I write this piece of code in one of my C# project:
public static class GetAppendReceiver
{
public static AppendReceiver<DataType> Get<DataType>(AppendReceiver<DataType>.DataProcessor processor0, int delayIn = 0)
{
throw new InvalidOperationException();
}
public static AppendReceiver<string> Get(AppendReceiver<string>.DataProcessor processor0, int delayIn = 0)
{
return new StringAppendReceiver(processor0, delayIn);
}
}
public abstract class AppendReceiver<DataType>
{
public delegate void DataProcessor(DataType data);
...
}
AppendReceiver<DataType> is an Abstract class, DataProcessor is a delegate type.
When calling GetAppendReceiver.Get with a string DataProcessor I expect the overloaded function to be called, but I get the InvalidOperationException.
Here is my call:
class ClassA<DataType>
{
public void RegisterAppendReceiver(AppendReceiver<DataType>.DataProcessor receiver)
{
appendReceivers.Add(GetAppendReceiver.Get(receiver, Delay));
}
}
Example of RegisterAppendReceiver call:
myObject.RegisterAppendReceiver(myMethod);
Where myMethod is defined like this:
public void writeMessage(string strMessageIn)
My question is why I get the wrong overload called, and how can I force the language to call the overload I want ?
Thanks for your help !
Eric Lippert answers this question concisely in his article Generics are not Templates
I don't want to copy the entire article. So the relevant point is this:
We do the overload resolution once and bake in the result.
So the C# compiler decides, at the time it compiles RegisterAppendReceiver, which overload of "GetAppendReceiver.Get" it is going to call. Since, at that point, the only thing it knows about DataType is that DataType can be anything at all, it compiles in the call to the overload that takes an AppendReceiver.DataProcessor, not an AppendReceiver.DataProcessor.
By comparison, the C++ compiler does not behave this way. Each and every time a generic call is made, the compiler does the substitution over again. This is one reason C++ compilers are much slower than C# compilers.
Related
I'm not sure exactly how to call this situation, but here it is:
I have 2 methods with different types, but from the "outside" they have the same signature. And when calling the method, I would like to invoke a specific method instead of the other - here is what I have:
public class SomeClass
{
public async Task<Response<T>> MyMethod<T>(string name, T myObj)
{
// some code here
}
public async Task<Response<T>> MyMethod<T>(string name, string myObj)
{
// some code here
}
}
For the compiler, the 2 methods above are different and have different signatures. But when calling:
var myClass = new SomeClass();
myClass.MyMethod("name", "something");
When calling MyMethod in the example, what's being called is MyMethod<T>(string name, T myObj), but what I would actually want to call is the second method. Is there a way to make it call a specific signature?
EDIT:
I found the if I give in one of the methods a different name to the second variable and then calling the method with the variable name as part of the call it does work, as in the following example:
public class SomeClass
{
public async Task<Response<T>> MyMethod<T>(string name, T myObj)
{
// some code here
}
public async Task<Response<T>> MyMethod<T>(string name, string myNewObj)
{
// some code here
}
}
var myClass = new SomeClass();
myClass.MyMethod<AnyOfMyTypes>("name", myNewObj: "something");
While this works, following Jon's response below, does it seem like something that is correct to do? As far as for the method name, I would like to keep it the same, and the other option is to change the signature by adding some dummy boolean variable.
The compiler certainly can't call the second method with that calling code, as it wouldn't know what to infer for T. If you specify a type argument though, it does call the second method:
myClass.MyMethod<string>("name", "something");
While that will work, I would strongly advise you to change the design if you possibly can. Rename one of the methods. I can reasonably call myself a C# expert, but I couldn't predict from inspection whether or not that would work. The overload resolution and type inference details in C# are really complicated, and it's unreasonable to expect every C# developer to know them inside out.
If you can give the methods different names, the code code is likely to be a lot simpler to read.
Following Jon's answer I'd suggest following solution to your problem:
private async Task<Response<T>> MethodForString<T>(string str)
{
// some code...
}
public async Task<Response<T>> Method<T>(T obj)
{
if (typeof(T) == typeof(string))
return MethodForString<T>(obj as string);
// some code...
}
Above is just a sample, but idea is simple: just check type of T and call appropriate method :)
This way, all your method call will remain exactly the same :)
The reason the first method is being called is because you have a generic type of T.
If you pass a string into this then it will hit the first method. It is better design to only have one method. If you want to be able to pass in any type then keep the first method, if you want just a string then keep just the second method. You should only have two methods if both have distinct clear purposes.
