Access Items From A C# List By Index

Introduction

A C# list has an underlying data structure of an array so the elements of the array can be accessed by an indexer. But only if that index exists on the array can the element's data be accessed. This is useful to know as there are times when we only want to access certain elements such as the last or first element so we don't want to loop through the entire list. C# provides two solid ways to access the data by the index. The indexer and the ElementAt function. They work in similar ways but the indexer only works on certain IEnumerable types ElementAt will work on any IEnumerable type which includes a list.

Get Item In List By Indexer

Using the indexer is the go-to method of getting an element's data. It is easy to use and many collection types in C# have an indexer. If you know the location or index address of the element you want to get data then you pass that integer value to the indexer and it will return the data almost instantaneously. See below for several use cases of how we can quickly get the first, last, and value in between with the indexer.

Get Item In List By Indexer Code Example

List<string> masterList = new List<string>() { "first", "second", "third", "fourth", "fifth" };
Console.WriteLine("Starting Value:" + masterList[0]);//Starting value is at index = 0
Console.WriteLine("Last Value:" + masterList[masterList.Count - 1]);//Last Value is at index = 4
Console.WriteLine("Second Value:" + masterList[1]);//Second Value is at index = 1

Code Output

Starting Value: first
Last Value: fifth
Second Value: second

This works well in getting exactly what we want. We don't have to loop through the list to get any individual elements.

Get Item By List Indexer Speed Test

We also want to test the speed of the indexer. In this test, we perform a set indexer operations 100 million times and then do that test 10 times to get the average. Let's examine the code below.

using System.Diagnostics;
int numberOfTests = 10;//Number of tests 
List<double> testSpeedList = new List<double>();
for (int i = 0; i < numberOfTests; i++)
{
    testSpeedList.Add(ListIndexerSpeedTest());
}
Console.WriteLine($"List Indexer Average speed:{Math.Round(testSpeedList.Average())}ms, In {numberOfTests} tests");
double ListIndexerSpeedTest()
{
    int numberOfFunctionCalls = 100000000;//Number of function calls made
    List<string> masterList = new List<string>() { "first", "second", "third", "fourth", "fifth" };//Items in the list
    Stopwatch stopwatch = new Stopwatch();
    stopwatch.Start();//Start the Stopwatch timer
    string alteredText = "";
    for (int i = 0; i < numberOfFunctionCalls; i++)
    {
        alteredText = masterList[0];//Get First item in the list
        alteredText = masterList[1];//Get the second item in the list
        alteredText = masterList[masterList.Count - 1];//Get the last item in the list
    }
    stopwatch.Stop();//Stop the Stopwatch timer
    Console.WriteLine($"alteredText:{alteredText}, Function calls:{numberOfFunctionCalls}, In {stopwatch.Elapsed.Minutes}m {stopwatch.Elapsed.Seconds}s {stopwatch.Elapsed.Milliseconds}ms");
    return stopwatch.Elapsed.TotalMilliseconds;
}

Code Output

alteredText:fifth, Function calls:100000000, In 0m 0s 613ms
alteredText:fifth, Function calls:100000000, In 0m 0s 580ms
alteredText:fifth, Function calls:100000000, In 0m 0s 574ms
alteredText:fifth, Function calls:100000000, In 0m 0s 574ms
alteredText:fifth, Function calls:100000000, In 0m 0s 574ms
alteredText:fifth, Function calls:100000000, In 0m 0s 573ms
alteredText:fifth, Function calls:100000000, In 0m 0s 574ms
alteredText:fifth, Function calls:100000000, In 0m 0s 573ms
alteredText:fifth, Function calls:100000000, In 0m 0s 575ms
alteredText:fifth, Function calls:100000000, In 0m 0s 574ms
List Indexer Average speed:579ms, In 10 tests

Results are in and the average speed to get items from the indexer 100 million times is about half a second. This is pretty fast. This will be the baseline to which we can compare.

Get Item In List By Indexer Readability

Indexer is one of those properties that you just have to know. It's not very readable but it comes up very often and you will pick up on it very quickly. It has been around a long time but more recently other C# properties are similar in structure so it could be confusing to new users.

Get Item In List By ElementAt

ElementAt is very similar to the Indexer for list. ElementAt allows to access an item from the list by an index value. ElementAt is available on any collection type that is an IEnumerable type. ElementAt reads well because it says what it does which is different than the Indexer. So let's look at an example.

List<string> masterList = new List<string>() { "first", "second", "third", "fourth", "fifth" };
Console.WriteLine("Starting Value: " + masterList.ElementAt(0));//Starting value is at index = 0
Console.WriteLine("Last Value: " + masterList.ElementAt(masterList.Count - 1));//Last Value is at index = 4
Console.WriteLine("Second Value: " + masterList.ElementAt(1));//Second Value is at index = 1

Code Output

Starting Value: first
Last Value: fifth
Second Value: second

This code also gives exactly what we want in a straightforward manner. Next, we'll test the speed of the function.

Get Item By List ElementAt Speed Test

We will also perform the same speed test as for Indexer. We would expect ElementAt to be slightly slower since it is a more general implementation across different collection types.

using System.Diagnostics;
int numberOfTests = 10;//Number of tests 
List<double> testSpeedList = new List<double>();
for (int i = 0; i < numberOfTests; i++)
{
    testSpeedList.Add(ElementAtSpeedTest());
}
Console.WriteLine($"ElementAt Average speed:{Math.Round(testSpeedList.Average())}ms, In {numberOfTests} tests");
double ElementAtSpeedTest()
{
    int numberOfFunctionCalls = 100000000;//Number of function calls made
    List<string> masterList = new List<string>() { "first", "second", "third", "fourth", "fifth" };
    Stopwatch stopwatch = new Stopwatch();
    stopwatch.Start();//Start the Stopwatch timer
    string alteredText = "";
    for (int i = 0; i < numberOfFunctionCalls; i++)
    {
        alteredText = masterList.ElementAt(0);
        alteredText = masterList.ElementAt(1);
        alteredText = masterList.ElementAt(masterList.Count - 1);
    }
    stopwatch.Stop();//Stop the Stopwatch timer
    Console.WriteLine($"alteredText:{alteredText}, Function calls:{numberOfFunctionCalls}, In {stopwatch.Elapsed.Minutes}m {stopwatch.Elapsed.Seconds}s {stopwatch.Elapsed.Milliseconds}ms");
    return stopwatch.Elapsed.TotalMilliseconds;
}

Code Output

alteredText:fifth, Function calls:100000000, In 0m 6s 467ms
alteredText:fifth, Function calls:100000000, In 0m 6s 359ms
alteredText:fifth, Function calls:100000000, In 0m 6s 364ms
alteredText:fifth, Function calls:100000000, In 0m 6s 396ms
alteredText:fifth, Function calls:100000000, In 0m 6s 445ms
alteredText:fifth, Function calls:100000000, In 0m 6s 356ms
alteredText:fifth, Function calls:100000000, In 0m 6s 387ms
alteredText:fifth, Function calls:100000000, In 0m 6s 408ms
alteredText:fifth, Function calls:100000000, In 0m 6s 436ms
alteredText:fifth, Function calls:100000000, In 0m 6s 351ms
ElementAt Average speed:6397ms, In 10 tests

This method is extremely slow. It doesn't compare to Indexer.

Get Item In List By ElementAt Readability

This method reads well as we're not wondering what is happening. It read an element at the following index which is good.

Conclusion

The best way to access an item on a list is to use the Indexer property. It is extremely fast and once used to the syntax, it becomes a go-to property to use on collection types. ElementAt although it reads well, is far too slow to recommend use, unless you have no choice.