var res = from insA in instancesA where insA.FId == instancesB.Id select insA.Data;

//[ID] [numeric](19, 0) IDENTITY(1,1) NOT NULL,
        private decimal _id;

        [Column(Name = "ID", Storage="_id", IsDbGenerated = true, IsPrimaryKey = true)]
        public decimal Id
        {
            get { return _id; }
            set { _id = value; }
        }

InstanceBType InstanceB;
DataContext db = new DataContext(LocalConfiguration.ConnectionString);
Table<InstanceAType> instancesA = db.GetTable<InstanceAType>();
var result = from insA in  instancesA where (InstanceB.Id == insA.FId) select insA.Data;
...
return result.ToList();



    [Table(Name = "...")]
    public class InstanceBType
    {

        //[ID] [numeric](19, 0) IDENTITY(1,1) NOT NULL,        
        private decimal _id;

        [Column(Name = "ID", Storage = "_id", DbType = "numeric(19,0)", IsPrimaryKey = true, IsDbGenerated = true, CanBeNull=false)]
        public decimal  Id
        {
            get { return _id; }
            set { _id = value; }
        }
        ...
   }

    public class InstanceAType
    {
           //[F_ID] [numeric](19, 0) NULL,
        private decimal _fId;

        [Column(Name = "F_ID", Storage = "_fId")]
        public decimal FId
        {
            get { return _fId; }
            set { _fId = value; }
        }

        private EntityRef< InstanceBType> _f;

        [Association(Name = "F_ID", IsForeignKey = true, ThisKey = "FId", Storage = "_f")]
        public InstanceBType Data
        {
            get { return _f.Entity; }
            set { _f.Entity = value; }
        }
        ...
   }