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# namespace

參考資料:(<https://docs.microsoft.com/zh-tw/dotnet/csharp/programming-guide/namespaces/using-namespaces>)

C# or .NET Framework 利用命名空間來組織多種類別。 宣告自己的命名類別，有助於在較大型的程式設計專案，控制類別和方法名稱的範圍。

## 存取命名空間

程式的開頭先設定好，應該就類似java的import。

```
using System;
```

在程式開始撰寫後，可以這樣寫:

```
Console.WriteLine("Hello world!");
```

就不需要寫成

```
System.Console.WriteLine("Hello world!");
```

## 命名空間別名

```
using Co = Company.Proj.Nested;
```

## 使用命名空間控制範圍

namespace 是用來宣告命名空間的。

從下列程式可觀察出，SampleClass此類別分別在命名空間SampleNamespace及其巢狀命名空間NestedNamespace中。根據底下Program的程式，可以了解是如何呼叫的。

```
namespace SampleNamespace
{
    class SampleClass
    {
        public void SampleMethod()
        {
            System.Console.WriteLine(
              "SampleMethod inside SampleNamespace");
        }
    }

    // Create a nested namespace, and define another class.
    namespace NestedNamespace
    {
        class SampleClass
        {
            public void SampleMethod()
            {
                System.Console.WriteLine(
                  "SampleMethod inside NestedNamespace");
            }
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            // Displays "SampleMethod inside SampleNamespace."
            SampleClass outer = new SampleClass();
            outer.SampleMethod();

            // Displays "SampleMethod inside SampleNamespace."
            SampleNamespace.SampleClass outer2 = new SampleNamespace.SampleClass();
            outer2.SampleMethod();

            // Displays "SampleMethod inside NestedNamespace."
            NestedNamespace.SampleClass inner = new NestedNamespace.SampleClass();
            inner.SampleMethod();
        }
    }
}
```

若有相同名稱

`.`會先找近的，遠的會被隱藏。 `::`會去找命名空間的(別名)。可以使用別名。 `global::`會去找全域命名空間的。不可以使用別名。

(文章內的敘述:在一般情況下，使用 :: 來參考命名空間別名，或使用 global:: 參考全域命名空間，以及使用 . 來限定類型或成員。)

`global`不是預先定義別名，只有在與`::`搭配使用才有特殊意義。


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