Linq Expression: Perform Distinct on a list dynamic property

Question

I've a list of objects on which I need to perform a Distinct() function. Currently I use the following code.

Return choices = displaylist
    .Select(Function(x) x.Zone)
    .ToList
    .Distinct()

The above code works well for me. However I need to expand this code into something that allows me to provide any property name and return me the distinct values of the list. My custom object has 20+ properties e.g. Zone, Region, Department, etc.

I need to make a flexible function that I give any property name and it returns distinct. Something like this pseudo:

Function GetDistincts(PROPERTYNAME as string)
    Return choices = displaylist
        .Select(Function(x) x.[PROPERTYNAME])
        .ToList
        .Distinct()
End Function

I believe this is job for Linq Expressions. But I'm no good at it. So need assistance here.

Thanks!

Answer 1

You can use the power of the extension methods and the Generic Types to write a generic extension method to obtain distinct result of every List(Of T):

VB

Dim objects = New List(Of TestObject)()
objects.Add(New TestObject(1, "Test1"))
objects.Add(New TestObject(1, "Test2"))
objects.Add(New TestObject(1, "Test3"))
objects.Add(New TestObject(2, "Test4"))
objects.Add(New TestObject(2, "Test5"))
objects.Add(New TestObject(2, "Test6"))

Dim t As List(Of Integer) = objects.GetDistict(Of Integer, TestObject)("Zone")

Public Class TestObject
    Public Property Zone As Integer
    Public Property Address As String

    Public Sub New(z As Integer, a As String)
        Zone = z
        Address = a
    End Sub
End Class

    Public NotInheritable Class DistinctExtension
    Private Sub New()
    End Sub
    <System.Runtime.CompilerServices.Extension> _
    Public Shared Function GetDistict(Of T, S)(source As List(Of S), name As String) As List(Of T)
        Return source.[Select](Function(x) DirectCast(x.[GetType]().GetProperty(name).GetValue(x), T)).Distinct().ToList()
    End Function

End Class

C#

var objects = new List<TestObject>();
            objects.Add(new TestObject(1,"Test1"));
            objects.Add(new TestObject(1,"Test2"));
            objects.Add(new TestObject(1,"Test3"));
            objects.Add(new TestObject(2,"Test4"));
            objects.Add(new TestObject(2,"Test5"));
            objects.Add(new TestObject(2,"Test6"));

        List<int> t = objects.GetDistict<int, TestObject>("Zone");

public static class DistinctExtension
{
    public static List<T> GetDistict<T,S>(this List<S> source, string name)
    {
        return source.Select(x => (T)x.GetType().GetProperty(name).GetValue(x)).Distinct().ToList();
    }

}

public class TestObject
{
    public int Zone { get; set; }
    public string Address { get; set; }

    public TestObject(int zone, string address)
    {
        Zone = zone;
        Address = address;
    }
}