How Can I Efficiently Add Items to a List During a C# `foreach` Loop?

Modifying Lists Within foreach Loops in C#: Best Practices

While C# .NET 4.0 and later versions don't throw exceptions when modifying enumerables inside foreach loops, directly adding items within the loop can lead to unpredictable behavior. This article explores efficient and reliable methods for adding items to a list during iteration.

The common approach of using a temporary IList as a buffer works, but isn't always the most efficient or elegant solution. The core issue stems from the immutable nature of collections accessed via foreach. As MSDN clarifies: foreach is designed for retrieving data; adding or removing elements during iteration is discouraged due to potential inconsistencies.

Why Avoid Direct Modification in foreach?

The foreach loop iterates over a snapshot of the collection's state at the loop's start. Adding items directly modifies the underlying collection, potentially causing the iterator to skip elements or encounter unexpected exceptions.

Recommended Approach: Using a for Loop

For adding or removing items while iterating, a for loop provides the most predictable and efficient solution. It offers direct control over the index and allows for safe modifications.

Concurrent Collections: A Special Case

The use of concurrent collections (like ConcurrentBag or ConcurrentQueue) is an exception. These collections are designed for thread-safe modifications, and adding items during iteration might be permissible, but this should be carefully considered and only used when thread safety is paramount.

In summary, while technically possible to add items during a foreach loop in newer C# versions, it's generally best practice to use a for loop for this purpose to maintain code clarity, efficiency, and avoid unexpected results. Concurrent collections provide an alternative for specific multi-threaded scenarios.

The above is the detailed content of How Can I Efficiently Add Items to a List During a C# `foreach` Loop?. For more information, please follow other related articles on the PHP Chinese website!