Select distinct values from a list using LINQ in C

Question

I have a collection of Employee  Class Employee      empName   empID   empLoc    empPL   empShift     My list contains   empName empID empLoc empPL empShift     E1 1 L1 EPL1 S1      E2 2 L2 EPL2 S2     E3 3 L3 EPL3 S3     E4 4 L1 EPL1 S1     E5 5 L5 EPL5 S5         E6 6 L2 EPL2 S2   I need to take the employees having distinct values    empLoc empPL empShift   Is there is any way to achieve this using LINQ

User · Answer

I was curious about which method would be faster:

Using Distinct with a custom IEqualityComparer or
Using the GroupBy method described by Cuong Le.

I found that depending on the size of the input data and the number of groups, the Distinct method can be a lot more performant. (as the number of groups tends towards the number of elements in the list, distinct runs faster).

Code runs in LinqPad!

    void Main()
    {
        List<C> cs = new List<C>();
        foreach(var i in Enumerable.Range(0,Int16.MaxValue*1000))
        {
            int modValue = Int16.MaxValue; //vary this value to see how the size of groups changes performance characteristics. Try 1, 5, 10, and very large numbers
            int j = i%modValue; 
            cs.Add(new C{I = i, J = j});
        }
        cs.Count ().Dump("Size of input array");

        TestGrouping(cs);
        TestDistinct(cs);
    }

    public void TestGrouping(List<C> cs)
    {
        Stopwatch sw = Stopwatch.StartNew();
        sw.Restart();
        var groupedCount  = cs.GroupBy (o => o.J).Select(s => s.First()).Count();
        groupedCount.Dump("num groups");
        sw.ElapsedMilliseconds.Dump("elapsed time for using grouping");
    }

    public void TestDistinct(List<C> cs)
    {
        Stopwatch sw = Stopwatch.StartNew();
        var distinctCount = cs.Distinct(new CComparerOnJ()).Count ();
        distinctCount.Dump("num distinct");
        sw.ElapsedMilliseconds.Dump("elapsed time for using distinct");
    }

    public class C
    {
        public int I {get; set;}
        public int J {get; set;}
    }

    public class CComparerOnJ : IEqualityComparer<C>
    {
        public bool Equals(C x, C y)
        {
            return x.J.Equals(y.J);
        }

        public int GetHashCode(C obj)
        {
            return obj.J.GetHashCode();
        }
    }

User · Answer

Try   var newList      from x in empCollection select new  Loc   x empLoc  PL   x empPL  Shift   x empShift    Distinct

User · Answer

You could implement a custom IEqualityComparer lt Employee gt    public class Employee       public string empName   get  set        public string empID   get  set        public string empLoc   get  set        public string empPL   get  set        public string empShift   get  set         public class Comparer   IEqualityComparer lt Employee gt                public bool Equals Employee x  Employee y                        return x empLoc    y empLoc                  amp  amp  x empPL    y empPL                  amp  amp  x empShift    y empShift                     public int GetHashCode Employee obj                        unchecked     overflow is fine                               int hash   17                  hash   hash   23    obj empLoc        GetHashCode                    hash   hash   23    obj empPL        GetHashCode                    hash   hash   23    obj empShift        GetHashCode                    return hash                                    Now you can use this overload of Enumerable Distinct   var distinct   employees Distinct new Employee Comparer         The less reusable  robust and efficient approach  using an anonymous type   var distinctKeys   employees Select e   gt  new   e empLoc  e empPL  e empShift                                 Distinct    var joined   from e in employees              join d in distinctKeys              on new   e empLoc  e empPL  e empShift   equals d              select e     if you want to replace the original collection employees   joined ToList

User · Answer

You can use GroupBy with anonymous type  and then get First   list GroupBy e   gt  new                              empLoc   e empLoc                             empPL   e empPL                             empShift   e empShift                                  Select g   gt  g First

User · Answer

You can try with this code  var result     from  item in List               select new                                   EmpLoc   item empLoc                   EmpPL  item empPL                   EmpShift  item empShift                                 ToList                  Distinct

[c#] Select distinct values from a list using LINQ in C#

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