How Does List<T>.Contains() Find Matching Items?

c# .net list collections contains
leora picture leora · Feb 13, 2012 · Viewed 12.2k times · Source

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

Yuck picture Yuck · Feb 13, 2012

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.

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