How to Use LINQ GroupBy in C# - Simple Guide

Use GroupBy in LINQ to group elements of a collection by a key selector function. It returns groups as IGrouping<TKey, TElement> which you can iterate to access grouped items. This helps organize data by categories or properties easily.

Syntax

The basic syntax of GroupBy in LINQ is:

• source.GroupBy(keySelector) groups elements by a key.
• keySelector is a function that selects the key to group by.
• The result is a collection of IGrouping<TKey, TElement> where each group has a key and elements.

csharp

var grouped = collection.GroupBy(item => item.KeyProperty);

Example

This example groups a list of words by their first letter and prints each group with its items.

csharp

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main()
    {
        List<string> words = new List<string> { "apple", "apricot", "banana", "blueberry", "cherry" };

        var groupedWords = words.GroupBy(word => word[0]);

        foreach (var group in groupedWords)
        {
            Console.WriteLine($"Words that start with '{group.Key}':");
            foreach (var word in group)
            {
                Console.WriteLine($"  {word}");
            }
        }
    }
}

Output

Words that start with 'a':
  apple
  apricot
Words that start with 'b':
  banana
  blueberry
Words that start with 'c':
  cherry

Common Pitfalls

Common mistakes when using GroupBy include:

• Not realizing GroupBy returns groups, not a flat list.
• Forgetting to iterate over groups and their elements separately.
• Using a key selector that returns non-unique or unexpected keys.
• Assuming groups are sorted; GroupBy does not guarantee order.

Always check the key selector logic and iterate groups properly.

csharp

/* Wrong: Trying to treat GroupBy result as a flat list */
var wrong = words.GroupBy(w => w[0]).ToList();
// wrong is a list of groups, not strings

/* Right: Iterate groups and then items */
foreach (var group in words.GroupBy(w => w[0]))
{
    Console.WriteLine(group.Key);
    foreach (var item in group)
    {
        Console.WriteLine(item);
    }
}

Quick Reference

Tips for using GroupBy effectively:

• Use a clear key selector function.
• Remember the result is groups, each with a Key and elements.
• Use ToDictionary if you want a dictionary from groups.
• Combine with OrderBy to sort groups or elements.

Key Takeaways

Use GroupBy with a key selector to group collection elements by a property.

GroupBy returns groups with a Key and a collection of elements, not a flat list.

Always iterate groups and their elements separately to access data.

GroupBy does not sort groups; use OrderBy if sorting is needed.

Check your key selector carefully to avoid unexpected grouping.