Difference between .ToString and "as string" in C#
Solution 1
If Session["SessionTheme"]
is not a string
, as string
will return null
.
.ToString()
will try to convert any other type to string by calling the object's ToString()
method. For most built-in types this will return the object converted to a string, but for custom types without a specific .ToString()
method, it will return the name of the type of the object.
object o1 = "somestring";
object o2 = 1;
object o3 = new object();
object o4 = null;
string s = o1 as string; // returns "somestring"
string s = o1.ToString(); // returns "somestring"
string s = o2 as string; // returns null
string s = o2.ToString(); // returns "1"
string s = o3 as string; // returns null
string s = o3.ToString(); // returns "System.Object"
string s = o4 as string; // returns null
string s = o4.ToString(); // throws NullReferenceException
Another important thing to keep in mind is that if the object is null
, calling .ToString()
will throw an exception, but as string
will simply return null
.
Solution 2
The as
keyword will basically check whether the object is
an instance of the type, using MSIL opcode isinst
under the hood. If it is, it returns the reference to the object, else a null reference.
It does not, as many say, attempt to perform a cast as such - which implies some kind of exception handling. Not so.
ToString()
, simply calls the object's ToString()
method, either a custom one implemented by the class (which for most in-built types performs a conversion to string) - or if none provided, the base class object
's one, returning type info.
Solution 3
Page.Theme = Session["SessionTheme"] as string;
tries to cast to a string
whereas
Page.Theme = Session["SessionTheme"].ToString();
calls the ToString()
method, which can be anything really. This method doesn't cast, it should return a string representation of this object.
Solution 4
I'm extending Philippe Leybaert's accepted answer a bit because while I have found resources comparing three of these, I've never found an explanation that compares all four.
(string)obj
obj as string
obj.ToString()
Convert.ToString(obj)
object o1 = "somestring";
object o2 = 1;
object o3 = new object();
object o4 = null;
Console.WriteLine((string)o1); // returns "somestring"
Console.WriteLine(o1 as string); // returns "somestring"
Console.WriteLine(o1.ToString()); // returns "somestring"
Console.WriteLine(Convert.ToString(o1)); // returns "somestring"
Console.WriteLine((string)o2); // throws System.InvalidCastException
Console.WriteLine(o2 as string); // returns null
Console.WriteLine(o2.ToString()); // returns "1"
Console.WriteLine(Convert.ToString(o2)); // returns "1"
Console.WriteLine((string)o3); // throws System.InvalidCastException
Console.WriteLine(o3 as string); // returns null
Console.WriteLine(o3.ToString()); // returns "System.Object"
Console.WriteLine(Convert.ToString(o3)); // returns "System.Object"
Console.WriteLine((string)o4); // returns null
Console.WriteLine(o4 as string); // returns null
Console.WriteLine(o4.ToString()); // throws System.NullReferenceException
Console.WriteLine(Convert.ToString(o4)); // returns string.Empty
From these results we can see that (string)obj
and obj as string
behave the same way as each other when obj
is a string or null; otherwise (string)obj
will throw an invalid cast exception and obj as string
will just return null. obj.ToString()
and Convert.ToString(obj)
also behave the same way as each other except when obj
is null, in which case obj.ToString()
will throw a null reference exception and Convert.ToString(obj)
will return an empty string.
So here are my recommendations:
(string)obj
works best if you want to throw exceptions for types that can't be assigned to a string variable (which includes null)obj as string
works best if you don't want to throw any exceptions and also don't want string representations of non-stringsobj.ToString()
works best if you want to throw exceptions for nullConvert.ToString(obj)
works best if you don't want to throw any exceptions and want string representations of non-strings
EDIT: I've discovered that Convert.ToString()
actually treats null
differently depending on the overload, so it actually matters that the variable was declared as an object
in this example. If you call Convert.ToString()
on a string
variable that's null
then it will return null
instead of string.Empty
.
Solution 5
First of all "any-object as string" and "any-object.ToString()" are completely different things in terms of their respective context.
string str = any-object as string;
1) This will cast any-object as string type and if any-object is not castable to string then this statement will return null without throwing any exception.
2) This is a compiler-service.
3) This works pretty much well for any other type other than string, ex: you can do it as any-object as Employee, where Employee is a class defined in your library.
string str = any-object.ToString();
1) This will call ToString() of any-object from type-defination. Since System.Object defines ToString() method any class in .Net framework has ToString() method available for over-riding. The programmer will over-ride the ToString() in any-object class or struct defination and will write the code that return suitable string representation of any-object according to responsibility and role played by any-object.
2) Like you can define a class Employee and over-ride ToString() method which may return Employee object's string representation as "FIRSTNAME - LASTNAME, EMP-CDOE" .
Note that the programmer has control over ToString() in this case and it has got nothing to do with casting or type-conversion.
Comments
-
jaywon almost 2 years
What is the difference between using the two following statements? It appears to me that the first "as string" is a type cast, while the second ToString is an actual call to a method that converts the input to a string? Just looking for some insight if any.
Page.Theme = Session["SessionTheme"] as string; Page.Theme = Session["SessionTheme"].ToString();
-
Wim over 14 yearsIt doesn't attempt to cast. It checks whether the class is an instance of the specified type and returns the reference if so, else null.
-
Wim over 14 years
ToString()
will 'try to convert any other type to string'? With most built-in types, yes. With your own custom stuff in Session, say a complex User object, all depends whether you've overridenToString()
and how you've implemented it in your class. If not implemented it returns the type info string. -
Philippe Leybaert over 14 yearsYes, of course. But the point here is to illustrate the difference between ToString() and "as string". If I want to provide a complete answer, it would be 2 pages long
-
Wim over 14 yearsMy comment explains it. That's not 2 pages long. It's about being accurate.
-
Philippe Leybaert over 14 yearsI added a few lines to my answer
-
Philippe Leybaert over 14 years@Wim: being accurate can sometimes confuse the OP. Which doesn't mean accuracy is not a good thing, but sometimes it's better to keep an answer short
-
jaywon over 14 yearsappreciate both your guys comments :) Thanks!
-
crashmstr over 8 yearsWhile this is useful, not sure how it directly answers the question of the difference between
as string
and.ToString()
. -
Fortune almost 6 yearswhat would you say about this
string s =o2?.ToString()