Question

how can i do in linq:

IEnumerable<DataRow> query = 
        from rec in dt.AsEnumerable() 
        where rec.Field<decimal>("column2") == 1 && foo(rec.Field<decimal>("column1"))
        select new {
            column1 = rec.Field<decimal>("column1"),
            column2 = rec.Field<decimal>("column2"),
            column3 = rec.Field<decimal>("column3")}  ;

this does not work. Im trying to select some columns as new datatable then join it later with some other datatable.

Answer 1

You can use http://code.msdn.microsoft.com/LinqEntityDataReader
and then select your values like this:

DataTable query = 
        (from rec in dt.AsEnumerable() 
        where rec.Field<decimal>("column2") == 1 && foo(rec.Field<decimal>("column1"))
        select new {
            column1 = rec.Field<decimal>("column1"),
            column2 = rec.Field<decimal>("column2"),
            column3 = rec.Field<decimal>("column3")}).ToDataTable();

A much more simple solution, that doesn’t require external libraries, would be to create a DataTable (if you don’t already have one), and populate it as follows:

// declaring a data table.. Replace it with whatever code you want
var table = new DataTable();
table.Columns.Add("column1", typeof(Decimal));
table.Columns.Add("column2", typeof(Decimal));
table.Columns.Add("column3", typeof(Decimal));

// Populate the table
(from rec in dt.AsEnumerable() 
where rec.Field<decimal>("column2") == 1 && foo(rec.Field<decimal>("column1"))
select new {
    column1 = rec.Field<decimal>("column1"),
    column2 = rec.Field<decimal>("column2"),
    column3 = rec.Field<decimal>("column3")})
    .Aggregate(table, (dt, r) => { dt.Rows.Add(r.column1, r.column2, r.column3); return dt; });

// at this point your table variable is populated