How can we update the values of a collection using LINQ in C#?

If the collection is a List, then we can make use of ForEach extension method which is available as part of LINQ.

Example

 Live Demo

using System;
using System.Collections.Generic;
namespace DemoApplication {
   class Program {
      static void Main(string[] args) {
         List<Fruit> fruits = new List<Fruit> {
            new Fruit {
               Name = "Apple",
               Size = "Small"
            },
            new Fruit {
               Name = "Orange",
               Size = "Small"
            }
         };
         foreach(var fruit in fruits) {
            Console.WriteLine($"Fruit Details Before Update. {fruit.Name}, {fruit.Size}");
         }
         fruits.ForEach(fruit => { fruit.Size = "Large"; });
         foreach (var fruit in fruits) {
            Console.WriteLine($"Fruit Details After Update. {fruit.Name}, {fruit.Size}");
         }
         Console.ReadLine();
      }
   }
   public class Fruit {
      public string Name { get; set; }
      public string Size { get; set; }
   }
}

Output

The output of the above code is

Fruit Details Before Update. Apple, Small
Fruit Details Before Update. Orange, Small
Fruit Details After Update. Apple, Large
Fruit Details After Update. Orange, Large

If we want to update the list items based on a condition, we can make use of Where() clause.

Example

 Live Demo

using System;
using System.Collections.Generic;
using System.Linq;
namespace DemoApplication {
   class Program {
      static void Main(string[] args) {
         IEnumerable<Fruit> fruits = new List<Fruit> {
            new Fruit {
               Name = "Apple",
               Size = "Small"
            },
            new Fruit {
               Name = "Orange",
               Size = "Small"
            },
            new Fruit {
               Name = "Mango",
               Size = "Medium"
            }
         };
         foreach(var fruit in fruits) {
            Console.WriteLine($"Fruit Details Before Update. {fruit.Name}, {fruit.Size}");
         }
         foreach (var fruit in fruits.Where(w => w.Size == "Small")) {
            fruit.Size = "Large";
         }
         foreach (var fruit in fruits) {
            Console.WriteLine($"Fruit Details After Update. {fruit.Name}, {fruit.Size}");
         }
         Console.ReadLine();
      }
   }
   public class Fruit {
      public string Name { get; set; }
      public string Size { get; set; }
   }
}

Output

The output of the above code is

Fruit Details Before Update. Apple, Small
Fruit Details Before Update. Orange, Small
Fruit Details Before Update. Mango, Medium
Fruit Details After Update. Apple, Large
Fruit Details After Update. Orange, Large
Fruit Details After Update. Mango, Medium

In the above we are filtering only the fruits having small size and updating the values. So, the where clause filters the record based on a condition.