c# - 在 C# 中，为什么 dictionary[0]++ 有效？

Question

考虑以下 C# 代码:

var d = new Dictionary<int, int>();
d[0] = 0;
d[0]++;

这段代码执行后d[0]的值是多少？我期望 d[0] == 0，因为 Dictionary<> 的 Item 属性返回一个值类型 int，大概在堆栈上，然后递增。然而，令人惊讶的是，当您实际运行这段代码时，您会发现 d[0] == 1。

上面的示例表现得好像索引器正在返回引用类型，但现在考虑以下内容:

var d = new Dictionary<int, int>();
d[0] = 0;
var a = d[0];
a++;

这段代码执行后d[0]的值是多少？这次我们得到了预期的 d[0] == 0，因此索引器肯定没有返回引用。

有人知道为什么我们会看到这种行为吗？

Answer 1

C# 规范 7.6.9 Postfix increment and decrement operators :

The run-time processing of a postfix increment or decrement operation of the form x++ or x-- consists of the following steps:

• if x is classified as a property or indexer access:
  • The instance expression (if x is not static) and the argument list (if x is an indexer access) associated with x are evaluated, and the results are used in the subsequent get and set accessor invocations.
  • The get accessor of x is invoked and the returned value is saved.
  • The selected operator is invoked with the saved value of x as its argument.
  • The set accessor of x is invoked with the value returned by the operator as its value argument.
  • The saved value of x becomes the result of the operation.

这实际上与值类型与引用类型语义无关，因为 -- 和 ++ 不应该改变实例，而是返回具有新值的新实例.

public static class Test {
    public static void Main() {
        TestReferenceType();
        TestValueType();
    }
    public static void TestReferenceType() {
        var d=new Dictionary<int,BoxedInt>();
        BoxedInt a=0;
        d[0]=a;
        d[0]++;
        d[1]=2;
        BoxedInt b=d[1];
        b++;
        Console.WriteLine("{0}:{1}:{2}:{3}",a,d[0],d[1],b);
    }
    public static void TestValueType() {
        var d=new Dictionary<int,int>();
        int a=0;
        d[0]=a;
        d[0]++;
        d[1]=2;
        int b=d[1];
        b++;
        Console.WriteLine("{0}:{1}:{2}:{3}",a,d[0],d[1],b);
    }
    public class BoxedInt {
        public int Value;
        public BoxedInt(int value) {
            Value=value;
        }
        public override string ToString() {
            return Value.ToString();
        }
        public static implicit operator BoxedInt(int value) {
            return new BoxedInt(value);
        }
        public static BoxedInt operator++(BoxedInt value) {
            return new BoxedInt(value.Value+1);
        }
    }
}

两种测试方法都将打印相同的字符串0:1:2:3。如您所见，即使使用引用类型，您也必须调用 set 访问器来观察字典中的更新值。