How to Safely Modify Lists Inside Nested `foreach` Loops in C#?

Safely modify foreachlists inside loops in C#

In C#/.NET 4.0, enumerable objects can be modified during a foreach loop without encountering exceptions. Paul Jackson's blog post discusses this in detail.

However, a common question arises: what is the best way to modify a list inside a foreach loop, especially in a nested foreach scenario?

Traditionally, IList is often used as a cache or buffer to accumulate changes until the outer foreach loop ends. But is there a better alternative?

Answer

It is important to note that the collection iterated over in the foreach loop is immutable by design. As stated in the MSDN documentation, foreach is designed to access data, not modify the source collection to prevent unpredictable side effects.

If you need to add or remove items from the source collection, it is recommended to use a for loop instead. This ensures predictable and controlled modifications to the collection.

The blog post linked by Poko suggests that this limit may not apply to concurrent collections introduced in C#/.NET 4.0.

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