Given a couple types like this:
interface I {}
class C : I {}
How can I do a static type cast? By this I mean: how can I change its type in a way that gets checked at compile time?
In C++ you can do static_cast<I*>(c). In C# the best I can do is create a temporary variable of the alternate type and try to assign it:
var c = new C();
I i = c; // statically checked
But this prevents fluent programming. I have to create a new variable just to do the type check. So I've settled on something like this:
class C : I
{
public I I { get { return this; } }
}
Now I can statically convert C to I by just calling c.I.
Is there a better way to do this in C#?
(In case anyone's wondering why I want to do this, it's because I use explicit interface implementations, and calling one of those from within another member function requires a cast to the interface type first, otherwise the compiler can't find the method.)
UPDATE
Another option I came up with is an object extension:
public static class ObjectExtensions
{
[DebuggerStepThrough]
public static T StaticTo<T>(this T o)
{
return o;
}
}
So ((I)c).Doit() could also be c.StaticTo<I>().Doit(). Hmm...probably will still stick with the simple cast. Figured I'd post this other option anyway.
Simply cast it:
(I)c
Edit Example:
var c = new C();
((I)c).MethodOnI();
Write an extension method that uses the trick you mentioned in your UPDATE:
public static class ObjectExtensions
{
public static T StaticCast<T>(this T o) => o;
}
To use:
things.StaticCast<IEnumerable>().GetEnumerator();
If things is, e.g., IEnumerable<object>, this compiles. If things is object, it fails.
// Compiles (because IEnumerable<char> is known at compiletime
// to be IEnumerable too).
"adsf".StaticCast<IEnumerable>().GetEnumerator();
// error CS1929: 'object' does not contain a definition for 'StaticCast'
// and the best extension method overload
// 'ObjectExtensions.StaticCast<IEnumerable>(IEnumerable)'
// requires a receiver of type 'IEnumerable'
new object().StaticCast<IEnumerable>().GetEnumerator();
Why Use a Static Cast?
One common practice during refactoring is to go ahead and make your changes and then verify that your changes have not caused any regressions. You can detect regressions in various ways and at various stages. For example, some types of refactoring may result in API changes/breakage and require refactoring other parts of the codebase.
If one part of your code expects to receive a type (ClassA) that should be known at compiletime to implement an interface (IInterfaceA) and that code wants to access interface members directly, it may have to cast down to the interface type to, e.g., access explicitly implemented interface members. If, after refactoring, ClassA no longer implements IIterfaceA, you get different types of errors depending on how you casted down to the interface:
C-style cast: ((IInterfaceA)MethodReturningClassA()).Act(); would suddenly become a runtime cast and throw a runtime error.
Assigning to an explicitly-typed variable: IInterfaceA a = MethodReturningClassA(); a.Act(); would raise a compiletime error.
Using the static_cast<T>-like extension method: MethodReturningClassA().StaticCast<IInterfaceA>().Act(); would raise a compiletime error.
If you expected your cast to be a downcast and to be verifiable at compiletime, then you should use a casting method that forces compiletime verification. This makes the intentions of the code’s original developer to write typesafe code clear. And writing typesafe code has the benefit of being more verifiable at compiletime. By doing a little bit of work to clarify your intention to opt into typesafety to both other developers, yourself, and the compiler, you magically get the compiler’s help in verifying your code and can catch repercussions of refactoring earlier (at compiletime) than later (such as a runtime crash if your code didn’t happen to have full test coverage).
var c = new C();
I i = c; // statically checked
equals to
I i = new C();
If you're really just looking for a way to see if an object implements a specific type, you should use as.
I i = whatever as i;
if (i == null) // It wasn't
Otherwise, you just cast it. (There aren't really multiple types of casting in .NET like there are in C++ -- unless you get deeper than most people need to, but then it's more about WeakReference and such things.)
I i = (I)c;
If you're just looking for a convenient way to turn anything implementing I into an I, then you could use an extension method or something similar.
public static I ToI(this I #this)
{
return #this;
}
Related
Is it in anyway possible ( preferably without using any third party libs), to create a function whose type is determined at runtime in C#?
e.g
public static void myfunc(var x)
{
System.Windows.Forms.MessageBox.Show(x); //just an example
}
NOTE: I want the runtime to determine the type of the parameter and do not want to later cast the parameter to another type, as would be necessary if I use generics. e.g I don't want:
myfunc<T>(T x)
// and then :
MessageBox.Show((string)m);
UPDATE:
I am actually making a function parser for my programming language, which translates to C# code. In my language, I wanted the parameter types to be determined at runtime always. I was looking for some good C# feature for easy translation.
e.g
in my language syntax:
function msg << x
MessageBox.Show x
end
needed to be translated to something that didn't ask for a type at compile time, but would need one at runtime.
e.g
public static void msg(var x)
{
System.Windows.Forms.MessageBox.Show(x);
}
The keyword introduced for runtime binding in C# 4 is dynamic.
public static void myfunc(dynamic x)
This allows you to make assumptions about x that are unchecked at compile time but will fail at runtime if those assumptions prove invalid.
public static void MakeTheDuckQuack(dynamic duck)
{
Console.WriteLine(duck.Quack());
}
The assumption made here is that the parameter will have a method named Quack that accepts no arguments and returns a value that can then be used as the argument to Console.WriteLine. If any of those assumptions are invalid, you will get a runtime failure.
