C# 中的对象转换

Question

what is the difference as well as the pros and cons between

 LinkButton lb = (LinkButton)ctl;

and

 LinkButton lb = ctl as LinkButton;

I tried using the first one and it gives me error, then I tried the other one with the keyword as, it work just fine.

Thank You in Advance.

Answer 1

The first is an explicit cast, and the second is a conversion. If the conversion fails for the as keyword, it will simply return null instead of throwing an exception.

This is the documentation for each:

• Casting and Type Conversions (C# Programming Guide)
• as (C# Reference)

Note in the linked documentation above, they state the as keyword does not support user-defined conversions. +1 to Zxpro :) This is what a user-defined conversion is:

User-Defined Conversions Tutorial

Answer 2

My usual guidance on using the as operator versus a direct cast are as follows:

1. If the cast must succeed (ie it would be an error to continue if the cast failed), use a direct cast.
2. If the cast might fail and there needs to be programmatic detection of this, use the as operator.

The above is true for reference types. For value types (like bool or int ), as does not work. In that case, you will need to use an is check to do a "safe cast", like this:

if (x is int y)
{
   // y is now a int, with the correct value

}
else
{
    // ...
}

I do not recommend trying to catch InvalidCastException , as this is generally the sign of a programmer error. Use the guidance above instead.

Answer 3

I believe that casting using the first method throws an exception if it can't cast the object properly (trying to cast the wrong type), whereas using the as keyword will simply set the variable to null if it couldn't cast it properly.

So make sure that if you use the as keyword cast, you check

if(lb == null)
    return null; // or throw new Exception()

and if you use the () cast, you surround it with

try
{
    LinkButton lb = (LinkButton)ctl;
}
catch(InvalidCastException ex)
{
    //TODO: Handle Exception
}

Answer 4

The second one is called safe cast, which, instead of throwing exception, will put "null" to your variable. So it does NOT work fine, but sets your LinkButton lb to null

Answer 5

请参阅 本自@乔恩飞碟双向： 什么是投语法和使用作为运营商之间的区别？