Lists Count vs Count

Question

Given a list  which method is preferred to determine the number of elements inside   var myList   new List lt string gt      myList Count myList Count

User · Answer

Always prefer Count and Length properties on a type over the extension method Count     The former is an O 1  for every type which contains them   The Count   extension method has some type check optimizations that can cause it to run also in O 1  time but will degrade to O N  if the underlying collection is not one of the few types it knows about

User · Answer

myList Count is a method on the list object  it just returns the value of a field so is very fast    As it is a small method it is very likely to be inlined by the compiler  or runtime   they may then allow other optimization to be done by the compiler  myList Count   is calling an extension method  introduced by LINQ   that loops over all the items in an IEnumerable  so should be a lot slower  However  In the Microsoft implementation  the Count extension method has a    special case    for Lists that allows it to use the list   s Count property  this means the Count   method is only a little slower than the Count property  It is unlikely you will be able to tell the difference in speed in most applications  So if you know you are dealing with a List use the Count property  otherwise if you have a  quot unknown quot  IEnumerabl  use the Count   method and let it optimise for you

User · Answer

If you by any chance wants to change the type of your collection you are better served with the Count   extension  This way you don t have to refactor your code  to use Length for instance

User · Answer

Count   is an extension method introduced by LINQ while the Count property is part of the List itself  derived from ICollection   Internally though  LINQ checks if your IEnumerable implements ICollection and if it does it uses the Count property  So at the end of the day  there s no difference which one you use for a List   To prove my point further  here s the code from Reflector for Enumerable Count    public static int Count lt TSource gt  this IEnumerable lt TSource gt  source        if  source    null                throw Error ArgumentNull  source              ICollection lt TSource gt  is2   source as ICollection lt TSource gt       if  is2    null                return is2 Count            int num   0      using  IEnumerator lt TSource gt  enumerator   source GetEnumerator                  while  enumerator MoveNext                          num                        return num

[c#] Lists: Count vs Count()

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