C#のエクステンションメソッドの使い方を徹底解説

public static class StringExtensions
{
    public static bool IsNullOrEmpty(this string str)
    {
        return string.IsNullOrEmpty(str);
    }
}

string example = null;
bool result = example.IsNullOrEmpty();  // true

public static class IntegerExtensions
{
    // ここにエクステンションメソッドを追加します
}

public static int Square(this int number)
{
    return number * number;
}

int number = 5;
int result = number.Square();  // 25

using MyExtensions;

int number = 5;
int result = number.Square();

public static class StringExtensions
{
    public static string ReverseString(this string str)
    {
        char[] charArray = str.ToCharArray();
        Array.Reverse(charArray);
        return new string(charArray);
    }
}

string example = "hello";
string reversed = example.ReverseString();  // "olleh"

public static class ListExtensions
{
    public static double Average(this List<int> list)
    {
        if (list.Count == 0) return 0;
        double sum = 0;
        foreach (int number in list)
        {
            sum += number;
        }
        return sum / list.Count;
    }
}

List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };
double average = numbers.Average();  // 3.0

public static class DateTimeExtensions
{
    public static string ToFormattedString(this DateTime dateTime, string format)
    {
        return dateTime.ToString(format);
    }
}

DateTime today = DateTime.Now;
string formattedDate = today.ToFormattedString("yyyy/MM/dd");  // "2024/07/04"

public static class ObjectExtensions
{
    public static void LogInfo(this object obj)
    {
        Console.WriteLine($"[{DateTime.Now}] Info: {obj.ToString()}");
    }
}

var myObject = new { Name = "Alice", Age = 30 };
myObject.LogInfo();  // [2024/07/04 12:00:00] Info: { Name = Alice, Age = 30 }

public static class HttpResponseMessageExtensions
{
    public static string CheckForError(this HttpResponseMessage response)
    {
        if (!response.IsSuccessStatusCode)
        {
            return $"Error: {response.StatusCode} - {response.ReasonPhrase}";
        }
        return "Success";
    }
}

HttpResponseMessage response = await httpClient.GetAsync("https://api.example.com/data");
string result = response.CheckForError();  // "Error: 404 - Not Found" または "Success"

public static class ListExtensions
{
    public static List<T> FilterBy<T>(this List<T> list, Func<T, bool> predicate)
    {
        List<T> filteredList = new List<T>();
        foreach (var item in list)
        {
            if (predicate(item))
            {
                filteredList.Add(item);
            }
        }
        return filteredList;
    }
}

List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };
List<int> evenNumbers = numbers.FilterBy(n => n % 2 == 0);  // { 2, 4 }

public static class ExceptionExtensions
{
    public static void HandleException(this Exception ex, Action<Exception> handler)
    {
        handler(ex);
    }
}

try
{
    // 何か例外を引き起こすコード
}
catch (Exception ex)
{
    ex.HandleException(e => Console.WriteLine($"An error occurred: {e.Message}"));
}

var result = numbers.Where(n => n > 10).OrderBy(n => n);

String.prototype.reverseString = function() {
    return this.split('').reverse().join('');
};

let example = "hello";
let reversed = example.reverseString();  // "olleh"

def reverse_string(self):
    return self[::-1]

str.reverse_string = reverse_string

example = "hello"
reversed = example.reverse_string()  # "olleh"

extension String {
    func reversedString() -> String {
        return String(self.reversed())
    }
}

let example = "hello"
let reversed = example.reversedString()  // "olleh"

fun String.reverseString(): String {
    return this.reversed()
}

val example = "hello"
val reversed = example.reverseString()  // "olleh"

public static class ListExtensions
{
    public static T MaxValue<T>(this List<T> list) where T : IComparable<T>
    {
        // メソッドの実装をここに書いてください
    }
}

List<int> numbers = new List<int> { 1, 3, 5, 2, 4 };
int max = numbers.MaxValue();  // 5

public static class StringExtensions
{
    public static string ToTitleCase(this string str)
    {
        // メソッドの実装をここに書いてください
    }
}

string example = "hello world";
string titleCase = example.ToTitleCase();  // "Hello World"

public static class ArrayExtensions
{
    public static T[] RemoveDuplicates<T>(this T[] array)
    {
        // メソッドの実装をここに書いてください
    }
}

int[] numbers = { 1, 2, 2, 3, 4, 4, 5 };
int[] uniqueNumbers = numbers.RemoveDuplicates();  // { 1, 2, 3, 4, 5 }

public static class ObjectExtensions
{
    public static string DumpProperties(this object obj)
    {
        // メソッドの実装をここに書いてください
    }
}

var person = new { Name = "Alice", Age = 30 };
string dump = person.DumpProperties();  
// "Name: Alice, Age: 30"

public static class ListExtensions
{
    public static T MaxValue<T>(this List<T> list) where T : IComparable<T>
    {
        if (list.Count == 0) return default(T);
        T max = list[0];
        foreach (var item in list)
        {
            if (item.CompareTo(max) > 0)
            {
                max = item;
            }
        }
        return max;
    }
}

public static class StringExtensions
{
    public static string ToTitleCase(this string str)
    {
        if (string.IsNullOrEmpty(str)) return str;
        return CultureInfo.CurrentCulture.TextInfo.ToTitleCase(str.ToLower());
    }
}

public static class ArrayExtensions
{
    public static T[] RemoveDuplicates<T>(this T[] array)
    {
        return array.Distinct().ToArray();
    }
}

public static class ObjectExtensions
{
    public static string DumpProperties(this object obj)
    {
        if (obj == null) return string.Empty;
        var properties = obj.GetType().GetProperties();
        var sb = new StringBuilder();
        foreach (var prop in properties)
        {
            sb.AppendFormat("{0}: {1}, ", prop.Name, prop.GetValue(obj));
        }
        return sb.ToString().TrimEnd(',', ' ');
    }
}