C# FirstOrDefault Method

FirstOrDefault is almost the same as First. The difference is how it handles empty collections.

If a collection is empty, it returns the default value for the type. This method simplifies code. In some programs it eliminates exceptions.

First

Example. The System.Linq namespace is required to use FirstOrDefault. The FirstOrDefault method will appear in Visual Studio's IntelliSense feature by typing the period after an identifier that describes a type that implements IEnumerable.

Note: This means it will appear after a List variable, array variable, or query expression.

Var

C# program that uses FirstOrDefault

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main()
    {
	//
	// This List has three elements.
	//
	var list = new List<string>() { "Cat", "Rat", "Mouse" };
	Console.WriteLine(list.FirstOrDefault());

	//
	// This query produces no results so FirstOrDefault is null.
	//
	var query1 = from element in list
		    where element.Length > 10
		    select element;
	Console.WriteLine(query1.FirstOrDefault() == null);

	//
	// This query produces one result, so FirstOrDefault is a string.
	//
	var query2 = from element in list
		     where element.Length > 3
		     select element;
	Console.WriteLine(query2.FirstOrDefault());

	//
	// This array has no elements, so FirstOrDefault is zero.
	//
	int[] array = new int[0];
	Console.WriteLine(array.FirstOrDefault());
    }
}

Output

Cat
True
Mouse
0

FirstOrDefault is invoked four times. The first time the method is invoked, it returns the value of the first string element in the List. The second time it is called, it returns null because the 'query1' expression returned no elements.

And: The third usage returns the string literal 'Mouse' because that is the first element that matches the 'where' clause in the query.

Finally: The program displays 0 because that is the result of FirstOrDefault when called on an empty integer array.

Internals. The FirstOrDefault method is a syntax extension found in a separate location in the base class library. It is a generic method which means it accepts a type parameter that indicates what types it acts upon.

Tip: You do not need to specify the type parameter because this is inferred during the C# compilation step.

It checks the acting collection for null, and if that is true it returns the result of the default(T) expression. Otherwise, it returns the element at index 0 of the element iterated to first.

Default expression. The C# language provides a default expression, which the FirstOrDefault method refers to. This expression is similar to the typeof operator. You can use default(int) or default(StringBuilder).

Info: The default value for a value type is equal to all zero bits. And the default value for a reference type is the null value.

Default

Summary. We explored the FirstOrDefault extension method. This method provides a shortcut to accessing the element that occurs first in the collection or query, while protecting against invalid accesses if there are no elements.

So: FirstOrDefault is another way to access the "best match" from a query expression that sorts and filters elements.

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