This
public class SomeClass
{
public async Task<Response<T>> MyMethod<T>(string name, T myObj)
{
// some code here
}
}
Or
public class SomeClass
{
public async Task<Response<T>> MyMethod<T>(string name, string myObj)
{
// some code here
}
}
Or
public class SomeClass
{
public async Task<Response<T>> MyMethod<T>(string name, string myObj)
{
// some code here
}
public async Task<Response<T>> MyMethod<T>(string name, int age, string address)
{
// some code here
}
}
Dont put a dummy variable into any of the methods, this will lead to problems debugging and be misleading for other developers.
The new Task.Run static method that's part of .NET 4.5 doesn't seem to behave as one might expect.
For example:
Task<Int32> t = Task.Run(()=>5);
compiles fine, but
Task<Int32> t = Task.Run(MyIntReturningMethod);
...
public Int32 MyIntReturningMethod() {
return (5);
}
complains that MyIntReturningMethod is returning the wrong type.
Perhaps I am just not understanding which overload of Task.Run is being called. But in my mind, my lambda code above looks a lot like a Func<Int32>, and MyIntReturningMethod is definitely compatible with Func<Int32>
Any ideas of what's going on?
Michael
(Of course, to get out of the problem, simply say Task.Run((Func<int>)MyIntReturningMethod).)
This has absolutely nothing to do with Task and so on.
One problem to be aware of here is that when very many overloads are present, the compiler error text will focus on just one "pair" of overloads. So that is confusing. The reason is that the algorithm to determine the best overload considers all overloads, and when that algorithm concludes that no best overload can be found, that does not produce a certain pair of overloads for the error text because all overloads may (or may not) have been involved.
To understand what happens, see instead this simplified version:
static class Program
{
static void Main()
{
Run(() => 5); // compiles, goes to generic overload
Run(M); // won't compile!
}
static void Run(Action a)
{
}
static void Run<T>(Func<T> f)
{
}
static int M()
{
return 5;
}
}
As we see, this has absolutely no reference to Task, but still produces the same problem.
Note that anonymous function conversions and method group conversions are (still) not the exact same thing. Details are to be found in the C# Language Specification.
The lambda:
() => 5
is actually not even convertible to the System.Action type. If you try to do:
Action myLittleVariable = () => 5;
it will fail with error CS0201: Only assignment, call, increment, decrement, await, and new object expressions can be used as a statement. So it is really clear which overload to use with the lambda.
On the other hand, the method group:
M
is convertible to both Func<int> and Action. Remember that it is perfectly allowed to not pick up a return value, just like the statement:
M(); // don't use return value
is valid by itself.
This sort-of answers the question but I will give an extra example to make an additional point. Consider the example:
static class Program
{
static void Main()
{
Run(() => int.Parse("5")); // compiles!
}
static void Run(Action a)
{
}
static void Run<T>(Func<T> f)
{
}
}
In this last example, the lambda is actually convertible to both delegate types! (Just try to remove the generic overload.) For the right-hand-side of the lambda arrow => is an expression:
int.Parse("5")
which is valid as a statement by itself. But overload resolution can still find a better overload in this case. As I said earlier, check the C# Spec.
Inspired by HansPassant and BlueRaja-DannyPflughoeft, here is one final (I think) example:
class Program
{
static void Main()
{
Run(M); // won't compile!
}
static void Run(Func<int> f)
{
}
static void Run(Func<FileStream> f)
{
}
static int M()
{
return 5;
}
}
Note that in this case, there is absolutely no way the int 5 could be converted into a System.IO.FileStream. Still the method group conversion fails. This might be related to the fact the with two ordinary methods int f(); and FileStream f();, for example inherited by some interface from two different base interfaces, there is no way to resolve the call f();. The return type is not part of a method's signature in C#.
I still avoid to introduce Task in my answer since it could give a wrong impression of what this problem is about. People have a hard time understanding Task, and it is relatively new in the BCL.
This answer has evolved a lot. In the end, it turns out that this is really the same underlying problem as in the thread Why is Func<T> ambiguous with Func<IEnumerable<T>>?. My example with Func<int> and Func<FileStream> is almost as clear. Eric Lippert gives a good answer in that other thread.
This was supposed to be fixed in .Net 4.0, but Task.Run() is new to .Net 4.5
.NET 4.5 has its own overload ambiguity by adding the Task.Run(Func<Task<T>>) method. And the support for async/await in C# version 5. Which permits an implicit conversion from T foo() to Func<Task<T>>.
