I have the following method with an overload:
public string GetName(object obj)
{
return obj.ToString();
}
public string GetName(CustomClass cc)
{
return cc.Name + " - " + cc.Description;
}
Now if I call the method with an untyped IEnumerable wich holds CustomClass the GetName(object obj) gets called, to fix this I have modified the method like this:
public string GetName(object obj)
{
if (obj is CustomClass)
return GetName(obj as CustomClass);
return obj.ToString();
}
I think its rather annoying to write 20 IF statements and catch all the other possibilities, is there an easier way to call the correct overload with an untyped IEnumerable enumeration?
Here is the code that calls the GetName(object obj):
IEnumerable rawData = GetData(); //DataBase method that fetches a CustomClass
foreach (var rawDataItem in rawData)
{
Debug.Print(GetName(rawDataItem)); //calls the GetName(object obj) overload
}
Pls dont tell me to override ToString from my CustomClass, help me fix this method calling problem.
Well, you could use dynamic typing. That will basically defer overload resolution until execution time:
foreach (dynamic rawDataItem in rawData)
{
Debug.Print(GetName(rawDataItem));
}
Note that there's potentially a performance cost here - it may well be minimal and insignificant, but it's worth being aware of.
EDIT: To handle the recursion side of things, you'd probably want two different names, e.g. GetName and GetNameImpl where GetName delegates to GetNameImpl which is what all the useful overloads are called. So you'd have:
// Note that dynamic as a parameter type is equivalent to object for callers.
// The dynamic part is only relevant within the method.
public string GetName(dynamic obj)
{
return GetNameImpl(obj);
}
// Fallback when no other overloads match
private string GetNameImpl(object obj)
{
...
}
private string GetNameImpl(IEnumerable obj)
{
// Maybe build up the name by calling GetName on each element?
}
Note that there's a potential problem with this: if you have two overloads for different interfaces and one type implements both interfaces (but there isn't a specific overload for that type itself) then you'll get an exception at execution time.
If you want callers to be able to call the overloads directly, you could just rename the dynamic one to GetNameDynamic and the others to GetName for example (and make them public).
I rarely find that dynamic is a good solution, but it would avoid the code duplication. I would try to step back and find a different design to be honest. You explicitly rejected it in the question, but polymorphism is the preferred way of handling this. You don't need to necessarily override ToString - you could make all of the custom types implement a particular interface, and use that where it's available, for example.
return GetName((dynamic)obj);
will postpone overload resolution till runtime.
Without dynamic typing, the classic OOP solution to supporting double dispatch (where the method called depends on both the concrete type of the object having the method and the concrete type of the passed object) is the visitor pattern.
Try this:
public string GetName(object obj)
{
if (!(obj is IEnumerable<object>))
return GetName(obj as CustomClass);
return obj.ToString();
}
Related
Using dynamic pattern perhaps? You can call any method/property using the dynamic keyword, right? How to check whether the method exist before calling myDynamicObject.DoStuff(), for example?
You could write something like that :
public static bool HasMethod(this object objectToCheck, string methodName)
{
var type = objectToCheck.GetType();
return type.GetMethod(methodName) != null;
}
Edit : you can even do an extension method and use it like this
myObject.HasMethod("SomeMethod");
via Reflection
var property = object.GetType().GetProperty("YourProperty")
property.SetValue(object,some_value,null);
Similar is for methods
It is an old question, but I just ran into it.
Type.GetMethod(string name) will throw an AmbiguousMatchException if there is more than one method with that name, so we better handle that case
public static bool HasMethod(this object objectToCheck, string methodName)
{
try
{
var type = objectToCheck.GetType();
return type.GetMethod(methodName) != null;
}
catch(AmbiguousMatchException)
{
// ambiguous means there is more than one result,
// which means: a method with that name does exist
return true;
}
}
Wouldn't it be better to not use any dynamic types for this, and let your class implement an interface.
Then, you can check at runtime wether an object implements that interface, and thus, has the expected method (or property).
public interface IMyInterface
{
void Somemethod();
}
IMyInterface x = anyObject as IMyInterface;
if( x != null )
{
x.Somemethod();
}
I think this is the only correct way.
