Generic Collections as parameters and return types in C#

Generic collections provide a convenient way to pass or retrieve data from methods. However, since .NET provides different types of collection, we should know which one to use for any given situation.


It is possible and sometimes recommended to use collections interfaces instead of the concrete implementation. This can provide more flexibility in coding.


IEnumerable is the base generic interface in .NET collections. All other collections have implemented this one.

It has only one method: GetEnumerator()

This allows the collection to be enumerated.

ICollection extends IEnumerable with methods for manipulating data, such as Add(T), Remove(T), Clear() and properties like Count.

IList and IDictionary are both implementing ICollection and extending it with their own properties and methods.

.NET Collections

IList exposes methods such as IndexOf(T) or RemoveAt(int) to access and manipulate data at specific locations (index) in list. IDictionary<TKey, TValue> also provides methods to alter data using existing “Key”. If the “Key” does not exist the compiler will throw an error. Therefore, the recommended method in such cases is TryGetValue(TKey, TValue).

Using collection interface as Parameter type

A method can define the accepting parameter type to be of interface type based on what it really needs to do with the collection.

The method defines it’s parameter type as IEnumerable when it only requires enumerating over that collection without altering it.

public void Add(IEnumerable collection)
    // enumerate using foreach

The caller can then pass any type of collection to this method since all collections implement an interface which in turn implements IEnumerable (In other words, every collection Is-A ienumerable).

var countries = new List


Using collection interface as Return type

A method can return a generic collection interface:

public ICollection List(Expression<Func<T, bool>> predicate)
    // return collection of T

The calling code can then cast that returned value to any class that implements that interface.

var countries = List(x => x == "Oceania").ToList();
var isAsutraliaInOceania = countries.IndexOf("Australia") > -1;

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