简单的减法和强制转换问题 [英] Simple subtraction and cast question

问题描述

为什么这个简单的减法不起作用？

Why won't this simple subtraction work?

int MyPageNumber = Convert.ToInt32(cboPageNumber.SelectedItem);
MyPageNumber += (MyPageNumber - 1); //does not work
int MyNewPageNumber = MyPageNumber - 1; /works

我还希望有人能告诉我为什么这给了我一个红线能够进行转换：

I was also hoping someone could tell me why this gives me a "red line" for not being able to do a cast:

short MyPageNumber = Convert.ToInt16(cboPageNumber.SelectedItem);
MyPageNumber += MyPageNumber - ((short) 1); //does not work says can't cast

我不了解什么？在示例中+将它转换为字符串吗？

What am I not understanding? Is the + turning it into a String in the examples?

谢谢。

推荐答案

确切地查看其作用：

MyPageNumber += (MyPageNumber - 1); 

将将MyPageNumber-1添加到现有值中。因此，如果MyPageNumber为5，则最终得到的是9，而不是您想要的4。

It adds MyPageNumber-1 to the existing value. So if MyPageNumber is 5, you end up with 9 instead of the 4 which you presumably want.

现在，第二个问题的情况基本上与

Now for the second problem, you've basically got a situation equivalent to this:

short x = 5;
short y = 10;
short z = x - y;

看起来不错，但是C＃实际上没有短裤的减法运算符-它只有一个在整数上，因此它会将 x 和 y 都隐式转换为 int 。结果也是 int ，您不能将其分配回 z 。您需要：

It looks okay, but C# doesn't actually have a subtraction operator for shorts - it only has it on ints, so it's implicitly converting both x and y to int. The result is then an int too, which you can't assign back to z. You need:

short z = (short) (x - y);

（或与您的实变量名称等效的变量）。

(or the equivalent with your real variable names).

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