partial class DogovorDataContext
    {

        public class OtborDoc {}
        public OtborDoc fOtborDoc = new OtborDoc();

}

  
        public IQueryable<mdm_DogovorAll> GetDogovorAll()
        {
            var query =
                   from c in this.Dogovor... select c;

            if (fOtborDoc["num"].active)
                query = (from c in query
                         where c.dg_num.Contains(fOtborDoc["dg_num"].value.ToString())
                         select c)
// и so on and so futh
}

Method 'otbor get_Item(System.String)' has no supported translation to SQL.

  
        public IQueryable<mdm_DogovorAll> GetDogovorAll( OtborDoc fOtborDoc)
        {
            var query =
                   from c in this.Dogovor... select c;

            if (fOtborDoc["num"].active)
                query = (from c in query
                         where c.dg_num.Contains(fOtborDoc["dg_num"].value.ToString())
                         select c)
// и so on and so futh
}