Should I Return Null or an Empty Collection from My Method?

Null vs. Empty Collections: Best Practices for Method Returns

When designing methods that return collections, the choice between returning null or an empty collection significantly impacts code clarity and robustness. This article advocates for consistently returning empty collections.

Why Empty Collections are Preferred

Returning an empty collection avoids the potential for NullReferenceException errors. Consider this scenario:

if (myInstance.CollectionProperty != null)
{
  foreach (var item in myInstance.CollectionProperty)
    // Process items
}

If CollectionProperty returns null when no results exist, the if check is mandatory, adding complexity. An empty collection eliminates this check, simplifying the code:

foreach (var item in myInstance.CollectionProperty)
  // Process items (no null check needed)

Eliminating Null Checks and Exception Handling

Consistently returning empty collections reduces the need for null checks throughout your codebase. This leads to cleaner, more maintainable code and reduces the risk of overlooking potential NullReferenceException scenarios.

Property Initialization Best Practices

For properties returning collections, initialize them during object creation to prevent null references. This is best achieved using the following pattern:

public List<Foo> Foos { get; private set; } = new List<Foo>();

This ensures the property always holds a valid (potentially empty) collection.

Method Return Values: The Right Approach

Methods returning enumerables should always return an empty enumerable instead of null. This can be elegantly implemented using the null-coalescing operator:

public IEnumerable<Foo> GetMyFoos()
{
  return InnerGetFoos() ?? Enumerable.Empty<Foo>();
}

Performance Considerations

Enumerable.Empty<t>()</t> offers a performance advantage over creating new empty collections each time. It provides a static, cached instance, minimizing object allocation overhead.

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