The foreach Loop Best Practices and Common Pitfalls

The foreach Loop Best Practices and Common Pitfalls


The foreach loop in .NET is a powerful and commonly used construct that simplifies iterating over collections. While its simplicity and readability are undeniable, there are nuances and potential pitfalls that every .NET developer should be aware of. This blog post looks into the correct use of the foreach loop, outlines best practices, and highlights common mistakes to avoid.

Understanding the Foreach Loop

The foreach loop iterates over each element in a collection, such as an array or a list, without the need for manual index management. Its basic syntax is:

foreach (var item in collection) 

    // Process item 

Key Benefits

  1. Simplicity: Automatically handles the iteration process.
  2. Readability: Makes the code more readable and concise.
  3. Safety: Reduces the risk of off-by-one errors and other common mistakes associated with manual iteration.

Best Practices in Using Foreach

Immutable Iteration

The foreach loop is designed for immutable iteration, so you should not modify the collection you’re iterating over. This helps to avoid unexpected behaviour or runtime exceptions.

Use with Enumerable Types

foreach can be used with any type that implements the IEnumerable or IEnumerable<T> interface, and this makes it incredibly versatile for iterating over most collections.

Efficient for Large Collections

For large collections, foreach can be more efficient than a for loop, especially with non-indexed collections like linked lists, where accessing elements by index is more expensive.

What it doesn’t do

In a foreach loop in .NET, the method specified in the loop declaration is not called on every iteration.

Instead, it’s called once before the loop starts. The foreach loop then iterates over the collection returned by that method.

Common Pitfalls and How to Avoid Them

Modifying the Collection

Modifying the collection while iterating over it with foreach can cause a InvalidOperationException. If you need to modify the collection, consider using a for loop or other strategies.

// Incorrect
foreach (var item in myList)
    myList.Remove(item); // This will throw an exception

// Correct
for (int i = myList.Count - 1; i >= 0; i--)
    if (someCondition)

Understanding Enumeration State in Foreach

The foreach loop maintains an enumeration state internally. It’s important to understand that the state is bound to the collection at the beginning of the iteration. Any changes to the collection do not affect the ongoing iteration.

Closure Traps

When using lambda expressions or anonymous methods inside a loop, be cautious of closures.

A common mistake is to capture the loop variable inadvertently.

List<Action> actions = new List<Action>();
foreach (var item in collection)
    actions.Add(() => Console.WriteLine(item)); // Captures the loop variable

// To avoid this, assign the loop variable to a local variable
foreach (var item in collection)
    var localItem = item;
    actions.Add(() => Console.WriteLine(localItem));

In this example, the lambda function () => Console.WriteLine(item) captures the variable item from the loop. However, it captures the reference to item, not its value at each iteration. As a result, all the functions in the functions list end up referring to the same variable item, which, after the loop completes, has its final value. So, when these functions are invoked, they all return the same value, which is the final value of item after the loop has completed, not the value of item at the time the lambda was created.

Performance Considerations of Foreach

While foreach is efficient in many scenarios, being aware of its performance in critical code sections. This is especially true when iterating over complex or custom collections where the enumerator can introduce overhead.

When processing large collections of items, it is important to ensure that the loop’s content is as efficient as possible; after all, if you iterate over thousands of items, this code section will run each time. I learned long ago that having many log lines in a loop may be where most of the time is spent at runtime.


The foreach loop in .NET is a fundamental tool in a developer’s arsenal, offering a simple and readable way to iterate over collections.

By adhering to best practices and being aware of common pitfalls, you can use foreach effectively and efficiently in your .NET applications.

Understanding the nuances of such fundamental constructs is key to writing efficient and maintainable code in the .NET framework.

Some of my other posts around collections include: –

Understanding the yield Keyword in C# and Enabling Foreach Functionality in C# Classes

The official dotnet documentation can be found here: –


Hi, my name is Stephen Finchett. I have been a software engineer for over 30 years and worked on complex, business critical, multi-user systems for all of my career. For the last 15 years, I have been concentrating on web based solutions using the Microsoft Stack including ASP.Net, C#, TypeScript, SQL Server and running everything at scale within Kubernetes.