FirstOrDefault

In this chapter you will learn:

C# Linq FirstOrDefault operator
FirstOrDefault with filter delegate
Get the first or the default
FirstOrDefault vs First
FirstOrDefault and string operation
FirstOrDefault with a Not Found Element

Get to know FirstOrDefault

FirstOrDefault returns null when an Element Is Found.

using System;/* j ava  2s .  co m*/
using System.Linq;
using System.Collections;
using System.Collections.Generic;

public class MainClass {
    public static void Main() {
        string[] presidents = {"G", "H", "a", "H", "over", "Jack"};
        string name = presidents.FirstOrDefault();
        Console.WriteLine(name == null ? "NULL" : name);
    }
}

FirstOrDefault with filter delegate

using System;/*from  j  a v  a2 s  . c  o  m*/
using System.Collections;
using System.Collections.Generic;
using System.Text;
using System.Linq;

public class MainClass{
   public static void Main(){
      int[] numbers = { 1, 3, 5, 7, 9 };
        var query = numbers.FirstOrDefault(n => n % 2 == 0);
   }
}

Get the first or the default

using System;//from   java  2s . c  o  m
using System.Collections.Generic;
using System.Linq;
using System.Text;

public class MainClass {
    public static void Main() {
        int[] numbers = { };

        int firstNumOrDefault = numbers.FirstOrDefault();

        Console.WriteLine(firstNumOrDefault);
    }
}

FirstOrDefault vs First

The following demonstrates First versus FirstOrDefault:

using System;/* j ava2s.c  o m*/
using System.Collections;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main()
    {
        int[] numbers = { 1, 2, 3, 4, 5 };
        int firstBigError = numbers.First(n => n > 10); // Exception 
        int firstBigNumber = numbers.FirstOrDefault(n => n > 10); // 0

        Console.WriteLine(firstBigError);
        Console.WriteLine(firstBigNumber);
    }
}

The output:

FirstOrDefault and string operation

using System;/*from   ja v a 2s. c  o m*/
using System.Linq;
using System.Collections;
using System.Collections.Generic;

public class MainClass {
    public static void Main() {
        string[] presidents = {"G", "H", "a", "H", "over", "Jack"};

        string name = presidents.FirstOrDefault(p => p.StartsWith("B"));
        Console.WriteLine(name == null ? "NULL" : name);
    }
}

FirstOrDefault with a Not Found Element

using System;/*from java  2s  .  co m*/
using System.Linq;
using System.Collections;
using System.Collections.Generic;

public class MainClass {
    public static void Main() {
        string[] presidents = {"G", "H", "a", "H", "over", "Jack"};
        string name = presidents.Take(0).FirstOrDefault();
        Console.WriteLine(name == null ? "NULL" : name);
    }
}

Next chapter...

What you will learn in the next chapter:

How to use GroupBy operator