The thing you're referring to is duck-typing, which is useful in scenarios where you already know that the object has the method, but the compiler cannot check for that.
This is useful in COM interop scenarios for instance. (check this article)
If you want to combine duck-typing with reflection for instance, then I think you're missing the goal of duck-typing.
To avoid AmbiguousMatchException, I would rather say
objectToCheck.GetType().GetMethods().Count(m => m.Name == method) > 0
public static object GetObject(int x)
{
return new object { };
}
public static object GetObject(string x)
{
return new object { };
}
public static void ProcessObject<T>(T x) where T : int, string <= got error here:
{
object o = GetObject(x);
}
Got error "A type used as a constraint must be an interface, a non-sealed class or a type parameter."
How can I rewrite the code to get it work without write ProcessObject(int x) and ProcessObject(string x) twice?
So what you have now (according to accepted answer and your comments) is:
public static void ProcessObject<T>(T x)
{
object o;
if (typeof(T) == typeof(int))
o = GetObject((int)(object)x);
else if (typeof(T) == typeof(string))
o = GetObject((string)(object)x);
else
throw new Exception();
// do stuff with o
}
I'd recommend making public int and string overloads, but to prevent code duplication, internally call another method:
public static void ProcessObject(int x)
{
ProcessObject(GetObject(x));
}
public static void ProcessObject(string x)
{
ProcessObject(GetObject(x));
}
private static void ProcessObject(object o)
{
// do stuff with o
}
This makes your public methods' input values clear: int and string are the only acceptable types, while still not duplicating your actual logic (// do stuff with o).
You might dislike that the two public ProcessObject methods are duplicates of each other, (on the surface anyway; under the hood, they're calling two different GetObject overloads) but I think it's the best option.
You cannot do what you are trying to do: first, it is not possible to list several classes in a generic constraint; second, the type that you can put in a constraint must be such that you could inherit it (or implement it if it is an interface). Both int and string fail this check. In cases like this, you would be better off with two separate overloads.
Just remove the where part
public static void ProcessObject<T>(T x)
{
object o = GetObject(x);
}
And also don't use object in your other methods, instead use T
it's impossible in C# take a look on Constraints on Type Parameters. Try to use dynamic
Generally speaking, if your object reacts differently based on the generic type argument, you probably shouldn't be using generics in this case. Generics are great for situations where you want to do always the same thing, no matter what the actual type used.
Therefore, generic constraints will only allow you to list one base class for a type argument. Any actual types passed to the respective type arguments are meant to be a part of the given inheritance hierarchy starting with the base class you specified, so users of your class can specify any type that matches the base class or any of its subclasses.
At the same time, you, the author of the generic class, can safely assume that the specified type has (at least) the interface of the base class indicated by the constraint. Hence, you may access any members of the base class.
If you want to allow either string or int, imagine what members that could be. Both are derived directly from System.Object, hence the restriction would make no sense as it is no restriction; every type is derived from System.Object.
Summarizing, if you really want to treat string and int differently, this is definitely a case for offering two overloads rather than one generic class.
Is there a way to invoke a generic function with a type known only at run time?
I'm trying to do something like:
static void bar()
{
object b = 6;
string c = foo<typeof(b)>();
}
static string foo<T>()
{
return typeof (T).Name;
}
Basically I want to decide on the type parameter only at run time, but the function I'm calling depends on the type parameter.
Also I know this can be done with reflections... but it's not the nicest solution to the problem...
I'm sort of looking for dynamic features in C#...
I'm writhing a bridge between two classes the first one is basically a big tree with different types of of objects (composite by interface) the other is a sort of a "super visitor".
the supper visitor accepts key-value dictioneries that map types to object it looks like:
dic.Add(object value)
and T is not necessarily the type of the value... a lot of times it isn't...
I know it's written poorly, but i can't fix it...
I can work around it, but only at runtime...
I already did it with reflections, but if there's a better way to do it without them i would be happy to learn...
Thank you
This is a bit of a hack but you can get dynamic to do the reflection work for you by something like,
class Program
{
static void Main(string[] args)
{
var b = 6;
var t = (dynamic)new T();
var n = t.Foo(b);
}
class T
{
public string Foo<T>(T a)
{
return typeof(T).Name;
}
}
}
Here the dynamic call will extract the type of b and use it as a type parameter for Foo().
