LINQ Basics

FirstOrDefault

FirstOrDefault finds the first matching value or returns a fallback default.

FirstOrDefault `FirstOrDefault` returns the first match or a default value when none matches.

FirstOrDefault

prefix
FirstOrDefaultExample.cs
Replay: real traced execution (multi-file project)
using System;
using System.Linq;

class Program
{
    static void Main()
    {
        string prefix = "b";
        string[] codes = { "ax", "by", "bz" };
        string match = codes.FirstOrDefault(code => code.StartsWith(prefix)) ?? "none";

        Console.WriteLine($"prefix={prefix}");
        Console.WriteLine($"match={match}");
    }
}

using System;
using System.Linq;

class Program
{
    static void Main()
    {
        string prefix = "a";
        string[] codes = { "ax", "by", "bz" };
        string match = codes.FirstOrDefault(code => code.StartsWith(prefix)) ?? "none";

        Console.WriteLine($"prefix={prefix}");
        Console.WriteLine($"match={match}");
    }
}

using System;
using System.Linq;

class Program
{
    static void Main()
    {
        string prefix = "z";
        string[] codes = { "ax", "by", "bz" };
        string match = codes.FirstOrDefault(code => code.StartsWith(prefix)) ?? "none";

        Console.WriteLine($"prefix={prefix}");
        Console.WriteLine($"match={match}");
    }
}

  1. prefix ← b, match ← by

    5{6    static void Main()7    {8        string prefix→ b = "b"; //@prefix="a", "z"9        string[] codes = { "ax", "by", "bz" };10        string match→ by = codes.FirstOrDefault(code => code.StartsWith(prefix)) ?? "none";1112        Console.WriteLine($"prefix={prefixb}");13        Console.WriteLine($"match={matchby}");14    }
    outputprefix=b
match=by

  1. prefix ← a, match ← ax

    5{6    static void Main()7    {8        string prefix→ a = "a";9        string[] codes = { "ax", "by", "bz" };10        string match→ ax = codes.FirstOrDefault(code => code.StartsWith(prefix)) ?? "none";1112        Console.WriteLine($"prefix={prefixa}");13        Console.WriteLine($"match={matchax}");14    }
    outputprefix=a
match=ax

  1. prefix ← z, match ← none

    5{6    static void Main()7    {8        string prefix→ z = "z";9        string[] codes = { "ax", "by", "bz" };10        string match→ none = codes.FirstOrDefault(code => code.StartsWith(prefix)) ?? "none";1112        Console.WriteLine($"prefix={prefixz}");13        Console.WriteLine($"match={matchnone}");14    }
    outputprefix=z
match=none