C as cast vs classic cast

Question

Possible Duplicate    Casting vs using the    as    keyword in the CLR       I recently learned about a different way to cast  Rather than using  SomeClass someObject    SomeClass  obj    one can use this syntax   SomeClass someObject   obj as SomeClass    which seems to return null if obj isn t a SomeClass  rather than throwing a class cast exception   I see that this can lead to a NullReferenceException if the cast failed and I try to access the someObject variable  So I m wondering what s the rationale behind this method  Why should one use this way of casting rather than the  old  one - it only seems to move the problem of a failed cast  deeper  into the code

User · Answer

There s nothing deep happening here   Basically  it s handy to test something to see if it s of a certain type  i e  use  as     You would want to check the result of the  as  call to see if the result is null   When you expect a cast to work and you want the exception to be thrown  use the  classic  method

User · Answer

In some cases  it s easily to deal with a null than an exception  In particular  the coalescing operator is handy   SomeClass someObject    obj as SomeClass     new SomeClass      It also simplifies code where you are  not using polymorphism  and  branching based on the type of an object   ClassA a  ClassB b  if   a   obj as ClassA     null           use a   else if   b   obj as ClassB     null           use b     As specified on the MSDN page  the as operator is equivalent to   expression is type    type expression    type null   which avoids the exception completely in favour of a faster type test  but also limits its use to types that support null  reference types and Nullable lt T gt

User · Answer

The as keyword is useful when you genuinely don t know what type the variable might be  If you have a single function that will follow different code paths depending upon the actual type of the parameter  then you have two choices   First  using a normal cast   if myObj is string        string value    string myObj           do something   else if myObj is MyClass        MyClass    MyClass myObj      This requires that you check the type of the object using is so that you don t try to cast it to something that will fail  This is also slightly redundant  as the is-type checking is done again in the cast  so that it can throw the exception if required    The alternative is to use as   string myString   myObj as string  MyClass myClass   myObj as MyClass   if myString    null       else if myClass    null         This makes the code somewhat shorter and also eliminates the redundant type checking

User · Answer

I suppose it is useful if the result of the cast will be passed to a method that you know will handle null references without throwing and ArgumentNullException or suchlike   I tend to find very little use for as  since   obj as T   Is slower than   if  obj is T          T obj      The use of as is very much an edge-case scenario for me  so I can t think of any general rules for when I would use it over just casting and handling the  more informative  casting exception further up the stack

User · Answer

You use the  as  statement to avoid the possibility of an exception  e g  you can handle the cast failure gracefully via logic   Only use the cast when you are sure that the object is of the desired type   I almost always use the  as  and then check for null

User · Answer

With the  classic  method  if the cast fails  an InvalidCastException is thrown  With the as method  it results in null  which can be checked for  and avoid an exception being thrown   Also  you can only use as with reference types  so if you are typecasting to a value type  you must still use the  classic  method   Note   The as method can only be used for types that can be assigned a null value  That use to only mean reference types  but when  NET 2 0 came out  it introduced the concept of a nullable value type  Since these types can be assigned a null value  they are valid to use with the as operator

User · Answer

Null comparison is MUCH faster than throwing and catching exception  Exceptions have significant overhead - stack trace must be assembled etc   Exceptions should represent an unexpected state  which often doesn t represent the situation  which is when as works better

User · Answer

The as operator is useful in a couple of circumstances      When you only need to know an object is of a specific type but don t need to specifically act on members of that type When you d like to avoid exceptions and instead explicitly deal with null You want to know if there is a CLR conversion between the objects and not just some user defined conversion    The 3rd point is subtle but important   There is not a 1-1 mapping between which casts will succeed with the cast operator and those which will succeed with the as operator   The as operator is strictly limited to CLR conversions and will not consider user defined conversions  the cast operator will      Specifically the as operator only allows for the following  from section 7 9 11 of the C  lang spec    An identity    6 1 1   implicit reference    6 1 6   boxing    6 1 7   explicit reference    6 2 4   or unboxing    6 2 5  conversion exists from the type of E to T  The type of E or T is an open type  E is the null literal

User · Answer

using as will return null if not a valid cast which allows you to do other things besides wrapping the cast in a try catch  I hate classic cast  I always use as cast if i m not sure  Plus  exceptions are expensive  Null checks are not

User · Answer

I think the best  rule  would be to only use the  as  keyword when it s expected that your subject won t be the object you re casting to   var x   GiveMeSomething     var subject   x as String   if subject    null         do what you want with a string   else        do what you want with NOT a string     However  when your subject SHOULD be of the type you re casting to  use a  classic cast    as you call it  Because if it isn t the type you re expecting  you ll get an exception which fits the exceptional situation

[c#] C# "as" cast vs classic cast

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