You can use dynamic keyword if you're using .NET 4. In a word, the type of the variable will be resolved at run time so it is a super generic type ;) You can read a article here or read the MSDN documentation
Saly refelction is THE solution to the problem, whether it is nice or not is irrelevant here. It is the runtime designed mechanism to achieve exactly this. As there is no parameter or generics to use as input, this is the only way to do it - it is also senseless. As in: your example is bad. Because in the example the type is hardcoded.
If the method where b exists has b as generic parameter, the type is available for passing to foo. If not - reflection is THE way to go, albeit the syntax looks clumsy. Only one time, though.
This I believe is the only way:
var foo = typeof(Foo<>).MakeGenericType(typeof (bar));
You can set up a class which takes a type parameter at run time which can be used in the methods in that class.
public class GenericClass<T>()
{
ICommonInterface TheObject;
public GenericClass(T theObject)
{
TheObject = theObject;
}
public string GetName()
{
return TheObject.Name;
}
}
But this is only really useful if the Types being passed in share interfaces so have common properties between them. In your example it seems that relection is the answer as depending on the type you want to access specific properties.
I have the following C# class:
public class MyType<T>
{
public void TryParse(string p_value)
{
T value ;
Parser.TryParse(p_value, out value);
// Do something with value
}
}
The point is to call the right Parser.TryParse method, depending on the generic type T.
This uses the following static class:
static public class Parser
{
static public void TryParse(string p_intput, out object p_output)
{
// Do something and return the right value
}
static public void TryParse(string p_intput, out double p_output)
{
// Do something and return the right value
}
static public void TryParse(string p_intput, out int p_output)
{
// Do something and return the right value
}
}
I expected this to work: In the worst case, the "object" TryParse would be called. Instead, I have two compilation errors:
CS1502: The best overloaded method match for 'Parser.TryParse(string, out object)' has some invalid arguments
CS1503: Argument 2: cannot convert from 'out T' to 'out object'
Question 1: I don't understand why this doesn't work: I can be naive, but aren't all C# objects supposed to derive from "object" ? Why T cannot be converted to object?
Question 2: How can I dispatch a method with generic type T into the right non-generic methods (i.e. MyType<T>.TryParse calling the right Parser.TryParse according to the right type of T) ?
Note
The question was edited to reflect the original question intent (as written in the title: How to dispatch C# generic method call into specialized method calls)
Actually, ref and out parameters do not allow type variation. So, to pass a variable to a method expecting an out object parameter, that variable must be declared as object.
From the specification (§10.6.1.2 and §10.6.1.3)
When a formal parameter is a reference parameter, the corresponding argument in a method invocation must consist of the keyword ref followed by a variable-reference (§5.3.3) of the same type as the formal parameter.
When a formal parameter is an output parameter, the corresponding argument in a method invocation must consist of the keyword out followed by a variable-reference (§5.3.3) of the same type as the formal parameter.
See: Why do ref and out parameters not allow type variation? for some insight into why.
Bonus question: How can I dispatch a method with generic type T into the right non-generic methods (i.e. MyType<T>.TryParse calling the right Parser.TryParse according to the right type of T)?
I'm going to turn it back around on you. Why are you doing this? If you are invoking MyType<T>.TryParse as, say, MyType<int>.TryParse, why not call Int32.TryParse directly? What is this extra layer buying you?
I know this is somewhat low-tech, but I have had the same problem, where I solved it by making a Dictionary<Type, Parser> containing the individual parsers. I will be interested in what answers this questions bring.
Regards,
Morten
Current solution
The current solution I use at work is based on dynamic dispatch, that is, the keyword dynamic as defined on C# 4.0.