That's syntax sugar that's pretty sweet for async/await but produces cavities here. The omission of the async keyword on the method declaration is not considered in the method overload selection, that opens another pandora box of misery with programmers forgetting to use async when they meant to. Otherwise follows the usual C# convention that only the method name and arguments in the method signature is considered for method overload selection.
Using the delegate type explicitly is required to resolve the ambiguity.
When you pass a Func<TResult> into a method Run<TResult>(Func<TResult>) you don't have to specify the generic on the methodcall because it can infer it. Your lambda does that inference.
However, your function is not actually a Func<TResult> whereas the lambda was.
If you do Func<Int32> f = MyIntReturningMethod it works. Now if you specify Task.Run<Int32>(MyIntReturningMethod) you would expect it to work also. However it can't decide if it should resolve the Func<Task<TResult>> overload or the Func<TResult> overload, and that doesn't make much sense because its obvious that the method is not returning a task.
If you compile something simple like follows:
void Main()
{
Thing(MyIntReturningMethod);
}
public void Thing<T>(Func<T> o)
{
o();
}
public Int32 MyIntReturningMethod()
{
return (5);
}
the IL looks like this....
IL_0001: ldarg.0
IL_0002: ldarg.0
IL_0003: ldftn UserQuery.MyIntReturningMethod
IL_0009: newobj System.Func<System.Int32>..ctor
IL_000E: call UserQuery.Thing
(Some of the extra stuff is from LINQ Pad's additions... like the UserQuery part)
The IL looks identical as if you do an explicit cast. So it seems like the compiler does't actually know which method to use. So it doesn't know what cast to create automatically.
You can just use Task.Run<Int32>((Func<Int32>)MyIntReturningMethod) to help it out a bit. Though I do agree that this seems like something the compiler should be able to handle. Because Func<Task<Int32>> is not the same as Func<Int32>, so it doesn't make sense that they would confuse the compiler.
Seems like an overload resolution problem. The compiler can't tell which overload you're calling (because first it has to find the correct delegate to create, which it doesn't know because that depends on the overload you're calling). It would have to guess-and-check but I'm guessing it's not that smart.
The approach of Tyler Jensen works for me.
Also, you can try this using a lambda expression:
public class MyTest
{
public void RunTest()
{
Task<Int32> t = Task.Run<Int32>(() => MyIntReturningMethod());
t.Wait();
Console.WriteLine(t.Result);
}
public int MyIntReturningMethod()
{
return (5);
}
}
Here's my stab at it:
public class MyTest
{
public void RunTest()
{
Task<Int32> t = Task.Run<Int32>(new Func<int>(MyIntReturningMethod));
t.Wait();
Console.WriteLine(t.Result);
}
public int MyIntReturningMethod()
{
return (5);
}
}
I have the following simple code
abstract class A
{
public abstract void Test(Int32 value);
}
class B : A
{
public override void Test(Int32 value)
{
Console.WriteLine("Int32");
}
public void Test(Double value)
{
Test((Int32)1);
}
}
When I ran this code the line Test((Int32)1) causes stack overflow due to infinite recursion. The only possible way to correctly call proper method (with integer parameter) I found is
(this as A).Test(1);
But this is not appropriate for me, because both methods Test are public and I am willing the users to be able to call both method?
Method overload resolution in C# does not always behave as you might expect, but your code is behaving according to the specification (I wrote a blog post about this a while ago).
In short, the compiler start off by finding methods that
Has the same name (in your case Test)
are declared in the type (in your case B) or one of its base types
are not declared with the override modifier
Note that last point. This is actually logical, since virtual methods are resolved in run-time, not compile time.
Finally, if the type (in this case B) has a method that is a candidate (which means that the parameters in your call can be implicitly converted to the parameter type of the candidate method), that method will be used. Your overridden method is not even part of the decision process.
If you want to call your overridden method, you will need to cast the object to its base type first.
Unfortunately in order to call the A::Test(int) through a B reference some sort of cast is needed. So long as the C# compiler sees the reference through B it will pick the B::Test(double) version.
A slightly less ugly version is the following
((A)this).Test(1);
Another thought though is have a private method with a different name that both feed into.
class B : A {
public override void Test(int i) {
TestCore(i);
}
public void Test(double d) {
TestCore(1);
}
private void TestCore(int i) {
// Combined logic here
}
}
I'm trying to call an extension method on my own class, but it fails to compile. Consider the following lines of code:
public interface IHelloWorld
{
}
public static class Extensions
{
public static string HelloWorld(this IHelloWorld ext)
{
return "Hello world!";
}
}
public class Test : IHelloWorld
{
public string SaySomething()
{
return HelloWorld();
}
}
Basically I'm extending on the interface. I keep getting this error:
The name 'HelloWorld' does not exist in the current context
Can anybody explains this to me? When I do a cast all seems well:
return ((Test)this).HelloWorld();
Any explanations?
