use parameter when populating a struct with dataset c#?

I have a struct in a web service in c#. When I use "Select * from TABLE1"; in a WebMethod I get a fully populated struct. But when I add a WHERE clause, I get null in response. Why is this? I have searched everywhere for a simple explanation but haven't found one.

How can I use a SELECT * FROM TABLE1 WHERE _id=" + id "'"; If I only want to retrieve a single post from the database it works fine, but not when I get a multiple row response.

Is there also a way of ordering a multiple-row response any way in c#?

Thanks in advance!

edit :

DataSet myDS = new DataSet();

   try开发者_运维技巧
   {
       myConnection.Open();

       // Fill dataset with account data
       //myCommand.Fill(myDS, "userdata");
       myAdapter.Fill(myDS, "toplista");

       myConnection.Close();

       int i = myDS.Tables["toplista"].Rows.Count;
       toplista[] mytoplista = new toplista[i];
       i = 0;


       foreach (DataRow row in myDS.Tables["toplista"].Rows)
       {
           mytoplista[i].name = row["_name"].ToString();
           mytoplista[i].points = int.Parse(row["_points"].ToString());
           mytoplista[i].level = row["_level"].ToString();

           i++;
       }
       return mytoplista;

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Am I understanding correctly that your struct represents a single tuple from your table? If that's the case, you should either be trying to populate an IEnumerable<MyStruct> or only getting the first matching row out of your table. Otherwise, what do you expect it to do with the rest of the data that gets returned?