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

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 theUniqueIDList1 =
            new List{  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 theUniqueIDList2 =
            new List{  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