Problème avec Excel-Dna et C#

Hi,
I am currently working on a program in C# which is looking for datas in a database with an excel-DNA addin. To simplify I have two columns in the SQL database, one is ID the other is Isin. I need an excel plugin in which I put he isin to get the id. (the program has other goals but my first problem is there). Please help me, I am getting crazy !
So here is my class :

public class MyFunctions
        {

      DBSQL B=new DBSQL();             // (here I have an object with the function get id and the connection to the server, I have this on another layer which is OK I think)


            [ExcelFunction(Description = "fonction de recherche")]
            public string idCL(string _isin)
                //module excel
            {
                String res;
                
                B.Connection();

                res= "id= " + B.getID(_isin);
                // (getID is in DBSQL)
                B.CloseConnection();

                return res;
            }
    }

Ok then I have another class DBSQL with the connection to the server and the getid function :

public long getID(String _isin)
        {
            SqlDataReader rd;
            long res = -9999;


            SqlCommand cd = new SqlCommand("select astid from blabla where isin='" + _isin + "'", MaConn);
            //requète au serveur
            try
            {
                rd = cd.ExecuteReader();

                if (rd.HasRows)
                {
                    while (rd.Read())
                        res = long.Parse(rd["astid"].ToString());
                }

And then I have my addin.dna :

<DnaLibrary Name="ClassLibrary Add-In" RuntimeVersion="v4.0">
  <ExternalLibrary Path="ClassLibrary1.dll" LoadFromBytes="true" Pack="true" />
  <Reference Path="AccesDB.dll" Pack="true" />
  
</DnaLibrary>

There are no errors on visual studio but it doesn't work. Excel returns : "NAME?"
Anyone can help please?