Question

I’m trying to get a list of rows in DataTableA where the value in Column 1 is not in Column1 of DataTableB.

I’m using the following LinQ query

//Not in Database
var query = from i in dtImport.AsEnumerable()
            where !dtProducts.AsEnumerable().Any(p => p[colP] == i[colI])
            select i;

Such that I want a list of products in the import table that aren’t already in the products table.

Which seems to skip past the line quickly when I’m debugging but then when I call anything relating to that query such as int rows = query.Count<DataRow>(); or DataTable dtResult = query.CopyToDataTable(); it seems to take a long time so I just stop the program.

So, What am I doing wrong?

Answer 1

Linq uses deferred execution. The query is executed when it is used (not when declared)
For better performance you can use a HashSet like the following;

var set = new HashSet<int>(dtProducts.AsEnumerable().Select(p => p.colP));
var result = dtImport.AsEnumerable().Where(i => !set.Contains(i[colI])).ToList();