为什么ConvertTo工作，但在某些情况下转换失败？ [英] Why does ConvertTo work, but casting fail in some cases?

问题描述

我总是好奇。

为什么这样工作：

  double Number = Convert.ToDouble（TextBox1.Text）; 
  

但这不是：

  double Number =（double）TextBox1.Text; 
  

解决方案

/ p>

当我在IDE中输入这行代码时：

  double Number =（double）TextBox1.Text; 
  

为什么编译器不能隐式地将它转换为：

  double Number = Convert.ToDouble（TextBox1.Text）; 
  

这里的问题是您正在使用 explicit cast 。你真正对编译器说的是...

即使我声明这个内存块为X，我希望你像Y一样对待它。编译器足够聪明，知道它是否可以完成。因为你试图转换的内存块是一个System.String，编译器知道没有可能的方法来对待它，因为它是一个System.Double。

静态转换方法程序化地解析一个值，并在所需类型的内存中创建一个全新的值，而不是简单地在内存中使用相同的字节，就像有其他的一样。

I was always curious.

Why does this work:

double Number = Convert.ToDouble(TextBox1.Text);

But this doesn't:

double Number = (double)TextBox1.Text;

解决方案

I think what you are asking is...

When I type this line of code into the IDE:

double Number = (double)TextBox1.Text;

Why can't the compiler turn it into this implicitly:

double Number = Convert.ToDouble(TextBox1.Text);

The issue here is that you are using an explicit cast. What you are literally saying to the compiler is...

Even though I declared this chunk of memory to be X, I want you to treat it like a Y. The compiler is smart enough to know if it can be done or not. Since the chunk of memory you are trying to convert is a System.String, the compiler knows that there is no possible way to treat it as if it were a System.Double.

The static Convert methods are programatically parsing a value out and creating a brand new value in memory of the desired type, not simply using the same bytes in memory as though there were something else.

这篇关于为什么ConvertTo工作，但在某些情况下转换失败？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！