Check for null in foreach loop

Question

Is there a nicer way of doing the following:
I need a check for null to happen on file.Headers before proceeding with the loop

if (file.Headers != null)
{
  foreach (var h in file.Headers)
  {
   //set lots of properties & some other stuff
  }
}

In short it looks a bit ugly to write the foreach inside the if due to the level of indentation happening in my code.

Is something that would evaluate to

foreach(var h in (file.Headers != null))
{
  //do stuff
}

possible?

Answer 1

Just as a slight cosmetic addition to Rune's suggestion, you could create your own extension method:

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

Then you can write:

foreach (var header in file.Headers.OrEmptyIfNull())
{
}

Change the name according to taste :)

Answer 2

Assuming that the type of elements in file.Headers is T you could do this

foreach(var header in file.Headers ?? Enumerable.Empty<T>()){
  //do stuff
}

this will create an empty enumerable of T if file.Headers is null. If the type of file is a type you own I would, however, consider changing the getter of Headers instead. null is the value of unknown so if possible instead of using null as "I know there are no elements" when null actually(/originally) should be interpreted as "I don't know if there are any elements" use an empty set to show that you know there are no elements in the set. That would also be DRY'er since you won't have to do the null check as often.

EDIT as a follow up on Jons suggestion, you could also create an extension method changing the above code to

foreach(var header in file.Headers.OrEmptyIfNull()){
  //do stuff
}

In the case where you can't change the getter, this would be my own preferred since it expresses the intention more clearly by giving the operation a name (OrEmptyIfNull)

The extension method mentioned above might make certain optimizations impossible for the optimizer to detect. Specifically, those that are related to IList using method overloading this can be eliminated

public static IList<T> OrEmptyIfNull<T>(this IList<T> source)
{
    return source ?? Array.Empty<T>();
}

Answer 3

Frankly, I advise: just suck up the null test. A null test is just a brfalse or brfalse.s; everything else is going to involve much more work (tests, assignments, extra method calls, unnecessary GetEnumerator(), MoveNext(), Dispose() on the iterator, etc).

An if test is simple, obvious, and efficient.

Answer 4

Using Null-conditional Operator and ForEach() which works faster than standard foreach loop.
You have to cast the collection to List though.

   listOfItems?.ForEach(item => // ... );

Answer 5

the "if" before the iteration is fine, few of those "pretty" semantics can make your code less readable.

anyway, if the indentation disturbs your, you can change the if to check:

if(file.Headers == null)  
   return;

and you'll get to the foreach loop only when there is a true value at the headers property.

another option I can think about is using the null-coalescing operator inside your foreach loop and to completely avoid null checking. sample:

List<int> collection = new List<int>();
collection = null;
foreach (var i in collection ?? Enumerable.Empty<int>())
{
    //your code here
}

(replace the collection with your true object/type)

Answer 6

The best answer in the year 2022 should be:

foreach (var h in file.Headers ?? Enumerable.Empty<T>())
{
    //do stuff
}

Replace T with your data type. if file.Headers is an array, use Array.Empty<T>() instead of Enumerable.Empty<T>()

Answer 7

I am using a nice little extension method for these scenarios:

  public static class Extensions
  {
    public static IList<T> EnsureNotNull<T>(this IList<T> list)
    {
      return list ?? new List<T>();
    }
  }

Given that Headers is of type list, you can do following:

foreach(var h in (file.Headers.EnsureNotNull()))
{
  //do stuff
}

Answer 8

For some cases I'd prefer slightly another, generic variant, assuming that, as a rule, default collection constructors return empty instances.

It would be better to name this method NewIfDefault. It can be useful not only for collections, so type constraint IEnumerable<T> is maybe redundant.

public static TCollection EmptyIfDefault<TCollection, T>(this TCollection collection)
        where TCollection: class, IEnumerable<T>, new()
    {
        return collection ?? new TCollection();
    }