How Does List.Contains() Find Matching Items?

Question

I have a list of car objects

 List<Car> cars = GetMyListOfCars();

and i want to see if a car is in the list

if (cars.Contains(myCar))
{
}

what does Contains use to figure out if myCar is in the list. Does it do a "ToString()" on my car object. Does it use the Equals() method, the gethashcode()?

I see i can pass in my own IEqualityComparer to force my own implementation but just wanted to understand what it does by default.

Answer 1

It uses Equals()

This method determines equality by using the default equality comparer, as defined by the object's implementation of the IEquatable(Of T).Equals method for T (the type of values in the list).

http://msdn.microsoft.com/en-us/library/bhkz42b3.aspx

Answer 2

Contains will return true as soon as it can - that is once the first item that fits the criteria is found.

A false will be returned after all items have been iterated over.

In regards to how it does that - it will use reference equality for reference types if you do not override Equals.

Answer 3

Straight from MSDN - List<T>.Contains:

This method determines equality by using the default equality comparer, as defined by the object's implementation of the IEquatable(Of T).Equals method for T (the type of values in the list).

This method performs a linear search; therefore, this method is an O(n) operation, where n is Count.

So in the end it depends on how T implements IEquatable.Equals(). For most objects this is going to be a reference comparison, unless overriden. Same location in memory is the same object.