The cast isn't necessary - the this part is. So this works fine:
return this.HelloWorld();
Section 7.6.5.2 explicitly talks about method invocations of the form
expr.identifier ( )
expr.identifier ( args )
expr.identifier < typeargs > ( )
expr.identifier < typeargs > ( args )
This invocation:
HelloWorld()
isn't of that form, as there's no expression involved.
It's not immediately clear to me why the language was designed that way (i.e. why the "implicit this" was excluded) and maybe Eric Lippert will add an answer to that effect later. (The answer may well be along the lines of "because it would have taken a long time to spec, implement and test, for relatively little benefit.") However, this answer at least shows that the C# compiler is sticking to the spec...
this.HelloWorld(); works with no casting.
Remember how Extension methods work:
You use an object and compiler would know the type then it could resolve it to the extension method. If no object is used, then it would not be able to resolve it.
Not really an answer, but too long to fit in the comment section...
Let's take the following example, that I think is pretty common:
public class DoubleSet : List<double>
{
public IEnumerable<double> Square()
{
return this.Select( x => x*x );
}
}
It is a perfectly valid point that the this is not necessary for the compiler to interpret the Select method properly.
However I think that in some ways, imposing the dot notation highlights the fact that we're dealing with an extension method, and that as such, the extension method will only access the members of the current instance through public accessors, even if you're calling it within the private scope of the class.
It makes explicit to the code reader that the extension method will treat the "this" instance as if it didn't know anything of its internal state. And indeed the class of the object is completely unknown to the extension method (as the extension method only knows the interface)
If the code was only:
public IEnumerable<double> Square()
{
return Select( x => x*x );
}
it would be much less obvious that you're dealing with IEnumerable.Select that is actually calling the IList.GetEnumerator and getting every element one by one to call the x => x*x function.
Say I have 2 classes, class A and class B. Class A creates an instance of Class B. Class A has a function that I would like to pass into a method from Class B.
class A {
void Main(string[] args) {
B classB=new B();
DelegateCaller(new delFunction(classB.TheFunction()); // <-- Won't compile (method name expected)
DelegateCaller(new delFunction(B.TheFunction()); // <-- Won't compile (object reference is req'd)
}
public delegate string delFunction();
public DelegateCaller(delFunction func) {
System.Console.WriteLine(func());
}
}
class B {
public string TheFunction() {
return "I'm Printing!!!";
}
}
I'm not sure if it a syntax issue or it's just something I can't do. Maybe I need to define the delegate in B, but reference it in A? What about B's this pointer?
It's just a syntax issue; get rid of the parentheses after classB.TheFunction - they indicate that you wish to invoke the method.
DelegateCaller(new delFunction(classB.TheFunction));
Do note that there is an implicit conversion available from a method-group, so you can just do:
DelegateCaller(classB.TheFunction);
Also note that creating your own delegate-type in this case is unnecessary; you could just use the in-built Func<string> type.
EDIT: As Darin Dimitrov points out, there is also the unrelated issue of calling an instance method as though it were a static method.
Try like this:
class A
{
static void Main()
{
B classB = new B();
DelegateCaller(classB.TheFunction);
}
public delegate string delFunction();
public static void DelegateCaller(delFunction func)
{
Console.WriteLine(func());
}
}
class B
{
public string TheFunction()
{
return "I'm Printing!!!";
}
}
Let me elaborate about the different changes I've made to your initial code:
TheFunction in class B needs to be public so that you can access it from class A
The DelegateCaller method in class A should be static and not necessarily return a value (declare it as void) if you want to call it from the static Main method.
The definition of the delFunction delegate should return a string.
Take the parenthesis off the end of TheFunction. You want the method, not the result of a call to the method.
If you want to capture an instance method for usage in a general purpose fashion you should use Delegate.CreateDelegate(Type,MethodInfo). This is nice as it allows you to create an "open delegate" meaning it isn't bound to an instance and can take any instance that is a ClassB. It makes reflection quite fast if you know the type information, as this method will perform much faster than the equivalent statement using MethodInfo.Invoke.
DelegateCaller(new delFunction(B.TheFunction());
Should be
DelegateCaller(new delFunction(B.TheFunction);
To use classB.TheFunction you would need to make TheFunction static. You pass in the function with no parens.