LINQ使用字符串列联接两个表

Question

I'm trying to compare two tables. I'd like to find out which rows exist in table1 , and don't exist in table2 . oData and oDataiSeries are DataTables, and properly populated.

var testing = from table1 in oData.AsEnumerable()
                          join table2 in oDataiSeries.AsEnumerable() on table1.Field<string>("SLOT") equals table2.Field<string>("SLOT")
                          where table1.Field<string>("SLOT") != table2.Field<string>("SLOT")
                          select table1;

testing ends up being null after the code runs. But if I were to do the following.

var testingtable1 = from table1 in oData.AsEnumerable()
                                select table1;

            var testingtable2 = from table2 in oDataiSeries.AsEnumerable()
                                select table2;

testingtable1 and testingtable2 will be populated.

Is it because I'm trying to compare strings?

in testingtable1 and testingtable2 , the column "SLOT" is String

Part of the problem: I'm using VS 2015, and while debugging (hover mouse over DataTable object that should contain results), the Table isn't showing the proper results. It's showing the value of oData, instead of the comparison between oData and oDataiSeries. I'm not sure if I'm just misunderstanding how VS2015 displays var tables, or if this is a bug. If I bind the results DataTable to a gridview object, it'll display different rows as compared to the debugger. I ended up using @D Stanley solution, with a bit of modification.

Answer 1

I'd like to find out which rows exist in table1, and don't exist in table2.

You don't need a join for that. Just look for records that don't have a match in the second table:

var orphans = oData.AsEnumerable()
                   .Where(d => !oDataiSeries.AsEnumerable()
                                            .Any(s => s.Field<string>("SLOT").Trim() == d.Field<string>("SLOT").Trim()));

or if you want to extract the values first to possibly improve performance, try

var series = oDataiSeries.AsEmumerable()
                         .Select(s => s.Field<string>("SLOT"));
var orphans = oData.AsEmumerable()
                   .Where(d => !oDataiSeries.Contains(d.Field<string>("SLOT"));

Is it because I'm trying to compare strings?

No, it's because join is an inner join by default, so ALL records will have matching values between the two datasets.