The code is something like (from memory) :
public class Parser
{
static public void TryParse<T>(string p_input, out T p_output)
{
// Because m_p is dynamic, the function to be called will
// be resolved at runtime, after T is known...
m_p.DoTryParse(p_input, out p_output) ;
}
// The dynamic keyword means every function called through
// m_p will be resolved at runtime, at the moment of the call
private dynamic m_p = new Parser() ;
// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
private void DoTryParse(string p_input, out double p_output)
{ /* Do something and return the right value */ }
private void DoTryParse(string p_input, out int p_output)
{ /* Do something and return the right value */ }
// etc.
private void DoTryParse<T>(string p_input, out T p_output)
{
// fallback method... There are no dedicated method for T,
// so p_output becomes the default value for T
p_output = default(T) ;
}
}
The elegant part is that it can't fail (the fallback function will be called, if none with a better signature match is found), and that it follows a simple pattern (overload the function).
Of course, the real-life, production code is somewhat different, and more complicated because, with but one public static method, I want to :
parse both reference objects (classes) and value objects (structs)
parse enums
parse nullable types
I want to offer the user the possibility to derive from Parser to offer its own overloads in addition to the default ones
But I guess the use of dynamic in the current solution is, in the end, the same thing as doing reflection as done in the original answer below. Only the "notation" changes.
Conclusion, I now have the following method :
public class Parser
{
static public void TryParse<T>(string p_input, out T p_output)
{
// etc.
}
}
which is able to parse anything, including in situations where T is not known at compile time (because the code is generic).
Original answer
Jason's answer was right about the first question (about the compiler errors). Still, I had no solution to my problem (dispatching from a generic method to non-generic methods according to the runtime generic type T).
I tried LukeH's answer, but it didn't work: The generic method is always called, no matter what (even when removing the out qualifier of the second parameter).
Morten's answer is the most sane one that should works, but it doesn't make use of reflection.
So, to solve my own problem, I used reflection. This needs the rewriting of the generic TryParse method:
public class MyType<T>
{
public void TryParse(string p_value)
{
T value = default(T);
// search for the method using reflection
System.Reflection.MethodInfo methodInfo = typeof(Parser).GetMethod
(
"TryParse",
new System.Type[] { typeof(string), typeof(T).MakeByRefType() }
);
if (methodInfo != null)
{
// the method does exist, so we can now call it
var parameters = new object[] { p_value, value };
methodInfo.Invoke(null, parameters);
value = (T)parameters[1];
}
else
{
// The method does not exist. Handle that case
}
}
}
I have the source code available if needed.
This problem intrigued me, so I did some research and found a nice thing by Paul Madox. This seems to do the trick.
public static T SafeParseAndAssign<T>(string val) where T: new()
{
try
{
T ValOut = new T();
MethodInfo MI = ValOut.GetType().
GetMethod("Parse", new Type[] { val.GetType() });
return (T)MI.Invoke(ValOut, new object[] { val });
}
catch
{
// swallow exception
}
return default(T);
}
I'm currently adding some new extended classes to this code:
foreach (BaseType b in CollectionOfExtendedTypes) {
if (b is ExtendedType1) {
((ExtendedType1) b).foo = this;
}
else if (b is ExtendedType2) {
((ExtenedType2) b).foo = this;
}
else {
b.foo = this;
}
}
and was curious if there is a way to use the is keyword functionality in a switch statement?
The latest version of C# (7) now includes this functionality
Type pattern
The type pattern enables concise type evaluation and conversion. When used with the switch statement to perform pattern matching, it tests whether an expression can be converted to a specified type and, if it can be, casts it to a variable of that type. Its syntax is:
case type varname
This really looks like a situation for a good polymorphic implementation. If you override the appropriate methods in the derived classes, you may not need the checks in the loop at all.
Nope. See
C# switch statement limitations - why?
In C# it's not possible to use the "is" keyword as part of a switch statement. All case labels in a switch must evaluate to constant expressions. "is" is not convertible to a constant expression.
I definately feel the pain though when it comes to switching on types. Because really the solution you outlined works but it's a conveluted way of saying for x do y, and a do b. It would be much more natular to write it more like the following
TypeSwitch.Do(
sender,
TypeSwitch.Case<Button>(() => textBox1.Text = "Hit a Button"),
TypeSwitch.Case<CheckBox>(x => textBox1.Text = "Checkbox is " + x.Checked),
TypeSwitch.Default(() => textBox1.Text = "Not sure what is hovered over"));
Here's a blog post I wrote on how to achieve this functionality.
http://blogs.msdn.com/jaredpar/archive/2008/05/16/switching-on-types.aspx
As mentioned in the answer from MikeT, you are able to use pattern mathing which requires C# 7.
