Extension FirstOrDefault IEnumerable IList int Array Lambda Modulo null Odd, Even Predicate

`LastOrDefault`, Last

LastOrDefault is a helpful C# method—it finds the last element in a collection that matches a condition. In many cases Last() does the same thing.

C# method info

With LastOrDefault() we get either the last matching element or the default value. Last meanwhile will throw an exception if no element exists.

`LastOrDefault`

This program shows the LastOrDefault method. The result is 3 for the first array. For the second array, which is empty, the result is 0 because the default int value is 0.

And The third call yields the value 3 because 3 is the final odd value in the source array.

Info The argument to the third LastOrDefault call is a predicate delegate instance. It is specified as a lambda expression.

Tip The LastOrDefault method will never throw an exception on a non-null collection reference. It will return the default value.

using System;
using System.Linq;

// Last or default.
int[] array1 = { 1, 2, 3 };
Console.WriteLine(array1.LastOrDefault());

// Last when there are no elements.
int[] array2 = { };
Console.WriteLine(array2.LastOrDefault());

// Last odd number.
Console.WriteLine(array1.LastOrDefault(element => element % 2 != 0));3
0
3

Last

This program calls the Last extension method on an array. Last() is found in System.Linq and can be used on arrays, Lists, or anything that implements IEnumerable.

Detail We demonstrate the Last extension method with a Predicate represented as a lambda expression.

Note The lambda specifies that only odd numbers are accepted. The modulo expression must not return 0.

using System;
using System.Linq;

int[] values = { 1, 2, 3, 4, 5, 6 };
int last = values.Last();
int lastOdd = values.Last(element => element % 2 != 0);

Console.WriteLine(last);
Console.WriteLine(lastOdd);6
5

Internals

How does the Last extension work? It determines if the collection type implements IList. The IList interface is implemented by arrays, Lists, and others.

And Suppose Last() is called on a IList. In this case, it uses the last index and returns the last element.

But For other IEnumerable instances, the Last implementation loops through the whole collection.

Summary

Last and LastOrDefault access elements. Last has optimizations for Lists and arrays, but is also effective for other collections. LastOrDefault, unlike Last, throws no exceptions.