Why does NET foreach loop throw NullRefException when collection is null

Question

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

User · Answer

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);
});

User · Answer

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

User · Answer

Because behind the scenes the foreach acquires an enumerator  equivalent to this   using  IEnumerator lt int gt  enumerator   returnArray getEnumerator          while  enumerator MoveNext              int i   enumerator Current             do some more stuff

User · Answer

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 lt T gt    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 lt T gt  AsNotNull lt T gt  this IEnumerable lt T gt  original         return original    Enumerable Empty lt T gt         You can then just call   foreach  int i in returnArray AsNotNull             do some more stuff

User · Answer

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 lt int gt          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    gt                   do some more stuff

User · Answer

SPListItem item  DataRow dr   datatable NewRow     dr  ID       Object Equals item  ID    null     item  ID   ToString     string Empty

User · Answer

Just write an extension method to help you out   public static class Extensions      public static void ForEachWithNull lt T gt  this IEnumerable lt T gt  source  Action lt T gt  action             if source    null                   return                 foreach var item in source                   action item

User · 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 lt int gt          System Console WriteLine string Format   0    i

User · Answer

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

User · Answer

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

User · Answer

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    gt                          do something with obj

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

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