Is there a performance impact when calling ToList()?

by

IEnumerable<T>.ToList()

Yes, IEnumerable<T>.ToList() does have a performance impact, it is an O(n) operation though it will likely only require attention in performance critical operations.

The ToList() operation will use the List(IEnumerable<T> collection) constructor. This constructor must make a copy of the array (more generally IEnumerable<T>), otherwise future modifications of the original array will change on the source T[] also which wouldn’t be desirable generally.

I would like to reiterate this will only make a difference with a huge list, copying chunks of memory is quite a fast operation to perform.

Handy tip, As vs To

You’ll notice in LINQ there are several methods that start with As (such as AsEnumerable()) and To (such as ToList()). The methods that start with To require a conversion like above (ie. may impact performance), and the methods that start with As do not and will just require some cast or simple operation.

Additional details on List<T>

Here is a little more detail on how List<T> works in case you’re interested 🙂

A List<T> also uses a construct called a dynamic array which needs to be resized on demand, this resize event copies the contents of an old array to the new array. So it starts off small and increases in size if required.

This is the difference between the Capacity and Count properties on List<T>. Capacity refers to the size of the array behind the scenes, Count is the number of items in the List<T> which is always <= Capacity. So when an item is added to the list, increasing it past Capacity, the size of the List<T> is doubled and the array is copied.

More Related Contents:

  1. Difference between List and array when initiating [closed]
  2. Comparing two byte arrays in .NET
  3. How do arrays in C# partially implement IList?
  4. Check if a string contains an element from a list (of strings)
  5. Fast way to convert a two dimensional array to a List ( one dimensional )
  6. Conversion from List to array T[]
  7. C# quickest way to shift array
  8. Easiest way to Rotate a List in c#
  9. How to sort a list / array of string version number?
  10. Move/Shift Objects in array up then move the first element to the last index [closed]
  11. Regarding time out in code
  12. Convert List to List
  13. Using LINQ to remove elements from a List
  14. Performance differences between debug and release builds
  15. Shorter syntax for casting from a List to a List?
  16. LINQ Ring: Any() vs Contains() for Huge Collections
  17. Why does “int[] is uint[] == true” in C#
  18. How to easily initialize a list of Tuples?
  19. Is string in array?
  20. How to get little endian data from big endian in c# using bitConverter.ToInt32 method?
  21. c# Array.FindAllIndexOf which FindAll IndexOf
  22. C#: How to convert a list of objects to a list of a single property of that object?
  23. C# list.Orderby descending
  24. ArgumentOutOfRangeException on initialized List
  25. Ternary operator is twice as slow as an if-else block?
  26. Convert from Oracle’s RAW(16) to .NET’s GUID
  27. How to delete an array in c#?
  28. How to sort list of Ip Addresses using c#
  29. Looking at each combination in jagged array
  30. Passing array to a SQL Server Stored Procedure

Leave a Comment