How to Optimize Join of Two DataTables Using LINQ In C#

Let's try your query by using  AsParallel() extension method to make the query run in parallel. However, this method can only be used when the source sequence is of type IEnumerable or IQueryable

Here is the updated code for you.

var JoinResult = (from p in dt.AsEnumerable().AsParallel()  
                  join t in dtTax.AsEnumerable()  
                  on p.Field("Tax Id") equals t.Field("Tax Id")  
                  select new  
                  {  
                      ProductName = p.Field("Product Name"),  
                      BrandName = p.Field("Brand Name"),  
                      ProductCategory = t.Field("Product Category"),  
                      TaxCharge = t.Field("Charge")   
                  }).ToList();

NOTE : If you feel that still it is not working properly, you can try below options 

1. You can use the AsCached() extension method to enable the query cache for a DataTable.
2. Instead of joining two DataTables, if you can create a HashSet or Dictionary of one DataTable and use it to lookup values in the second DataTable. This can be faster than a join operation, especially when the second DataTable is small.

Exmaple of using HashSet.

        // Create the first DataTable
        DataTable firstTable = new DataTable();
        firstTable.Columns.Add("ID", typeof(int));
        firstTable.Columns.Add("Name", typeof(string));
        firstTable.Rows.Add(1, "Anupam M");
        firstTable.Rows.Add(2, "Test Name");

        // Create the second DataTable
        DataTable secondTable = new DataTable();
        secondTable.Columns.Add("ID", typeof(int));
        secondTable.Columns.Add("Age", typeof(int));
        secondTable.Rows.Add(1, 30);
        secondTable.Rows.Add(2, 56);

        // Create a HashSet of the first DataTable's ID column
        HashSet firstTableIDs = new HashSet(firstTable.AsEnumerable().Select(r => r.Field("ID")));

        // Use the HashSet to lookup values in the second DataTable
        var result = secondTable.AsEnumerable()
            .Where(r => firstTableIDs.Contains(r.Field("ID")))
            .Select(r => new { ID = r.Field("ID"), Age = r.Field("Age") });

Hope this will help you.