Habib是对的——因为字符串是引用类型。

但更重要的是，您不必每次使用它时都检查是否为null。如果有人给你的函数传递一个空引用，你可能应该抛出一个ArgumentNullException。

事情是这样的——无论如何，如果你试图对字符串调用. toupper()，框架都会为你抛出一个NullReferenceException。记住，即使你测试你的参数为空，这种情况仍然会发生，因为传递给你的函数作为参数的对象上的任何属性或方法都可能求值为空。

也就是说，检查空字符串或null是一件常见的事情，因此他们提供String.IsNullOrEmpty()和string . isnullowhitespace()仅用于此目的。

既然你提到了ToUpper()，这种用法是我如何找到这个线程的，我将分享这个快捷方式(字符串??" ") .ToUpper ():

    private string _city;
    public string City
    {
        get
        {
            return (this._city ?? "").ToUpper();
        }
        set
        {
            this._city = value;
        }
    }

似乎比:

        if(null != this._city)
        { this._city = this._city.ToUpper(); }

因为字符串是一个引用类型，引用类型的默认值是null。

最根本的原因/问题是CLS规范(定义了语言如何与。net交互)的设计者没有定义一种方法，通过这种方法，类成员可以指定它们必须直接被调用，而不是通过callvirt，而调用方不执行空引用检查;它也没有提供一种定义不受“正常”装箱约束的结构的方法。

Had the CLS specification defined such a means, then it would be possible for .net to consistently follow the lead established by the Common Object Model (COM), under which a null string reference was considered semantically equivalent to an empty string, and for other user-defined immutable class types which are supposed to have value semantics to likewise define default values. Essentially, what would happen would be for each member of String, e.g. Length to be written as something like [InvokableOnNull()] int String Length { get { if (this==null) return 0; else return _Length;} }. This approach would have offered very nice semantics for things which should behave like values, but because of implementation issues need to be stored on the heap. The biggest difficulty with this approach is that the semantics of conversion between such types and Object could get a little murky.

An alternative approach would have been to allow the definition of special structure types which did not inherit from Object but instead had custom boxing and unboxing operations (which would convert to/from some other class type). Under such an approach, there would be a class type NullableString which behaves as string does now, and a custom-boxed struct type String, which would hold a single private field Value of type String. Attempting to convert a String to NullableString or Object would return Value if non-null, or String.Empty if null. Attempting to cast to String, a non-null reference to a NullableString instance would store the reference in Value (perhaps storing null if the length was zero); casting any other reference would throw an exception.

Even though strings have to be stored on the heap, there is conceptually no reason why they shouldn't behave like value types that have a non-null default value. Having them be stored as a "normal" structure which held a reference would have been efficient for code that used them as type "string", but would have added an extra layer of indirection and inefficiency when casting to "object". While I don't foresee .net adding either of the above features at this late date, perhaps designers of future frameworks might consider including them.

Habib是对的——因为字符串是引用类型。

但更重要的是，您不必每次使用它时都检查是否为null。如果有人给你的函数传递一个空引用，你可能应该抛出一个ArgumentNullException。

事情是这样的——无论如何，如果你试图对字符串调用. toupper()，框架都会为你抛出一个NullReferenceException。记住，即使你测试你的参数为空，这种情况仍然会发生，因为传递给你的函数作为参数的对象上的任何属性或方法都可能求值为空。

也就是说，检查空字符串或null是一件常见的事情，因此他们提供String.IsNullOrEmpty()和string . isnullowhitespace()仅用于此目的。

为什么c#的设计者选择使用null作为的默认值 字符串?

因为字符串是引用类型，引用类型的默认值是null。引用类型的变量存储对实际数据的引用。

在这种情况下，让我们使用default关键字;

string str = default(string); 

STR是一个字符串，所以它是一个引用类型，所以默认值是null。

int str = (default)(int);

STR是int型，所以它是一个值类型，所以默认值是0。