使用LINQ，如何获得以IEnumerable返回的联接表<DataRow>

Question

I use SQL a lot, and I'm trying to transfer the same joining logic to LINQ queries against a DataSet. The DataSet is a bunch of tables which are pulled from SQL queries further up the line.

I managed to get this join working -

        IEnumerable<DataRow> query =
            from a in ds.Tables["Names"].AsEnumerable()
            join b in ds.Tables["NameHasAffiliate"].AsEnumerable()
                on a.Field<int>("PK") equals b.Field<int>("fk_MainTable_PK")
            select a;

This doesn't compile:

select a.Field<int>("PK"), a.Field<string>("Name")

and nor does

select new { PK = a.Field<int>("PK"), Name = a.Field<string>("Name") }

It's definitely querying correctly (I see the expected amount of rows and duplicated table a data) but this is only returning the table a columns - obviously because of select a .

I've tried changing to select new { a, b } and also wrapping the query up in () to add an .ToList() at the end, but neither compiles to give me the IEnumerable version of the query for a simple converstion to table aftewards using

DataTable boundTable = query.CopyToDataTable<DataRow>();

How can I select ALL the columns like this? Or rather, if I'm joining a lot of tables, how can I specify which columns from each table?

Answer 1

This is because returning anything other than a DataRow will not work in your example, since you are defining the query to be of type IEnumerable<DataRow> . So if you are returning a field or collection of fields, then you need to change the expected return type.

I tried the following and it works just fine.

var ds = new DataSet();

ds.Tables.Add("Names");
ds.Tables["Names"].Columns.Add("PK", typeof(Int32));
ds.Tables["Names"].Columns.Add("Name", typeof(String));
ds.Tables["Names"].Rows.Add("1", "NameValue1");

ds.Tables.Add("NameHasAffiliate");
ds.Tables["NameHasAffiliate"].Columns.Add("fk_MainTable_PK", typeof(Int32));
ds.Tables["NameHasAffiliate"].Columns.Add("AffiliateValue", typeof(String));
ds.Tables["NameHasAffiliate"].Rows.Add("1", "AffiliateValue1");



var query = 
    ds.Tables["Names"].AsEnumerable()
        .Join(
            ds.Tables["NameHasAffiliate"].AsEnumerable(), 
            n => n.Field<int>("PK"), a => a.Field<int>("fk_MainTable_PK"),
            (n, a) => new { Key = n.Field<Int32>("PK"), Name = n.Field<String>("Name")})
        .ToList();