Given classes defined as
class Duck
{
public string Quack()
{
return "Quack!";
}
}
class FakeDuck
{
public string Quack()
{
return "Moo!";
}
}
And method calls
MakeTheDuckQuack(new Duck());
MakeTheDuckQuack(new FakeDuck());
MakeTheDuckQuack(42);
The first two succeed, as runtime binding succeeds, and the third results in an exception, as System.Int32 does not have a method named Quack.
Generally speaking, you would want to avoid this if possible, as you're essentially stipulating that an argument fulfill an interface of some sort without strictly defining it. If you are working in an interop scenario, then perhaps this is what you have to do. If you are working with types that you control, then you would be better served trying to achieve compile time safety via interfaces and/or base classes. You can even use different strategies (such as the Adapter Pattern) to make types you do not control (or cannot change) conform to a given interface.
If you need to know the type... then you need to know the type. You can't have your cake and eat it too.
First off, the cast in your example is unnecessary as all objects implement ToString(). Instead of telling us what you think you need, tell us what problem you are trying to solve. There is almost certainly a solution either via generics or the use of the dynamic keyword (though dynamic is rarely needed), but we need more info. If you add more I'll update this answer.
You could use a type of object or, if you don't know how many items are available, you could use a params object array, i.e. params object[] cParams.
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
Compile-time and runtime casting c#
As I understand it, the following code will always compile, and will additionally always fail at run-time by throwing an InvalidCastException.
Example:
public class Post { }
public class Question : Post { }
public class Answer : Post
{
public void Fail()
{
Post p = new Post();
Question q = (Question)p; // This will throw an InvalidCastException
}
}
My questions are...
If my assumptions are off, then can someone provide an example demonstrating how they're off?
If my assumptions are correct, then why doesn't the compiler warn against this error?
There are a couple of reasons why this conversion is allowed.
First, as people have said in other answers, the cast operator means "I know more than you do; I guarantee you that this conversion will succeed and if I am wrong, throw an exception and crash the process". If you are lying to the compiler, bad things are going to happen; you in fact are not making that guarantee, and the program is crashing as a result.
Now, if the compiler can tell that you are lying to it, then it can catch you in the lie. The compiler is not required to be arbitrarily clever in catching you in your lies to it! The flow analysis needed to determine that an expression of type Base is never going to be of type Derived is complex; considerably more complex than the logic we already implement to catch things like unassigned local variables. We have better ways to spend our time and effort than in improving the compiler's ability to catch you out in obvious lies.
The compiler therefore typically reasons only about types of expressions, not about possible values. Solely from the type analysis it is impossible to know whether or not the conversion will succeed. It might succeed, and so it is allowed. The only casts that are disallowed are the ones that the compiler knows will always fail from the type analysis.
Second, it is possible to say (Derived)(new Base()) where Derived is a type that implements type Base and have it not fail at runtime. It is also possible for (Base)(new Base()) to fail with an invalid cast exception at runtime! True facts! These are extraordinarily rare situations but they are possible.
For more details, see my articles on the subject:
Chained user-defined explicit conversions in C#
Chained user-defined explicit conversions in C#, Part Two
Chained user-defined explicit conversions in C#, Part Three
A Post could, under some cases, be cast to a Question. By performing the cast, you're telling the compiler, "This will work, I promise. If it doesn't you're allowed to throw an invalid cast exception."
For example, this code would work fine:
Post p = new Question();
Question q = (Question)p;
A cast is expressly stating that you know better than the compiler what this actually is. You may want to do something like the as or is keywords?
The point is that p could be a Question, as question inherits from Post.
Consider the following:
public class Post { }
public class Question : Post { }
public class Banana { }
static class Program {
public static void Main(params string[] args) {
Post p = new Question();
Question q = (Question)p; // p IS a Question in this case
Banana b = (Banana)p; // this does not compile
}
}
When you do an explicit cast, you are telling the compiler "I know something you don't".
You are in essence overriding the normal logic of the compiler - p might be a Question (so, the compiler will compile), you are telling the compiler that you know it is (even though it isn't, hence runtime exception).
