Different Cast Types in C#

Question

Possible Duplicates:
Casting: (NewType) vs. Object as NewType
Why is the C# “as” operator so popular?

Hey,

I know this may be a silly question but this doubt came to me today.

What is the difference between doing

String text = (String) variable;

and

String text = variable as String;

?

http://stackoverflow.com/questions/2139798/why-is-the-c-as-operator-so-popular — Mehrdad Afshari, Feb 25 '10 at 18:43

score 6 · Accepted Answer · answered Feb 25 '10 at 18:45

A cast can do three things:

An as operation is almost always a reference conversion, the only exception being unboxing to a nullable type:

object x = "hello";
int? y = x as int?; // y is null afterwards

Then there's the behaviour with conversions which fail at execution time. So the differences are:

Casts performing reference conversions or unboxing will throw InvalidCastException on failure; as will result in the null value of the target type instead
Casts can perform user-defined conversions; as can't
Casts can unbox to non-nullable value types; as can only be used for unboxing if the target type is a nullable value type

score 4 · Answer 2 · answered Feb 25 '10 at 18:42

4

as will return null if variable isn't actually of that type (String in this case). The cast will throw an exception.

answered Feb 25 '10 at 18:42

i_am_jorf

1

as will return null if variable isn't actually of that type **or a derived type** (String in this case). The cast will throw an exception. – Mehrdad Afshari Feb 25 '10 at 18:44
1

Beat me to it. Keep in mind that `as` will not work on value types. `int i = obj as int;` will not compile. The `as` keyword must be used with a reference type or nullable type as @Mehrdad Afshari said. – Matt Davis Feb 25 '10 at 18:44
@Matt: it works. You need to use a nullable type: `int? i = obj as int?;` – Mehrdad Afshari Feb 25 '10 at 18:45

score 2 · Answer 3 · answered Feb 25 '10 at 18:50

Here is the link to Eric Lippert's blog on casting in C#. I'd summarize it, but it's pretty short and he'll explain it much better than me.

And here is his post on the cast operator:

score 0 · Answer 4 · answered Feb 25 '10 at 18:55

There are a lot of different ways to cast in C#.

This will try to cast the reference to a String reference. If the cast fails, it throws an exception:

string text = (String) variable;

This will try to cast the reference to a String reference. If the cast fails, it will return a null reference to be assigned to the variable:

string text = varible as String;

This will cast a string reference to an object reference, which is a safe casting as String inherits from Object:

object text = (object)"1337";

Casting to a parent class can also be done implicitly:

object text = "1337";

This will box a value inside an object, then unbox it to a plain value again:

int value = 42;
object boxed = (object)value;
int valueAgain = (int)boxed;

The boxing can also be done implicitly:

int value = 42;
object boxed = value;
int valueAgain = (int)boxed;

This will make a widening conversion from byte to int:

byte a = 42;
int b = (int)a;

The same works as an implicit conversion:

byte a = 42;
int b = a;

This will make a narrowing conversion from int to byte, throwing away the overflow:

int a = 512;
byte b = (byte)a; // b now contains 0

4 Answers4