Use IEnumerable over List for DTO, Models properties with collections

Over the past year or so I have seen developers using List<T> for poco properties, public method signatures and results.

Here I will attempt to explain why it’s generally a better idea to use IEnumerable<T> in most DTO’s, public method result and parameter signatures over List<T>

1. Coding by abstraction rather than concrete implementation

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An IEnumerable<T> represents the interface contract to iterate a series of items for reading, now from there you have ICollection<T> which provides abstractions for modifying a collection, and at a higher level that inherits both interfaces have List<T> which is a concrete implementation to add and modify that collection of items.

It’s generally considered a best practice to code by interface (abstraction) rather than concrete implementation in order to reduce coupling.

Example 1:

Consider the following:

public void ProcessItems(List<int> integers){
    ....  
}

Means that this method will only accept a list of integers that allows sorting, adding, deleting of that collection directly on the list.

vs the decoupled version which provides portability of types.

public void ProcessItems(IEnumerable<int> integers){
    ....  
}

What about IList<T> as thats an interface? …. well yes, though it inherits IEnumerable<T>, ICollection<T> and other interfaces to provide modification of a collection.

Example 2:

public class AModel
{
  public List<string> Names {get; set;}
  public List<string> Items {get; set;}
}

these props are tightly coupled to List<T> (or decendant classes) and the collection itself can be directly modified.

vs the decoupled properties

public class AModel
{
  public IEnumerable<string> Names {get; set;}
  public IEnumerable<string> Items {get; set;}
}

now these props are decoupled from the implementation at a much lower level, and become more portable.

2. Immutable collection of elements

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While the objects within IEnumerable<T> may change, the actual collection of items wont, without changing the entire collection. it reduces errors and promotes writing clean, concise, intent-revealing code.

3. LINQ

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LINQ is an incredibly powerful and flexible expression language within .NET that naturally operates with IEnumerable<T>, take for instance the .Where(), .Select() methods (among a host of others) that accept and return IEnumerable<T>.

These can also be lazily evaluated if you like, for instance a .ToList() would execute the final result.