Question

My code is working however i am trying to find a more efficient way to accomplish this. Like a linq solution without the foreach loop something that would be better. I dont have any performance issues..

  List<Table> tables = getTables();
  foreach (TableCategories category in categories)
  {
   category.Tables=tables.FindAll(tbl => tbl.CategoryId == category.Id);
  }

For database i use pure ADO.NET not linq as i am using postgres.

Answer 1

How about:

var tablesByCategory = getTables().ToLookup(t => t.CategoryId);
foreach(var category in categories)
{
    category.Tables = tablesByCategory[category.Id];
}

This is more efficient (assuming you have a lot of tables) because you build a lookup at the beginning in O(n) time, and then each lookup in the for loop is O(1) time, so you end up with O(m + n) instead of O(m*n) complexity.