1) Your assumption is off. Somebody could always implement an explicit conversion operator for Question to convert from Post:
public class Question`
{
// some class implementation
public static explicit operator Question(Post p)
{
return new Question { Text = p.PostText };
}
}
2) An Explicit cast is your way of telling the compiler that you know better than it does. If you want something to use when you're not sure if a cast will succeed or not and don't want a runtime exception, use the is and as operators.
The compiler treats p as a variable, so, it doesn't try to track it's value. If it did, it'd take so long for analyzing the entire application. Some static analysis tools do like FxCop.
The Compiler sees a Post, but it didn't track the assignment, and it knows that maybe:
Post p = new Question();
So, it passes it normally.
You know you cannot do:
Question q = p;
The difference is in this one you are trying to tell the compiler to use what it knows to validate this, and it knows the Post is not necessarily a Question.
In the original version you are telling the compiler "I know it is, and I will set this explicitly, get off my way and I'll take the exception if what I know is wrong", so, it listens to you and gets off your way!
You assumptions are correct: It will compile and it will fail at runtime.
In your small example it is obvious that the cast will fail, but the compiler have no way of knowing this. Since Post is a supertype of Question you could assign a Question to p, and since you make the cast you do declare a willingness take some responsibility from the compiler. Had you been trying to assign a string or something else not part of the same inheritance branch the compiler should warn you. Conversely you can always try to cast object to any type.
But having the compiler complaining about your specific example would mean that no casts would be allowed.
Wow Jeremy, I ran into this exact problem recently! So I made this handy extension method that maps two models that share a few identical properties. The intention was to use it when class A inherits from class B to map class B to class A. Hope you find it helpful!
public static class ObjectHelper
{
public static T Cast<T>(this Object source)
{
var destination = (T)Activator.CreateInstance(typeof(T));
var sourcetype = source.GetType();
var destinationtype = destination.GetType();
var sourceProperties = sourcetype.GetProperties();
var destionationProperties = destinationtype.GetProperties();
var commonproperties = from sp in sourceProperties
join dp in destionationProperties on new { sp.Name, sp.PropertyType } equals
new { dp.Name, dp.PropertyType }
select new { sp, dp };
foreach (var match in commonproperties)
{
match.dp.SetValue(destination, match.sp.GetValue(source, null), null);
}
return destination;
}
}
FYI, it will probably only work if the two objects exists in the same assembly.
Much of the code came from here: Mapping business Objects and Entity Object with reflection c#
I am experimenting with fluent extension methods.
I have the following simple extension method to perform a safe cast.
public static T As<T>(this Object source)
where T : class
{
return source as T;
}
This worked well, but when I tried to make it intuitive to use valuetypes with an overload
public static T As<T>(this ValueType source)
where T : struct
{
return (T)source;
}
I ran into problems. The method resolution logic always chooses the first method above, and gives a syntax error (accurately) that the struct is not a class.
Is there a way to handle the above, or should I go the route of removing the constraint while testing for and handling all types in the same method?
==== Edit: to answer questions ====
I am compiling this against the 3.5 framework. I'm not really trying to accomplish anything in particular; this is just an experiment with the above. My interest was piqued and I threw together some code.
I'm not particularly concerned with it remaining a 'safe' cast. That is how it started, and can be kept safe with default() -- but that's not really the focus of the question and code to ensure 'safeness' would just obscure.
As to the expressiveness, no value.As<int>() is not any more expressive than (int)value; but why should the user of the method have to 'just know' it only works with reference types? My trying to work it in was more about the expected behavior of the method than expressive writing.
The code snippet value.As<DateTime>(), gives the error "The type 'System.DateTime' must be a reference type in order to use it as parameter 'T' in the generic type or method ....As(object)". From the error message I see it is resolving to use the top method above as it is the one requiring the reference type.
In .NET 4, the second overload is chosen based on your code sample. (Also just tested against .NET 3.5, same result.)
int myInt = 1;
long myLong = myInt.As<long>(); // chooses ValueType version
However, this only gets us to the scene of the next crash. The (T)source; results in an invalid cast exception. You could get around that by writing the method as
public static T As<T>(this ValueType source)
where T : struct
{
return (T)Convert.ChangeType(source, typeof(T));
}
However, I wonder what you're actually looking to achieve, as I do not see the immediate benefit. (And for that matter, this isn't safe like the source as T object version.) For example, how is
long myLong = myInt.As<long>();
Any more expressive or easier to use than
long myLong = (long)myInt;
Consider the following example program:
using System;
public delegate string MyDelegateType(int integer);
partial class Program
{
static string MyMethod(int integer) { return integer.ToString(); }
static void Main()
{
Func<int, string> func = MyMethod;
// Scenario 1: works
var newDelegate1 = new MyDelegateType(func);
newDelegate1(47);
// Scenario 2: doesn’t work
dynamic dyn = func;
var newDelegate2 = new MyDelegateType(dyn);
newDelegate2(47);
}
}
The first one works as expected — the conversion to MyDelegateType succeeds. The second one, however, throws a RuntimeBinderException with the error message:
Cannot implicitly convert type 'System.Func<int,string>' to 'MyDelegateType'
Is there anything in the C# specification that allows for this behaviour, or is this a bug in Microsoft’s C# compiler?
