How Can I Safely Remove Elements from a List During Iteration in C#?

Efficiently Removing Elements from a C# List During Iteration

Modifying a list while iterating through it using standard methods often leads to InvalidOperationException errors. This article presents clean solutions to safely remove elements from a List<T> based on a condition.

The common problem arises when using List<T>.Remove() or List<T>.RemoveAt() within a foreach loop or a standard for loop. Index shifting causes issues.

Solution 1: Reverse Iteration

Iterating backward using a for loop avoids index issues:

var list = new List<int>(Enumerable.Range(1, 10));
for (int i = list.Count - 1; i >= 0; i--) {
    if (list[i] > 5) {
        list.RemoveAt(i);
    }
}
list.ForEach(i => Console.WriteLine(i));

This method starts from the end, removing elements without affecting subsequent indices.

Solution 2: RemoveAll() Method

A more concise and readable approach is using the RemoveAll() method with a predicate:

var list = new List<int>(Enumerable.Range(1, 10));
Console.WriteLine("Before:");
list.ForEach(i => Console.WriteLine(i));
list.RemoveAll(i => i > 5);
Console.WriteLine("After:");
list.ForEach(i => Console.WriteLine(i));

This elegantly removes all elements satisfying the given condition (i > 5 in this case). It's generally preferred for its clarity and efficiency.

Both methods provide reliable ways to remove elements from a list during iteration, preventing exceptions and ensuring data integrity. The RemoveAll() method is often the preferred choice for its simplicity and readability.

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