How to change what default(T) returns in C#?

Question

I would like to change how default(T) behaves for certain classes. So instead of returning null for my reference types I would like to return a null object.

Kind of like

kids.Clear();
var kid = kids.Where(k => k.Age < 10).SingleOrDefault(); 

if (kid is NullKid)
{
  Console.Out.WriteLine("Jippeie");
}

Anyone know if this is at all possible?

sloth · Answer 1 · 2012-10-09T06:53:00.157

13

Anyone know if this is at all possible?

It is simply not possible at all.

But maybe you want to use DefaultIfEmpty instead:

kids.Clear(); 
var kid = kids.Where(k => k.Age < 10).DefaultIfEmpty(NullKid).Single(); 

if (kid == NullKid)
{  
    Console.Out.WriteLine("Jippeie");
}

edited Oct 09 '12 at 06:53

answered Feb 23 '11 at 09:02

sloth

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score 10 · Answer 2 · answered Feb 23 '11 at 08:55

10

You can't change default(T) - it's always null for reference types and zero for value types.

answered Feb 23 '11 at 08:55

Will Dean

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1

This answer is not altogether correct in its particulars, since the default value of a bool is not zero but false (to name just one value type whose default value is not zero; see [this article for a list](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/default-values). The canonical reference is [Default value expressions in the C# language specification](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/expressions#default-value-expressions). – Heretic Monkey Nov 11 '20 at 15:11

Oliver · Answer 3 · 2012-07-04T07:25:38.330

4

How about this:

var kid = kids.Where(k => k.Age < 10).SingleOrDefault() ?? new Kid();

edited Jul 04 '12 at 07:25

answered Feb 23 '11 at 08:59

Oliver

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Shorter equivalent: `var kid = kids.SingleOrDefault(k => k.Age < 10) ?? new Kid();` – smdrager Sep 21 '15 at 20:57

score 3 · Answer 4 · answered Feb 23 '11 at 08:55

3

I don't think, it is possible. What you could do however, is to create your own extension method SingleOrCustomDefault or something like that.

answered Feb 23 '11 at 08:55

Daniel Hilgarth

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That's definitely the cleanest approach. – Jon Skeet Feb 23 '11 at 09:02

Cheng Chen · Answer 5 · 2011-02-23T09:03:28.237

I think you've already got the answer in your question: if/switch statement. Something like this:

if (T is Dog) return new Dog(); 
   //instead of return default(T) which is null when Dog is a class

You can make your own extension method like this:

public static T SingleOrSpecified<T>(this IEnumerable<T> source, Func<T,bool> predicate, T specified)
{
    //check parameters
    var result = source.Where(predicate);
    if (result.Count() == 0) return specified;
    return result.Single();   //will throw exception if more than 1 item
}

Usage:

var p = allProducts.SingleOrSpeficied(p => p.Name = "foo", new Product());

I think, the question remains: how can someone do this from inside of ...SingleOrDefault() !? — ralf.w., Feb 23 '11 at 08:56

How to change what default(T) returns in C#?

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