Good catch Timwi.
Our support for dynamic method groups is weak. For example, consider this simpler case:
class C
{
public void M() {}
}
class P
{
static void Main()
{
dynamic d = new C();
C c = new C();
Action a1 = c.M; // works
Action a2 = d.M; // fails at runtime
The d.M is interpreted as a property get (or field access) by the dynamic runtime, and when it resolves as a method group, it fails at runtime.
The same thing is happening in your case, it is just a bit more obscure. When you say MyDelegate x = new MyDelegate(someOtherDelegate); that is treated by the compiler just as if you'd said MyDelegate x = someOtherDelegate.Invoke;. The dynamic runtime piece does not know to do that transformation, and even if it did, it couldn't handle resolving the method group that is the result of the .Invoke portion of the expression.
Is there anything in the C# specification that allows for this behaviour, or is this a bug in Microsoft’s C# compiler?
The spec does not call out that this should be a runtime error, and does imply that it should be handled correctly at runtime; clearly the implementation does not do so. Though it is a shortcoming of the implementation I wouldn't call this a "bug" because we deliberately made the behaviour you've discovered. We did not have the resources to make these kinds of expressions work exactly right, so we left them as errors. If we ever get a good way to represent method groups in the dynamic runtime, we might implement it.
Similarly there is no way in dynamic code to represent the notion of "this dynamic thing is a lambda expression where the types of the parameters are to be determined at runtime". If we have a good way to represent those in the future, we might do the work.
Sam talked a bit about this back in 2008; see his article on it:
http://blogs.msdn.com/b/samng/archive/2008/11/02/dynamic-in-c-ii-basics.aspx
I've run in to this limitation too. Although I couldn't answer the why better than Eric Lippert, there is a straight forward workaround.
var newDelegate2 = new MyDelegateType(x => dyn(x));
It implicitly gets the static signature from the delegate and the dynamic invocation works without any more info. This works for delegates and, as a bonus, dynamic callable objects.
I know that methods declared with void does not return anything.
But it seems that in C#, void is more than just a keyword, but a real type.
void is an alias for System.Void, like int that is for System.Int32.
Why am I not allowed to use that type? It does not make any sense, but these are just some thoughts about the logic.
Neither
var nothing = new System.Void();
(which says I should use void (Not an alias?))
nor
var nothing = new void();
compiles.
It is also not possible to use something like that:
void GiveMeNothing() { }
void GiveMeNothingAgain()
{
return GiveMeNothing();
}
So what's the point with System.Void?
From the documentation:
The Void structure is used in the
System.Reflection namespace, but is
rarely useful in a typical
application. The Void structure has no
members other than the ones all types
inherit from the Object class.
There's no reason really to use it in code.
Also:
var nothing = new void();
This doesn't compile for me. What do you mean when saying it "works"?
Update:
A method void Foo() does not return anything. System.Void is there so that if you ask (through Reflection) "what is the type of the return value of that method?", you can get the answer typeof(System.Void). There is no technical reason it could not return null instead, but that would introduce a special case in the Reflection API, and special cases are to be avoided if possible.
Finally, it is not legal for a program to contain the expression typeof(System.Void). However, that is a compiler-enforced restriction, not a CLR one. Indeed, if you try the allowed typeof(void) and look at its value in the debugger, you will see it is the same value it would be if typeof(System.Void) were legal.
void/System.Void is different from int/System.Int32, it's a special struct in C#, used for reflection only. See this example:
class Program
{
public static void Main(string[] args)
{
Type voidType = typeof(Program).GetMethod("Main").ReturnType;
}
}
There must be some type used to describe the return type of Main method here, that's why we have System.Void.
We have used the following code
public Type GetType(object o)
{
var type = o == null ? typeof(void) : o.GetType();
}
so that we can use the null object pattern. It's pretty good.
This allows us to do stuff like
GetType(o).GetProperties().Select( .....
instead of putting guard clauses everywhere.
Beyond not returning a value, very little definition is given of void (although void* gets some attention) in the language spec. This isn't really a language concern - although the CLI may define it further.
Ultimately though: because it has no meaning to do new void()