Select()

• Purpose: Projects each element in a sequence into a new form.
• Returns: A Sequence (IEnumerable) where each element is transformed.
• Use case: Modify each element in a collection.

Example:

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 };
       // Square each number
       var squaredNumbers = numbers.Select(n => n * n).ToList();
       Console.WriteLine("Squared Numbers: " + string.Join(", ", squaredNumbers));
       // Output: Squared Numbers: 1, 4, 9, 16, 25
   }
}

SelectMany()

• Purpose: Flattens nested collections into a single sequence.
• Returns: A single IEnumerable<T> instead of nested lists.
• Use case: Useful when each item in a collection is a single item, and we need a flat structure.

Example:

using System;
using System.Linq;
using System.Collections.Generic;
class Program
{
   static void Main()
   {
       List<List<int>> numberGroups = new List<List<int>>
       {
           new List<int> { 1, 2, 3 },
           new List<int> { 4, 5 },
           new List<int> { 6, 7, 8, 9 }
       };
       // Flatten the lists
       var flattenedNumbers = numberGroups.SelectMany(group => group).ToList();
       Console.WriteLine("Flattened Numbers: " + string.Join(", ", flattenedNumbers));
       // Output: Flattened Numbers: 1, 2, 3, 4, 5, 6, 7, 8, 9
   }
}

Differences:

• Select() maintains the nested structure
• SelectMany() flattens the collection

Zip()

• Purpose: Merges two collections in pairs into a new sequence.
• Returns: A sequence where the elements of both collections are combined.
• Use case: Useful for parallel processing of two lists.

Example: Merging two Lists

using System;
using System.Linq;
using System.Collections.Generic;
class Program
{
   static void Main()
   {
       List<string> names = new List<string> { "Alice", "Bob", "Charlie" };
       List<int> scores = new List<int> { 85, 92, 78 };
       // Merge names and scores
       var studentScores = names.Zip(scores, (name, score) => $"{name}: {score}").ToList();
       Console.WriteLine("Student Scores: " + string.Join(", ", studentScores));
       // Output: Student Scores: Alice: 85, Bob: 92, Charlie: 78
   }
}

Key Points about Zip()

• This works only if both lists have the same length.
• If one list is shorter, the extra elements of the longer list are ignored.

Also, Read: LINQ where() and OfType() using C#.