Direct casting vs as casting

There are two primary ways to cast in C#, Direct casting and ‘as’ casting.

Explicit/Implicit casting

Here are some examples of Direct/Implicit casting

byte source = 1;
int value = source;

object source = 1; //this assignment defaults to an int
int value = (int)source;
/* or */
byte value = (byte)source;

Implicit casts can be used when converting from one type to another without any loss of information. In this example from a byte to a higher order numeric value. Byte to Int32, Int32 to Double. You cannot, however, cast down from Double to Byte without doing an explicit cast.

The second example shows an explicit cast from an integer to a byte. The explicit cast here is required because there could be a loss of information going from a larger int (4 bytes) to a byte (1 byte).

Casting using the as operator

The types must be nullable or reference objects in this case.

object animal = new Dog();
var value = animal as Dog;

/* value, is this instance, will be `null` */
var value = animal as Plankton;

As casting basically does a check to see if the type is convertible and if so returns the value. If not it returns null.

Are these the same?

In essense no. Using ILSpy we can see that the generated IL for explicit casts uses the castclass, where as the as operator uses isinst.

IL_0007: isinst CastClassVsIsInst.Dog

IL_0007: castclass CastClassVsIsInst.Dog

From a little bit of testing it seems that ‘as’ is also faster. Others could confirm this but in tight loops isinst performs significantly faster than castclass.

Charts courtesy of Jon Galloway

Here’s my quick-and-dirty test.

Feel free to fork it and make improvements. There might be instances where castclass is faster.

It’s not perfect but enough to convince me that isinst is a bit faster in tight loops than castclass

-Ben Dornis