how to subtract two datatables with linq

To get difference in rows use GetChanges method of dataTable.
A Sample below:

public static DataTable CompareTwoDataTable(DataTable dt1, DataTable dt2)
{

dt1.Merge(dt2);

DataTable d3 = dt2.GetChanges();

return d3;
}

This one is working as expected:

var a = new int[] {1,2,3,4,5};
var b = new int[] {3,4,5,6,7};

var c = a.Except(b);

But both are IEnimerable<int>, in your case, you will get two EnumerableRowCollection<DataRow> collections. I think you will not be able to simply compare them with the defaults. You will need to use the second overload[^], where you can specify your EqualityComparer. Here is a good starting point to implement it: http://www.iramellor.com/LINQ-Distinct-a-DataTable-and-the-IEqualityComparerT[^]