Is if(items != null) superfluous before foreach(T item in items)?

Question

You still need to check if (items != null) otherwise you will get NullReferenceException. However you can do something like this:

List<string> items = null;  
foreach (var item in items ?? new List<string>())
{
    item.Dump();
}

but you might check performance of it. So I still prefer having if (items != null) first.

Based on Eric's Lippert suggestion I changed code to:

List<string> items = null;  
foreach (var item in items ?? Enumerable.Empty<string>())
{
    item.Dump();
}

Using C# 6 you could use the new null conditional operator together with List<T>.ForEach(Action<T>) (or your own IEnumerable<T>.ForEach extension method).

List<string> items = null;
items?.ForEach(item =>
{
    // ...
});

The real takeaway here should be a sequence should almost never be null in the first place. Simply make it an invariant in all of your programs that if you have a sequence, it is never null. It is always initialized to be the empty sequence or some other genuine sequence.

If a sequence is never null then obviously you don't need to check it.

Actually there is a feature request on that @Connect: http://connect.microsoft.com/VisualStudio/feedback/details/93497/foreach-should-check-for-null

And the response is quite logical:

I think that most foreach loops are written with the intent of iterating a non-null collection. If you try iterating through null you should get your exception, so that you can fix your code.

You could always test it out with a null list... but this is what I found on the msdn website

foreach-statement:
    foreach   (   type   identifier   in   expression   )   embedded-statement 

If expression has the value null, a System.NullReferenceException is thrown.

It is not superflous. At runtime items will be casted to an IEnumerable and its GetEnumerator method will be called. That will cause a dereferencing of items that will fail

You can encapsulate the null check in an extension method and use a lambda:

public static class EnumerableExtensions {
  public static void ForEach<T>(this IEnumerable<T> self, Action<T> action) {
    if (self != null) {
      foreach (var element in self) {
        action(element);
      }
    }
  }
}

The code becomes:

items.ForEach(item => { 
  ...
});

If can be even more concise if you want to just call a method that takes an item and returns void:

items.ForEach(MethodThatTakesAnItem);