[c#] Why does .NET foreach loop throw NullRefException when collection is null?

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.

Just write an extension method to help you out:

public static class Extensions
{
   public static void ForEachWithNull<T>(this IEnumerable<T> source, Action<T> action)
   {
      if(source == null)
      {
         return;
      }

      foreach(var item in source)
      {
         action(item);
      }
   }
}

A foreach loop calls the GetEnumerator method.
If the collection is null, this method call results in a NullReferenceException.

It is bad practice to return a null collection; your methods should return an empty collection instead.

I think the explanation of why exception is thrown is very clear with the answers provided here. I just wish to complement with the way I usually work with these collections. Because, some times, I use the collection more then once and have to test if null every time. To avoid that, I do the following:

    var returnArray = DoSomething() ?? Enumerable.Empty<int>();

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

This way we can use the collection as much as we want without fear the exception and we don't polute the code with excessive conditional statements.

Using the null check operator ?. is also a great approach. But, in case of arrays (like the example in the question), it should be transformed into List before:

    int[] returnArray = DoSomething();

    returnArray?.ToList().ForEach((i) =>
    {
        // do some more stuff
    });

SPListItem item;
DataRow dr = datatable.NewRow();

dr["ID"] = (!Object.Equals(item["ID"], null)) ? item["ID"].ToString() : string.Empty;

Because behind the scenes the foreach acquires an enumerator, equivalent to this:

using (IEnumerator<int> enumerator = returnArray.getEnumerator()) {
    while (enumerator.MoveNext()) {
        int i = enumerator.Current;
        // do some more stuff
    }
}

Because a null collection is not the same thing as an empty collection. An empty collection is a collection object with no elements; a null collection is a nonexistent object.

Here's something to try: Declare two collections of any sort. Initialize one normally so that it's empty, and assign the other the value null. Then try adding an object to both collections and see what happens.

It is being answer long back but i have tried to do this in the following way to just avoid null pointer exception and may be useful for someone using C# null check operator ?.

     //fragments is a list which can be null
     fragments?.ForEach((obj) =>
        {
            //do something with obj
        });

It is the fault of Do.Something(). The best practice here would be to return an array of size 0 (that is possible) instead of a null.

Another extension method to work around this:

public static void ForEach<T>(this IEnumerable<T> items, Action<T> action)
{
    if(items == null) return;
    foreach (var item in items) action(item);
}

Consume in several ways:

(1) with a method that accepts T:

returnArray.ForEach(Console.WriteLine);

(2) with an expression:

returnArray.ForEach(i => UpdateStatus(string.Format("{0}% complete", i)));

(3) with a multiline anonymous method

int toCompare = 10;
returnArray.ForEach(i =>
{
    var thisInt = i;
    var next = i++;
    if(next > 10) Console.WriteLine("Match: {0}", i);
});

There is a big difference between an empty collection and a null reference to a collection.

When you use foreach, internally, this is calling the IEnumerable's GetEnumerator() method. When the reference is null, this will raise this exception.

However, it is perfectly valid to have an empty IEnumerable or IEnumerable<T>. In this case, foreach will not "iterate" over anything (since the collection is empty), but it will also not throw, since this is a perfectly valid scenario.

Edit:

Personally, if you need to work around this, I'd recommend an extension method:

public static IEnumerable<T> AsNotNull<T>(this IEnumerable<T> original)
{
     return original ?? Enumerable.Empty<T>();
}

You can then just call:

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