How do you sort a generic list in C# and allow NULL items to appear first in the list?

Question

I have a generic list of objects in C#, for example sake, here's what the object might be.

public class Thing {
    public string Name { get; set; }
    public DateTime EditDate { get; set; }
}

var things = new List<Thing>();

Now I want to call:

thing.Sort((t1, t2) => t1.EditDate.CompareTo(t2.EditDate));

However, some of my EditDate properties are null. I want these objects with a null EditDate property to show up at the top of my list (i.e. closer to the zero index). How can I do this?

Answer 1

List has a sort function that takes a delegate.

Now that I've read the question

TheList.Sort(int delegate(Type a, Type b)
{
    if (a.memeber == null) return -1;
    if (b.memeber == null) return 1;
    if (a.memeber<b.memeber) return -1;
    if (b.memeber<a.memeber) return 1;
    return 0;
}

Answer 2

You can create a somewhat more complex lambda, such as:

things.Sort((t1, t2) => 
{
 if (t1 == null)
 {
   return (t2 == null) ? 0 : -1;
 }
 if (t2 == null)
 {
   return 1;
 }
 return t1.EditDate.CompareTo(t2.EditDate);
});

EndDate cannot be null as it is a value type. However, if you had a specific value of EndDate that you consider as null, you could modify the above code to take that into account in a similar fashion to how I've coped with t1 or t2 being null.

Answer 3

Have Thing implement IComparable such that Things with null editDates have a higher order than those with an edit date. Then modify the sort expression to sort on the objects and not the edit dates.

Answer 4

The answer from ffpf should work.

But if you have some time read through this great post about sorting with nulls from the BCL team

http://blogs.msdn.com/bclteam/archive/2008/10/06/the-compare-contract-kim-hamilton.aspx