When it comes to iterating over collections in C#, the performance difference between foreach and for loops primarily depends on the collection type being traversed.
For arrays and Lists, a traditional for loop with indexing can be marginally faster because it avoids the overhead of creating an enumerator object, especially in performance-critical scenarios.
The foreach loop internally creates an IEnumerator, which adds a small memory allocation and method call overhead.
However, for most modern applications, this performance difference is negligible and often optimized away by the JIT compiler.
The readability benefits of foreach typically outweigh the minor performance gains of for loops in non-critical code paths.
Collections like LinkedList or those implementing only IEnumerable actually perform better with foreach since they don't support efficient random access.
The rule of thumb: use foreach for readability in most cases, and only switch to for loops when benchmarking shows a meaningful performance improvement in your specific high-performance scenarios.
Example
// Collection to iterate List<int> numbers = Enumerable.Range(1, 10000).ToList(); // Using for loop public void ForLoopExample(List<int> items) { int sum = 0; for (int i = 0; i < items.Count; i++) { sum += items[i]; } // For loop can be slightly faster for List<T> and arrays // because it avoids creating an enumerator } // Using foreach loop public void ForEachLoopExample(List<int> items) { int sum = 0; foreach (int item in items) { sum += item; } // More readable and works well for any collection type // Preferred for most scenarios where performance isn't critical } // For a LinkedList, foreach is typically faster public void LinkedListExample(LinkedList<int> linkedItems) { int sum = 0; // This would be inefficient with a for loop since LinkedList // doesn't support efficient indexing foreach (int item in linkedItems) { sum += item; } }
String interpolation, introduced in C# 6.0, provides a more readable and concise way to format strings compared to traditional concatenation (+) or string.Format(). Instead of manually inserting variables or placeholders, you can use the $ symbol before a string to directly embed expressions inside brackets.
string name = "Walt"; string job = 'Software Engineer'; string message = $"Hello, my name is {name} and I am a {job}"; Console.WriteLine(message);
This would produce the final output of:
Hello, my name is Walt and I am a Software Engineer
String interpolation can also be chained together into a multiline string (@) for even cleaner more concise results:
string name = "Walt"; string html = $@" <div> <h1>Welcome, {name}!</h1> </div>";
Raw string literals in C# provide a flexible way to work with multiline strings, with some interesting rules around how quotes work.
The key insight is that you can use any number of double quotes (three or more) to delimit your string, as long as the opening and closing sequences have the same number of quotes.
"""
// Three quotes - most common usage string basic = """ This is a basic multiline string """; // Four quotes - when your content has three quotes string withThreeQuotes = """" Here's some text with """quoted""" content """"; // Five quotes - when your content has four quotes string withFourQuotes = """"" Here's text with """"nested"""" quotes """""; // Six quotes - for even more complex scenarios string withFiveQuotes = """""" Look at these """""nested""""" quotes! """""";
The general rule is that if your string content contains N consecutive double quotes, you need to wrap the entire string with at least N+1 quotes. This ensures the compiler can properly distinguish between your content and the string's delimiters.
// Example demonstrating the N+1 rule string example1 = """ No quotes inside """; // 3 quotes is fine string example2 = """" Contains """three quotes""" """"; // Needs 4 quotes (3+1) string example3 = """"" Has """"four quotes"""" """""; // Needs 5 quotes (4+1)
// Indentation example string properlyIndented = """ { "property": "value", "nested": { "deeper": "content" } } """; // This line's position determines the indentation
This flexibility with quote counts makes raw string literals extremely versatile, especially when dealing with content that itself contains quotes, like JSON, XML, or other structured text formats.
Reading a file line by line is useful when handling large files without loading everything into memory at once.
✅ Best Practice: Use File.ReadLines() which is more memory efficient.
foreach (string line in File.ReadLines("file.txt")) { Console.WriteLine(line); }
Why use ReadLines()?
Reads one line at a time, reducing overall memory usage. Ideal for large files (e.g., logs, CSVs).
Alternative: Use StreamReader (More Control)
For scenarios where you need custom processing while reading the contents of the file:
using (StreamReader reader = new StreamReader("file.txt")) { string? line; while ((line = reader.ReadLine()) != null) { Console.WriteLine(line); } }
Why use StreamReader?
Lets you handle exceptions, encoding, and buffering. Supports custom processing (e.g., search for a keyword while reading).
When to Use ReadAllLines()? If you need all lines at once, use:
string[] lines = File.ReadAllLines("file.txt");
Caution: Loads the entire file into memory—avoid for large files!
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