C#, LINQ, Table Join 關聯欄位條件需加工處理

執行會發生錯誤

var query = from data in dbContextTT.AA
            join t2 in dbContextTT.BB on "ABC=" + data.A1 equals t2.B1
            select data;

可以正常執行

var query = from data in dbContextTT.AA
             join t2 in dbContextTT.BB on data.A1 equals t2.B1.Replace("ABC=", "")
             select data;