How to Use LINQ Select in C#: Syntax and Examples

Use LINQ Select in C# to transform each element in a collection into a new form by applying a function. It projects each item from the source sequence into a new sequence of results using a lambda expression inside Select.

Syntax

The Select method takes a lambda expression that defines how to transform each element in the source collection. It returns a new collection with the transformed elements.

• source.Select(item => transformation): Applies the transformation to each item.
• The lambda expression defines the output for each element.
• The result is an IEnumerable<U> of the transformed type.

csharp

var result = source.Select(item => item.Transformation());

Example

This example shows how to use Select to convert a list of numbers into their squares.

csharp

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

class Program
{
    static void Main()
    {
        List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };
        var squares = numbers.Select(x => x * x);

        foreach (var square in squares)
        {
            Console.WriteLine(square);
        }
    }
}

Output

1
4
9
16
25

Common Pitfalls

Common mistakes when using Select include:

• Forgetting that Select does not modify the original collection but returns a new one.
• Not enumerating the result, so no output is produced.
• Using Select when you want to filter items; use Where instead.

csharp

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

class Program
{
    static void Main()
    {
        List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };

        // Wrong: expecting original list to change
        numbers.Select(x => x * x);
        foreach (var num in numbers)
        {
            Console.WriteLine(num); // Prints original numbers, not squares
        }

        // Right: assign the result
        var squares = numbers.Select(x => x * x);
        foreach (var square in squares)
        {
            Console.WriteLine(square); // Prints squares
        }
    }
}

Output

1
2
3
4
5
1
4
9
16
25

Quick Reference

• Select: Projects each element into a new form.
• Input: IEnumerable<T> source collection.
• Output: IEnumerable<U> transformed collection.
• Use: source.Select(x => transformation).
• Remember: Does not change original collection.

Key Takeaways

LINQ Select transforms each element in a collection into a new form using a lambda expression.

Select returns a new collection and does not modify the original source.

Always assign or enumerate the result of Select to see the transformed data.

Use Select for projection, not filtering; use Where to filter elements.

The output type can differ from the input type depending on the transformation.