The differences between IEnumerable and List in C#

Let’s explore the differences between IEnumerable and List in C#:

1. IEnumerable:

   • Behavior Description: IEnumerable describes behavior rather than providing a concrete implementation.
   • Deferred Execution: When you use IEnumerable, you allow the compiler to defer work until later. The actual execution occurs only when you enumerate the collection (e.g., using a foreach loop or LINQ methods).
   • Optimization Opportunity: By specifying behavior only, you give LINQ a chance to optimize the program during enumeration.
   • Example:

     public IEnumerable<Animal> AllSpotted()
     {
         return from a in Zoo.Animals where a.Coat.HasSpots select a;
     }
     

     Here, the query is defined, but execution is deferred until you actually enumerate the result.

2. List:

   • Implementation: List is an implementation of the IEnumerable behavior.
   • Immediate Execution: When you use ToList(), you force the compiler to materialize the results immediately (i.e., create a list with all the elements).
   • Adding and Removing Items: With a List<T>, you can add and remove items dynamically.
   • Example:

     List<Animal> sel = (from animal in Animals
                         join race in Species on animal.SpeciesKey equals race.SpeciesKey
                         select animal).Distinct().ToList();
     

     Here, sel is a list containing the distinct animals.

3. Summary:

   • Use IEnumerable when you want to describe behavior and allow deferred execution.
   • Use List when you need a concrete collection with immediate execution and the ability to modify its contents.
   • Any modifications will change underlying collection if we are using IEnumerable as it keep reference of parent collection underlying collection.