c# - 直接铸造与“ as”运算符？(Direct casting vs 'as' operator?)

Question

Consider the following code:

(考虑以下代码：)

void Handler(object o, EventArgs e)
{
   // I swear o is a string
   string s = (string)o; // 1
   //-OR-
   string s = o as string; // 2
   // -OR-
   string s = o.ToString(); // 3
}

What is the difference between the three types of casting (okay, the 3rd one is not a casting, but you get the intent).

(三种类型的转换之间有什么区别（好吧，第三个类型不是转换，但是您有意图）。)

Which one should be preferred?

(应该首选哪一个？)

  ask by nullDev translate from so

Answer 1

string s = (string)o; // 1

Throws InvalidCastException if o is not a string .

(如果o不是string则抛出InvalidCastException 。)

Otherwise, assigns o to s , even if o is null .

(否则，将o分配给s ，即使o为null 。)

string s = o as string; // 2

Assigns null to s if o is not a string or if o is null .

(分配null到s ，如果o是不是一个string ，或者o为null 。)

For this reason, you cannot use it with value types (the operator could never return null in that case).

(因此，您不能将其与值类型一起使用（在这种情况下，运算符永远不会返回null ）。)

Otherwise, assigns o to s .

(否则，将o分配给s 。)

string s = o.ToString(); // 3

Causes a NullReferenceException if o is null .

(如果o为null则导致NullReferenceException 。)

Assigns whatever o.ToString() returns to s , no matter what type o is.

(将o.ToString()返回的值o.ToString() s ，无论o是什么类型。)

---

Use 1 for most conversions - it's simple and straightforward.

(大多数转换使用1-简单明了。)

I tend to almost never use 2 since if something is not the right type, I usually expect an exception to occur.

(我倾向于几乎从不使用2，因为如果某些类型不正确，我通常希望发生异常。)

I have only seen a need for this return-null type of functionality with badly designed libraries which use error codes (eg return null = error, instead of using exceptions).

(我只看到这种返回null类型的功能需要使用错误代码（例如，返回null =错误，而不是使用异常）的错误设计的库。)

3 is not a cast and is just a method invocation.

(3不是强制转换，而只是方法调用。)

Use it for when you need the string representation of a non-string object.

(在需要非字符串对象的字符串表示形式时使用它。)