Union Vs Concat in Linq

by

Union returns Distinct values. By default it will compare references of items. Your items have different references, thus they all are considered different. When you cast to base type X, reference is not changed.

If you will override Equals and GetHashCode (used to select distinct items), then items will not be compared by reference:

class X
{
    public int ID { get; set; }

    public override bool Equals(object obj)
    {
        X x = obj as X;
        if (x == null)
            return false;
        return x.ID == ID;
    }

    public override int GetHashCode()
    {
        return ID.GetHashCode();
    }
}

But all your items have different value of ID. So all items still considered different. If you will provide several items with same ID then you will see difference between Union and Concat:

var lstX1 = new List<X1> { new X1 { ID = 1, ID1 = 10 }, 
                           new X1 { ID = 10, ID1 = 100 } };
var lstX2 = new List<X2> { new X2 { ID = 1, ID2 = 20 }, // ID changed here
                           new X2 { ID = 20, ID2 = 200 } };

var a5 = lstX1.Cast<X>().Union(lstX2.Cast<X>());  // 3 distinct items
var a6 = lstX1.Cast<X>().Concat(lstX2.Cast<X>()); // 4

Your initial sample works, because integers are value types and they are compared by value.

More Related Contents:

  1. Get all checked nodes of a treeview
  2. Distinct by property of class with LINQ [duplicate]
  3. Using LINQ to concatenate strings
  4. C# – code to order by a property using the property name as a string [duplicate]
  5. How to get values from IGrouping
  6. LEFT JOIN in LINQ to entities?
  7. Can I specify my explicit type comparator inline?
  8. Find the most occurring number in a List
  9. Entity Framework (.NET Full Framework) Ordering Includes [duplicate]
  10. How to compare only date components from DateTime in EF?
  11. Except has similar effect to Distinct?
  12. Entity Framework 6 Code First Custom Functions
  13. Merge multiple Lists into one List with LINQ
  14. LINQ to Entities does not recognize the method ‘System.Web.Mvc.FileResult’
  15. Order by dynamic parameter
  16. Can I split an IEnumerable into two by a boolean criteria without two queries?
  17. Sum of TimeSpans in C#
  18. Linq Syntax – Selecting multiple columns
  19. Sequence contains no matching element
  20. Outer Variable Trap
  21. Will using LINQ to SQL help prevent SQL injection
  22. How to count the number of elements that match a condition with LINQ
  23. EF Core queries all columns in SQL when mapping to object in Select
  24. Filling a DataSet or a DataTable from a LINQ query result set
  25. Issues Doing a String Comparison in LINQ
  26. Select all columns after JOIN in LINQ
  27. Convert / Cast IEnumerable to IEnumerable
  28. Where can I find the System.Linq.Dynamic dll?
  29. C# Searching for files and folders except in certain folders
  30. What is the difference between LINQ ToDictionary and ToLookup

Leave a Comment