静态 Class 与受保护的构造函数

Question

我在 class 中收到一条警告消息，比如

Add a Protected constructor or the static keyword to the class declaration

解决方案

我尝试了以下两种方法后错误消失了，

`static` class 没有 `constructor`

public static class Program    {
    }

非 `static` class with `protected` using constructor

public  class Program
    {
        protected Program() { }
    }

问题：

那么我上面的解决方案中提到的 Static Class 与 Protected Constructor 之间有什么区别？哪个最好用？

Answer 1

实例化 class 类型时调用静态构造函数。当创建 class 的实例时，将调用受保护的构造函数。受保护的部分意味着只有继承 class 的 classes 才能调用它。

Answer 2

您可能在 class 中只有静态成员，并且代码分析器假定您的意图是无法创建 class 的实例，因此它要求您要么class 静态

public static class Program {
    //...static members
}

或者放一个 protected/private 构造器

public class Program {
    protected Program { //OR private

    }

    //...static members
}

以防止 class 的实例被初始化。

A static class is basically the same as a non-static class, but there is one difference: a static class cannot be instantiated.

引用Static Classes and Static Class Members (C# Programming Guide)

受保护的构造函数意味着只有派生的classes可以调用构造函数

并且私有构造函数不允许任何其他 classes 使用私有构造函数

初始化 class

Answer 3

A static class 不需要实例来访问其成员。 static class 不能有实例成员（例如，静态 class 上不允许 public int MyNumber;，因为静态 class 上只允许静态成员）。不过，非静态 class 允许实例成员和静态成员。具有 protected 构造函数的 class 只能拥有由其自身创建的实例或从其继承的实例。

public class Program
{
    protected Program()
    {
        // Do something.
    }

    public static Program Create()
    {
        // 100% Allowed.
        return new Program();
    }

    public void DoSomething()
    {

    }
}

public static class AnotherClass
{
    public static Program CreateProgram()
    {
        // Not allowed since Program's constructor is protected.
        return new Program();
    }
}

public class SubProgram : Program
{
    protected SubProgram()
    {
        // Calls Program() then SubProgram().
    }

    public new static Program Create()
    {
        // return new Program(); // We would need to move the SubProgram class INSIDE the Program class in order for this line to work.
        return new SubProgram();
    }
}

Program.Create();               // Can be called since Create is public and static function.
Program.DoSomething()           // Can't be called because an instance has not been instantiated.
var test = Program.Create();
test.DoSomething();             // Can be called since there is now an instance of Program (i.e. 'test').
AnotherClass.CreateProgram();   // Can't be called since Program's constructor is protected.
SubProgram.Create();            // Can be called since SubProgram inherits from Program.

至于性能，这种区别与性能并没有太大关系。

Answer 4

Static Constructor：class类型实例化时调用一次，用于初始化静态成员。不创建 class 的实例。

受保护构造函数：只能由class或继承它的class调用的构造函数。

最好的做法是，如果您只希望继承的 classes 能够创建您的 class.你可以两者兼得。

public class MyClass
{
    static readonly long _someStaticMember;
    private bool _param;
    static MyClass()
    {
        //Do Some Logic
        _someStaticMember = SomeValueCalculated;
    }
    protected MyClass(bool param)
    {
        _param = param;
    }
}
public class ChildClass: MyClass
{
    public ChildClass(bool param) : base(param);
}
public class NotChildClass
{
    public MyClass someObject = new MyClass(true); //Will Fail
}

静态 Class 与受保护的构造函数

Static Class vs Protected Constructor

c#

class

constructor

static

access-modifiers

解决方案

`static` class 没有 `constructor`

非 `static` class with `protected` using constructor

问题：

静态 Class 与受保护的构造函数

Static Class vs Protected Constructor

c#

class

constructor

static

access-modifiers

解决方案

static class 没有 constructor

非 static class with protected using constructor

问题：

`static` class 没有 `constructor`

非 `static` class with `protected` using constructor