VB.NET 中 DirectCast() 和 CType() 的区别
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Difference between DirectCast() and CType() in VB.NET
提问by Caleb Hearth
I am an experienced C/C++/C# programmer who has just gotten into VB.NET. I generally use CType (and CInt, CBool, CStr) for casts because it is fewer characters and was the first way of casting which I was exposed to, but I am aware of DirectCast and TryCast as well.
我是一位经验丰富的 C/C++/C# 程序员，刚刚接触 VB.NET。我通常使用 CType（和 CInt、CBool、CStr）进行转换，因为它的字符较少，并且是我接触到的第一种转换方式，但我也知道 DirectCast 和 TryCast。
Simply, are there any differences (effect of cast, performance, etc.) between DirectCast and CType? I understand the idea of TryCast.
简单地说，DirectCast 和 CType 之间有什么区别（转换效果、性能等）？我理解 TryCast 的想法。
回答by Joel Coehoorn
The first thing to note is VB.NET does not have a direct analog to C#'s (type)instancecasting mechanism. I bring this up because it's useful as a starting point in comparing the two VB.NET operators (and they are operators, not functions, even though they have function semantics).
首先要注意的是，VB.NET 没有直接模拟 C# 的(type)instance转换机制。我提出这个是因为它作为比较两个 VB.NET 运算符（它们是运算符，而不是函数，即使它们具有函数语义）的起点很有用。
DirectCast()is more strict than the C# casting operator. It only allows you to cast when the item being cast already isthe type you are casting to. I believe it will still unbox value types, but otherwise it won't do any conversion. So, for example, you can't cast from shortto int, like you could with a C# (int)cast. But you can cast from an IEnumerableto an array, if your underlying IEnumerableobject variable really is an Array. And of course you can cast from Objectto anything, assuming the type of your object instance really is somewhere below your cast type in the inheritance tree.
DirectCast()比 C# 转换运算符更严格。它只允许您在被转换的项目已经是您要转换到的类型时进行转换。我相信它仍然会取消装箱值类型，但否则它不会进行任何转换。因此，例如，您不能像使用 C# 转换那样从shortto 转换。但是，如果您的底层对象变量确实是一个. 当然，您可以从任何类型转换为任何类型，假设您的对象实例的类型确实在继承树中的类型低于您的转换类型。int(int)IEnumerableIEnumerableArrayObject
This is desirable because that makes it faster. There's a little less conversion and type checking that needs to take place.
这是可取的，因为这使它更快。需要进行的转换和类型检查要少一些。
CType()is less strict than the C# casting operator. It will do things you just can't do with a simple (int)-style cast, like convert a string to an integer. It has as much power as calling Convert.To___()in C#, where the ___is the target type of your cast.
CType()不如 C# 强制转换运算符严格。它会做一些你不能用简单的(int)风格转换做的事情，比如将字符串转换为整数。它与Convert.To___()在 C# 中调用一样强大，其中___是您的演员表的目标类型。
This is desirable because it's very powerful. However, this power comes at the cost of performance; it's not as fast as DirectCast()or C#'s cast operator because it might need to do quite a lot of work to finish the cast. Generally you should prefer DirectCast()when you can.
这是可取的，因为它非常强大。然而，这种能力是以牺牲性能为代价的。它不如DirectCast()C# 的转换运算符快，因为它可能需要做很多工作才能完成转换。一般来说，你应该选择DirectCast()什么时候可以。
Finally, you missed one casting operator: TryCast(), which is a direct analog to C#'s asoperator.
最后，您错过了一个强制转换运算符: TryCast()，它直接类似于 C# 的as运算符。
回答by Abhay
With CTypeyou can write some thing like this Ctype("string",Integer). But with DirectCastthe above statement would give compile time error.
随着CType你可以写一些像这样的事情Ctype("string",Integer)。但是用DirectCast上面的语句会给出编译时错误。
Dim a As Integer = DirectCast("1", Integer) 'Gives compiler error
Dim b As Integer = CType("1", Integer) 'Will compile
回答by Hemantha
DirectCastis more restrictive than CType.
DirectCast比 更严格CType。
For example, this will throw an error:
例如，这将引发错误：
Sub Main()
Dim newint As Integer = DirectCast(3345.34, Integer)
Console.WriteLine(newint)
Console.ReadLine()
End Sub
It will also be shown in the Visual Studio IDE.
它也将显示在 Visual Studio IDE 中。
This however, does not throw an error:
但是，这不会引发错误：
Sub Main()
Dim newint As Integer = CType(3345.34, Integer)
Console.WriteLine(newint)
Console.ReadLine()
End Sub
