C# – Why does .NET foreach loop throw NullRefException when collection is null

cnet

So I frequently run into this situation… where Do.Something(...) returns a null collection, like so:

int[] returnArray = Do.Something(...);

Then, I try to use this collection like so:

foreach (int i in returnArray)
{
    // do some more stuff
}

I'm just curious, why can't a foreach loop operate on a null collection? It seems logical to me that 0 iterations would get executed with a null collection… instead it throws a NullReferenceException. Anyone know why this could be?

This is annoying as I'm working with APIs that aren't clear on exactly what they return, so I end up with if (someCollection != null) everywhere…

Edit: Thank you all for explaining that foreach uses GetEnumerator and if there is no enumerator to get, the foreach would fail. I guess I'm asking why the language/runtime can't or won't do a null check before grabbing the enumerator. It seems to me that the behavior would still be well defined.

Best Answer

Well, the short answer is "because that's the way the compiler designers designed it." Realistically, though, your collection object is null, so there's no way for the compiler to get the enumerator to loop through the collection.

If you really need to do something like this, try the null coalescing operator:

int[] array = null;

foreach (int i in array ?? Enumerable.Empty<int>())
{
   System.Console.WriteLine(string.Format("{0}", i));
}

Related Solutions

C# – How to parse a string into a nullable int

int.TryParse is probably a tad easier:

public static int? ToNullableInt(this string s)
{
    int i;
    if (int.TryParse(s, out i)) return i;
    return null;
}

Edit @Glenn int.TryParse is "built into the framework". It and int.Parse are the way to parse strings to ints.

C# – Why is there no ForEach extension method on IEnumerable

There is already a foreach statement included in the language that does the job most of the time.

I'd hate to see the following:

list.ForEach( item =>
{
    item.DoSomething();
} );

Instead of:

foreach(Item item in list)
{
     item.DoSomething();
}

The latter is clearer and easier to read in most situations, although maybe a bit longer to type.

However, I must admit I changed my stance on that issue; a ForEach() extension method would indeed be useful in some situations.

Here are the major differences between the statement and the method:

Type checking: foreach is done at runtime, ForEach() is at compile time (Big Plus!)
The syntax to call a delegate is indeed much simpler: objects.ForEach(DoSomething);
ForEach() could be chained: although evilness/usefulness of such a feature is open to discussion.

Those are all great points made by many people here and I can see why people are missing the function. I wouldn't mind Microsoft adding a standard ForEach method in the next framework iteration.

Best Answer

Related Solutions

C# – How to parse a string into a nullable int

C# – Why is there no ForEach extension method on IEnumerable

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