How Can I Avoid the 'Possible Multiple Enumeration of IEnumerable' Warning in C#?

Dealing with the Perils of Multiple Enumeration of IEnumerable

When working with IEnumerable collections, developers often encounter the warning "Possible multiple enumeration of IEnumerable," which arises when the collection is iterated multiple times. This issue can be problematic, especially when working with certain types of collections, such as those representing database queries or other expensive operations.

Consider the following code snippet:

public List<object> Foo(IEnumerable<object> objects)
{
    if (objects == null || !objects.Any())
        throw new ArgumentException();

    var firstObject = objects.First();
    var list = DoSomeThing(firstObject);        
    var secondList = DoSomeThingElse(objects);
    list.AddRange(secondList);

    return list;
}</object></object>

One way to avoid the multiple enumeration warning is to modify the method parameter to accept a List

Another option is to convert the IEnumerable to a List at the beginning of the method:

public List<object> Foo(IEnumerable<object> objects)
{
    var objectList = objects.ToList();
    // ...
}</object></object>

While this technique is effective in preventing the warning, it can feel awkward and unnecessarily verbose.

The key to understanding this issue lies in the semantics conveyed by the method signature. Passing an IEnumerable parameter implies that the caller anticipates the method to enumerate the collection only once. By accepting an IList or ICollection, you explicitly communicate your expectation of the collection to callers, reducing the risk of misunderstandings and costly mistakes.

In situations where using an IEnumerable is essential, it is advisable to perform the ToList() conversion at the beginning of the method. This ensures that subsequent iterations do not encounter the multiple enumeration warning.

It is unfortunate that .NET does not provide an interface that encapsulates the functionality of IEnumerable, Count, and an indexer without the addition or removal methods. Such an interface would greatly simplify this problem and provide a cleaner and more concise solution.

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