Direct casting vs 'as' operator?
Consider the following code:
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). Which one should be preferred?
string s = (string)o; // 1
Throws InvalidCastException if o
is not a string
. Otherwise, assigns o
to s
, even if o
is null
.
string s = o as string; // 2
Assigns null
to s
if o
is not a string
or if o
is null
. For this reason, you cannot use it with value types (the operator could never return null
in that case). Otherwise, assigns o
to s
.
string s = o.ToString(); // 3
Causes a NullReferenceException if o
is null
. Assigns whatever o.ToString()
returns to s
, no matter what type o
is.
Use 1 for most conversions - it's simple and straightforward. I tend to almost never use 2 since if something is not the right type, I usually expect an exception to occur. I have only seen a need for this return-null type of functionality with badly designed libraries which use error codes (e.g. return null = error, instead of using exceptions).
3 is not a cast and is just a method invocation. Use it for when you need the string representation of a non-string object.
-
string s = (string)o;
Use when something should definitely be the other thing. -
string s = o as string;
Use when something might be the other thing. -
string s = o.ToString();
Use when you don't care what it is but you just want to use the available string representation.