA1"));
              signature = Convert.ToBase64String(signaturebytes);


          }
          catch (Exception ex)
          {

          }
          finally
          {
              if (objGetCertificate != null)
                  objGetCertificate.Reset();
              if (privateKey != null)
                  privateKey.Dispose();

          }
          return signature;

What I have tried:

I have tried to concatenate the signaturebytes retrieved from both the inputs but the services is not returning any data.