C# LINQ Index — Complete Guide

In this tutorial, you'll learn about C# LINQ Index. We cover key concepts, practical examples, and best practices.

You need the index of each element while iterating — for numbering, logging position, or matching with another sequence. You declare an int i = 0 outside the loop and increment it manually. The LINQ Index method (C# 9+) pairs each element with its index.

Wrong

int i = 0;
foreach (var item in items)
{
    Console.WriteLine($"{i}: {item}");
    i++;
}

Output: Works. But the mutable counter is prone to errors (forgetting to increment, wrong initial value).

Right

foreach (var (index, item) in items.Index())
{
    Console.WriteLine($"{index}: {item}");
}

Output: Same result. No manual counter. Index() returns IEnumerable<(int Index, T Item)>.

Index works with any LINQ method:

var numbered = items
    .Where(x => x > 0)
    .Index()
    .Select(t => $"{t.Index}: {t.Item}")
    .ToList();

Prevention

• Use Index() to avoid manual counter variables in loops.
• Use deconstruction var (index, item) to access the tuple.
• Use .Index() before projection: .Index().Select(t => ...).
• Use .Select((item, i) => ...) instead of Index when you prefer the lambda approach.
• Use Index with deferred execution — the index is computed during iteration, not stored.
• Use Zip when you need to match indices across two sequences.

Common Mistakes with linq index

1. Overlapping type class instances that cause GHC to reject the program with ambiguous dispatch errors
2. Non-exhaustive pattern matches that compile with warnings then crash at runtime
3. Misunderstanding that String is [Char] with poor performance for large text operations

These mistakes appear frequently in real-world CSHARP code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

Index is used in Doda Browser to number search results and log entries chronologically. For more LINQ, visit DodaTech.