LINQ Distinct Query

Question

I have a C# application that loads a List of CLR objects called \"Tasks\". Each Task has the following properties:

public int ID { get; set; }
public int Typ         


        

Answer 1

The simplest way to do this is to use GroupBy:

var uniqueTasks = allTasks.GroupBy(p => p.TypeID);

This will give you a set of groupings that map a TypeID to the tasks with that ID.

If you want a set of tasks, you can write

var uniqueTasks = allTasks.GroupBy(p => p.TypeID).Select(g => g.First());

Answer 2

You need to implement your own comparer:

public class TaskComparer : IEqualityComparer<Task>
{

    #region IEqualityComparer<Task> Members

    public bool Equals(Task x, Task y)
    {
        return x.TypeID == y.TypeID && x.TypeName == y.TypeName;
    }

    public int GetHashCode(Task obj)
    {
        return obj.TypeID.GetHashCode() + obj.TypeName.GetHashCode();
    }

    #endregion
}

Then use it like this:

var uniqueTasks = allTasks.Distinct(new TaskComparer());

EDIT: Hacked out some GetHashCode() thanks to Slaks who pointed out that GetHashCode is absolutely necessary (I guess it builds a HashTable internally)

Answer 3

Let me make sure I understand the problem: you have a sequence of tasks called allTasks, and you would like a sequence without duplicates of all the ids, yes?

Transform the sequence of tasks into a sequence of ids:

var ids = allTasks.Select(p=>p.TypeId);

Now you have a sequence of ids. You wish to filter out the duplicates from that sequence:

var distinctIds = ids.Distinct();

And you're done. Is that what you were after?

Answer 4

 var uniqueTasks = allTasks.Select(t=>t.TypeName).Distinct().ToList();