擴(kuò)展方法允許在不修改類型或創(chuàng)建派生類的情況下為其“添加”方法。它們是定義在靜態(tài)類中的靜態(tài)方法，通過實(shí)例方法語法調(diào)用，第一個參數(shù)使用this關(guān)鍵字指定所擴(kuò)展的類型。例如，可為string類型定義IsNullOrEmpty擴(kuò)展方法，并像實(shí)例方法一樣調(diào)用。定義步驟包括：1. 創(chuàng)建靜態(tài)類；2. 定義靜態(tài)方法；3. 在第一個參數(shù)前加this；4. 使用實(shí)例方法語法調(diào)用。擴(kuò)展方法適用于增強(qiáng)現(xiàn)有類型的可讀性、操作無法修改的類型或構(gòu)建工具庫，常見于LINQ中。注意其不能訪問私有成員，且與同名實(shí)例方法沖突時后者優(yōu)先。應(yīng)合理組織命名空間并添加注釋以確?？删S護(hù)性。

Extension methods in C# let you "add" methods to existing types without modifying them, creating them as derived classes, or recompiling the original type. This is especially useful when you don't have access to the source code of the type you're extending — for example, if it's part of a framework like .NET.

Here’s how they work and how you can use them effectively.

What Are Extension Methods?

An extension method is a static method defined in a static class, but it's called using instance method syntax. The first parameter of the method specifies which type the method operates on, and it's preceded by the this keyword.

For example:

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

Once defined, you can call it like this:

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

This makes your code more readable and feels like a natural part of the type.

How to Define an Extension Method

Creating one involves just a few steps:

• Create a static class to hold your method(s).
• Define a static method inside that class.
• Use this before the first parameter to specify the type being extended.
• Call the method as if it were an instance method.

Here’s a breakdown:

public static class ListExtensions
{
    public static void PrintAll(this List<string> list)
    {
        foreach (var item in list)
        {
            Console.WriteLine(item);
        }
    }
}

Now any List<string></string> can call .PrintAll() directly.

You’re not actually modifying the type — you’re just giving the illusion that you did. The compiler handles the rest behind the scenes.

When Should You Use Them?

Extension methods are best used when:

• You want to enhance readability or usability of existing types.
• You're working with types you can’t modify (like third-party or framework classes).
• You're building utility libraries that operate on common data structures.

They're widely used in LINQ (Language Integrated Query), where methods like .Where(), .Select(), and .OrderBy() are all implemented as extensions on IEnumerable<t></t>.

A good rule of thumb:

• If the method logically belongs to a type but wasn’t included originally, an extension might be appropriate.
• Avoid using them for complex logic or state changes — those should live in proper classes.

Important Things to Keep in Mind

While extension methods are powerful, there are some gotchas:

• They can’t access private members of the type they extend.
• If a method with the same name and signature exists in the actual type, the instance method wins.
• Overuse can lead to confusion or cluttered IntelliSense, making code harder to read and maintain.

Also, make sure to:

• Organize your extension methods into meaningful static classes.
• Use clear naming conventions so it's obvious what they do.
• Add XML comments for better documentation support.

One thing people often forget is that extension methods need to be in scope — meaning the namespace must be referenced via a using directive, or the method won’t show up in IntelliSense.

So, extension methods give you a clean way to add functionality to existing types without inheritance or source code access. They’re not magic — just syntactic sugar that helps keep your code expressive and clean.

基本上就這些。

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