Here's an example for your code:
foreach (BaseType b in CollectionOfExtendedTypes) {
switch (b) {
case ExtendedType1 et1:
// Do stuff with et1.
et1.DoStuff();
break;
case ExtendedType2 et2:
// Do stuff with et2.
et2.DoOtherStuff();
break;
default:
// Do something else...
break;
}
}
While it is not possible to use switch statement for checking types, it is not impossible to reduce the problem to a more manageable codebase.
Depending on the specific situation and requirement I would consider.
Using a IDictionary<Type, T> to store the result in a dictionary. T could itself be a delegate that you can call on. This will work if you don't need to worry about inheritance - catering for inheritance will take a little more work.
Using the type name of the class (which is string) inside the switch statement. This uses switch (b.GetType().Name) and there is no option for deep inheritance structure.
You could add a method getType() to BaseType that is implemented by each concrete subclass to return a unique integral ID (possibly an enum) and switch on that, yes?
Not really, switches match a variable (string or int (or enum) ) with a constant expression as the switch statement.
http://msdn.microsoft.com/en-us/library/06tc147t(VS.71).aspx
Type-cases and object oriented code don't seem to get on that well together in my experience. The approach I prefer in this situation is the double dispatch pattern. In short:
Create a listener type with an empty virtual method Process(ExtendedTypeN arg) for each extended type you will be dispatching over.
Add a virtual method Dispatch(Listener listener) to the base type that takes a listener as argument. Its implementation will be to call listener.Process((Base) this).
Override the Dispatch method in each extended type to call the appropriate overload of Process in the listener type.
Extend the listener type by overriding the appropriate Process method for each subtype you are interested in.
The argument shuffling dance eliminates the narrowing cast by folding it into the call to Dispatch -- the receiver knows its exact type, and communicates it by calling back the exact overload of Process for its type. This is also a big performance win in implementations such as .NET Compact Framework, in which narrowing casts are extremely slow but virtual dispatch is fast.
The result will be something like this:
public class Listener
{
public virtual void Process(Base obj) { }
public virtual void Process(Derived obj) { }
public virtual void Process(OtherDerived obj) { }
}
public class Base
{
public virtual void Dispatch(Listener l) { l.Process(this); }
}
public class Derived
{
public override void Dispatch(Listener l) { l.Process(this); }
}
public class OtherDerived
{
public override void Dispatch(Listener l) { l.Process(this); }
}
public class ExampleListener
{
public override void Process(Derived obj)
{
Console.WriteLine("I got a Derived");
}
public override void Process(OtherDerived obj)
{
Console.WriteLine("I got an OtherDerived");
}
public void ProcessCollection(IEnumerable collection)
{
foreach (Base obj in collection) obj.Dispatch(this);
}
}
In C#, I believe the switch statement only works with integers and strings.
There's another thing to think about besides the way that the compiler handles switch statements, and that's the functioning of the is operator. There's a big difference between:
if (obj is Foo)
and
if (obj.GetType() == typeof(Foo))
Despite the name, the is operator tells you if an object is compatible with a given type, not if it is of the given type. This leads to not-entirely-obvious bugs (though this one's pretty obvious) that look like:
if (obj is System.Object)
{
//this will always execute
}
else if (obj is Foo)
{
//this will never execute
}
Many of the suggestions here point you in the direction of using the object's type. That's fine if what you really want is logic associated with each type. But if that's the case, walk carefully when using the is operator.
Also: though you can't modify these base types, that doesn't mean that you can't use Owen's suggestion. You could implement extension methods:
public enum MyType { Foo, Bar, Baz };
public static class MyTypeExtension
{
public static MyType GetMyType(this Foo o)
{
return MyType.Foo;
}
public static MyType GetMyType(this Bar o)
{
return MyType.Bar;
}
public static MyType GetMyType(this Baz o)
{
return MyType.Baz;
}
}
Then you can use a switch statement:
switch (myObject.GetType())
{
case MyType.Foo:
// etc.