To get string type value from store procedure

public string ExecuteDataSetSP(string spName, Boolean withOutPara)
  {

      SqlDataAdapter daDataAdapter = new SqlDataAdapter();
      DataSet dsDataSet = new DataSet();

      // Initialize the Exception ...

          // Prepare DataAdapter ...
          daDataAdapter.SelectCommand = new SqlCommand();
          daDataAdapter.SelectCommand.Connection = cnnConnection;
          daDataAdapter.SelectCommand.CommandText = spName;
          daDataAdapter.SelectCommand.CommandType = CommandType.StoredProcedure;
          // Pass Parameters ...


              // Add output parameters ...
              daDataAdapter.SelectCommand.Parameters.Add("@o_ErrorMesg", SqlDbType.VarChar, 250).Direction = ParameterDirection.Output;

          // Open Database Connection ...
          cnnConnection.Open();
          // Fill the DataSet ...
          daDataAdapter.Fill(dsDataSet);
          // Read values of Output Parameters

              // Read Values of Output Paramters and Stored into Property
              mErrorMsg = (string)(daDataAdapter.SelectCommand.Parameters["@o_ErrorMesg"].Value);


      return mErrorMsg ;
  }

//Suppose your command object is cmd and dataset object is dt
SqlDataAdapter sda=new SqlDataAdapter(cmd);
sda.Fill(ds);
string strMyVal=ds.Tables[0].Rows[0][0].toString();
//now strMyVal is ready with your string value returned from database.