LINQ query with Distinct and Union

Question

I currently have 2 queries that are returning lists of MyModel like this:

var q1 = ....
         select new MyModel()
         {
             TheData1 = ...
         


        

Answer 1

I think frenchie wants a list of MyModel back instead of just the TheUniqueID.

You need to create a MyModelTheUniqueIDComparer class and pass a new instance of it as a second argument into Union:

class MyModelTheUniqueIDComparer : IEqualityComparer<MyModel>
{
    public bool Equals(MyModel x, MyModel y)
    {
        return x.TheUniqueID == y.TheUniqueID;
    }

    // If Equals() returns true for a pair of objects 
    // then GetHashCode() must return the same value for these objects.

    public int GetHashCode(MyModel myModel)
    {
        return myModel.TheUniqueID.GetHashCode();
    }
}

Then you can call to get the result:

var result = q1.Union(q2, new MyModelTheUniqueIDComparer());

See http://msdn.microsoft.com/en-us/library/bb358407.aspx for a more details.

Update:

Try this:

public class A
{
    public string TheData1 { get; set; }
    public string TheData2 { get; set; }
    public string UniqueID { get; set; }
}

public class AComparer : IEqualityComparer<A>
{

    #region IEqualityComparer<A> Members

    public bool Equals(A x, A y)
    {
        return x.UniqueID == y.UniqueID;
    }

    public int GetHashCode(A obj)
    {
        return obj.UniqueID.GetHashCode();
    }

    #endregion
}

And test with this:

var listOfA = new List<A>();
var q1 = from a in listOfA
                 select new A()
             {
                 TheData1 = "TestData",
                 TheData2 = "TestData",
                 UniqueID = a.UniqueID
             };

var anotherListOfA = new List<A>();
var q2 = from a in anotherListOfA
                 select new A()
                 {
                     TheData1 = "TestData",
                     TheData2 = "TestData",
                     UniqueID = a.UniqueID
                 };

q1.Union(q2, new AComparer());

Make sure you have using System.Linq;

Answer 2

Inefficient single line answer with no IEqualityComparerer

Using MoreLinq source code as inspiration, this will give a unique list:

Short answer (the OrderBy isn't necessary but if not used the answer comes out as 2,3,6,9,11,4,7,12):

var concattedUniqueList = theUniqueIDList1.Concat(theUniqueIDList2)
            .GroupBy(f=>f.UniqueID, f=>f).Select(g => g.First()).OrderBy(f=>f.UniqueID);

Complete answer:

//INPUT
//theUniqueIDList1 = 2,3,6,9,11 
//theUniqueIDList2 = 2,4,7,9,12
//OUTPUT
//2,3,4,6,7,9,11,12
public class MyModel
{
    public string TheData1 { get; set; }
    public string TheData2 { get; set; }
    public int UniqueID { get; set; }
}

public static void GroupByEx1()
    {
        // Create a list of Models.
        List<MyModel> theUniqueIDList1 =
            new List<MyModel>{  new MyModel { TheData1="Barley",    UniqueID=2 },
                                    new MyModel { TheData1="Boots",     UniqueID=3 },
                                    new MyModel { TheData1="Whiskers",  UniqueID=6 },
                                    new MyModel { TheData1="Daisy",     UniqueID=9 },
                                    new MyModel { TheData1="Preti",     UniqueID=11 } };
        List<MyModel> theUniqueIDList2 =
            new List<MyModel>{  new MyModel { TheData1="Barley",    UniqueID=2 },
                                    new MyModel { TheData1="Henry",     UniqueID=4 },
                                    new MyModel { TheData1="Walsh",     UniqueID=7 },
                                    new MyModel { TheData1="Daisy",     UniqueID=9 },
                                    new MyModel { TheData1="Ugly",  UniqueID=12 } };
        
        var concattedUniqueList = theUniqueIDList1.Concat(theUniqueIDList2)
            .OrderBy(f=>f.UniqueID).GroupBy(f=>f.UniqueID, f=>f).Select(g => g.First());

        foreach (var item in concattedUniqueList)
        {
            Console.WriteLine("UniqueId: {0}({1})", item.UniqueID, item.TheData1);
        }
    }
    
void Main()
{
    GroupByEx1();               
    //2,3,4,6,7,9,11,12
}

Note: compared to using an IEqualityComparer for speed - 10000 times for each 698 ns for Concat 100 ns for IEqualityComparer

developed in LinqPad

Answer 3

Union creates an Enumerable with unique values from both collections. In other words, you don't need Distinct.

edit: example of Union here

edit2: forgot that it's not the list of UniqueIDs that you're concatenating. I removed the suggested code since it was wrong. You should be able to do a simple Union if you implement an IEqualityComparer, but that might be overkill.

Answer 4

As was pointed out if you are combining the lists with .Union() you will have to define uniqueness by using the overload passing an IEqualityComparer for your type.

var result = q1.Union(q2, myEqualityComparer);

otherwise, and easier you could use DistinctBy( x=> x.TheUniqueId) from the MoreLinq project:

var result = q1.Concat(q2).DistinctBy(c => c.TheUniqueID);