How to JOIN two data tables using LINQ and Where condition?

I have two data tables as following

    TableA      TableB
    UID         QID
    QID         Value1
                Value2

I want to join two data tables and get Value1 and Value 2. I tried the following, but I am not getting any data back.

string str = "AAA";
var result = (from t1 in TableA.AsEnumerable()
    //join between two tables
    join t2 in TableB.AsEnumerable() on t1.Field("UID") equals t2.Field("QID")
    //where conditions
    where t2.Field("QID") == str && t1.Field("QID") == "ABCD"
    //select clause
    select t2.Field("Value1"));

This is my query with out using LINQ and its working.

  Select t2.Value1, t2.value2 from  TableA t1 JOIN  TableB t2  ON t2.QID = t1.UID WHERE t2.QID = 'AAAAA' AND t1.QID = 'XXXX'

Thanks for your help.