I want to define a typedlist using in the code. There are 2 tables. One is Bolum the other is KkBolum. BolumKodu and OrgFirmaKodu fields are primary keys in the "Bolum" table and foreign keys in the "KkBolum" table.

Here is example.

           DataTable results = new DataTable();
           ResultsetFields fields = new ResultsetFields(5);

            fields.DefineField(BolumFields.OrgFirmaKodu, 0);
            fields.DefineField(BolumFields.BolumKodu, 1);
            fields.DefineField(BolumFields.BolumAdi, 2);
            fields.DefineField(KkBolumFields.OrgFirmaKodu, 3);
            fields.DefineField(KkBolumFields.BolumKodu, 4);

            IRelationPredicateBucket bucket = new RelationPredicateBucket();
            bucket.Relations.Add(BolumEntity.Relations.KkBolumEntityUsingOrgFirmaKoduBolumKodu, "KkBolum", "Bolum", JoinHint.None);

            _adapter.FetchTypedList(fields, results, bucket, 0, null, true, null);

but there is an error : "An item with the same key has already been added"

The two lines below use the same field name, I think you need to add an alias for the second duplicate field name.

fields.DefineField(BolumFields.BolumKodu, 1);
fields.DefineField(KkBolumFields.BolumKodu, 4);

//try this instead
fields.DefineField(KkBolumFields.BolumKodu, 4, "BolumKodu2");

I have tried it but then it gives other error. It can't find any field name with that allias.

How about something like this. The fields need to have the alias defined with them, or don't reference the second set of columns.

            DataTable results = new DataTable();
            ResultsetFields fields = new ResultsetFields(5);

            fields.DefineField(BolumFields.OrgFirmaKodu, 0, "Bolum");
            fields.DefineField(BolumFields.BolumKodu, 1, "Bolum");
            fields.DefineField(BolumFields.BolumAdi, 2, "Bolum");
            fields.DefineField(KkBolumFields.OrgFirmaKodu, 3, "KkBolum");
            fields.DefineField(KkBolumFields.BolumKodu, 4, "KkBolum");

            IRelationPredicateBucket bucket = new RelationPredicateBucket();
            bucket.Relations.Add(BolumEntity.Relations.KkBolumEntityUsingOrgFirmaKoduBolumKodu, "KkBolum", "Bolum", JoinHint.None);

            _adapter.FetchTypedList(fields, results, bucket, 0, null, true, null);