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.

ToString returns the type's name when not overridden so it wouldn't be a good way to compare items :) — vc 74, Feb 13 '12 at 17:05
By the way, you can't pass your own `IEqualityComparer` into the `List.Contains` method. You *can* pass your own comparer into LINQ's `Contains` extension method, which works quite happily against `List`. — LukeH, Feb 13 '12 at 17:10

score 18 · Accepted Answer · answered Feb 13 '12 at 17:00

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.

One exception to this is if you search for `null`, in which case `Contains` just does an `item == null` test directly rather than using the comparer. — LukeH, Feb 13 '12 at 17:02

score 5 · Answer 2 · answered Feb 13 '12 at 17:02

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

score 1 · Answer 3 · answered Feb 13 '12 at 17:00

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.

How Does List.Contains() Find Matching Items?

3 Answers3

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