为什么的ConvertTo的工作，但铸造在某些情况下会失败呢？ [英] Why does ConvertTo work, but casting fail in some cases?

问题描述

我总是好奇

为什么这项工作：

 双数= Convert.ToDouble（TextBox1.Text）; 
  

但是，这并不：

 双数=（双）TextBox1.Text; 
  

解决方案

我想你问的是...

当我键入这行代码到IDE：

 双数=（双）TextBox1.Text; 
  

为什么不能编译器把它变成这个含蓄：

 双数= Convert.ToDouble（TextBox1.Text）; 
  

这里的问题是，你正在使用的明确的转换。你是从字面上说给编译器...

尽管我宣布的内存这个块是X，我要你把它像一个Y的编译器是足够聪明，知道它是否可以做或没有。因为你正试图转换的内存块的以System.String，编译器知道有把它当作好像它是一个System.Double没有可能的方式。

静态转换方法编程出解析值并创造所需类型的存储器一个全新的值，而不是简单地使用相同的字节存储器好象有别的。

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.

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