Can I pass a method with an out parameter as a Func?
public IList<Foo> FindForBar(string bar, out int count) { }
// somewhere else
public IList<T> Find(Func<string, int, List<T>> listFunction) { }
Func needs a type so out won't compile there, and calling listFunction requires an int and won't allow an out in.
Is there a way to do this?
ref and out are not part of the type parameter definition so you can't use the built-in Func delegate to pass ref and out arguments. Of course, you can declare your own delegate if you want:
delegate V MyDelegate<T,U,V>(T input, out U output);
Why not create a class to encapsulate the results?
public class Result
{
public IList<Foo> List { get; set; }
public Int32 Count { get; set; }
}
The Func family of delegates (or Action for that matter) are nothing but simple delegate types declared like
//.NET 4 and above
public delegate TResult Func<out TResult>()
public delegate TResult Func<in T, out TResult>(T obj)
//.NET 3.5
public delegate TResult Func<T1, T2, TResult>(T1 obj1, T2 obj2)
public delegate TResult Func<T1, T2, T3, TResult>(T1 obj1, T2 obj2, T3 obj3)
etc. Delegates as such can have out/ref parameters, so in your case its only a matter of custom implementation by yourself as other answers have pointed out. As to why Microsoft did not pack this by default, think of the sheer number of combinations it would require.
delegate TResult Func<T1, T2, TResult>(T1 obj1, T2 obj2)
delegate TResult Func<T1, T2, TResult>(out T1 obj1, T2 obj2)
delegate TResult Func<T1, T2, TResult>(T1 obj1, out T2 obj2)
delegate TResult Func<T1, T2, TResult>(out T1 obj1, out T2 obj2)
for just two parameters. We have not even touched ref. It would actually be cumbersome and confusing for developers.
You could wrap it in a lambda/delegate/function/method that exposed the right interface and called FindForBar, but I suspect that FindForBar has count as an out parameter as a reason, so you'd need to be sure throwing that information away was ok/safe/desirable/had the right results (you'd need to be sure of this even if you could just directly pass in FindForBar).
Related
For my delegate (with signature Func<T1, Func<T2, Task<TResult>>) to be applicable it has to confirm to the signature Func<T1, Task<TResult>>. I would like to have a function such as:
public static Func<T1, Task<Func<T2, TResult>> TaskToOuterScope<T1, T2, TResult>(Func<T1, Func<T2, Task<TResult>>> f)
{
// throw new NotImplementedException();
}
How to implement this TaskToOuterScope-function?
I tried:
public static Func<T1, Task<Func<T2, TResult>> TaskToOuterScope<T1, T2, TResult>(Func<T1, Func<T2, Task<TResult>>> f)
{
return async (T1 arg1) => await (async (T2 arg2) => await f(arg1)(arg2));
}
However, this gives CS4001 Cannot await 'lambda expression'.
Googling gave pages that are either about currying (the signature Func<T1, Func<T2, Task<TResult>> is the result from currying) or about async/await but non about the combination or more specifically about moving task to outer scope in higher order functions.
Is conversion from Func<T1, Func<T2, Task<TResult>> to Func<T1, Task<Func<T2, TResult>>> possible?
And if so, how can I do it?
You can't.
Task<TResult> is a task whose result depends on two parameters, t1 and t2. You can see that you cannot transform it to a task Task<Func<T2, TResult>> whose result only depends on one parameter t1.
Whatever asynchronous task is being executed requires (by the definition you have given) those two parameters upfront before it starts executing. It cannot take t1 then, given t2 at a later point, return its result synchronously.
I am writing an asynchronous class to simplify function operation. Below is my code:
using System;
namespace AsyncLibery
{
public class AsyncLibery
{
delegate void Exec(); // No input parameter, no return value
delegate void ExecWithParams<T>(T T1); //One input parameter, no return value
delegate void ExecWithParams<T1, T2>(T1 t1,T2 t2); //Two input parameters, no return value
delegate void ExecWithParams<T1, T2, T3>(T1 t1, T2 t2, T3 t3);//Three input parameters, no return value
delegate void ExecWithParams<T1, T2, T3, T4>(T1 t1, T2 t2, T3 t3, T4 t4);//Four input parameters,no return value
delegate T ExecWithReturnType<T>(); //No input parameter, one return value
delegate T2 ExecWithReturnType<T1, T2>(T1 t1); //One input parameter, one return value
delegate T3 ExecWithReturnType<T1, T2, T3>(T1 t1, T2 t2);//Two input parameters, one return value
delegate T4 ExecWithReturnType<T1, T2, T3, T4>(T1 t1, T2 t2, T3 t3);//Three input parameters, one return value
delegate T5 ExecWithReturnType<T1, T2, T3, T4, T5>(T1 t1, T2 t2, T3 t3, T4 t4);// Four input parameters, one return value.
#region no input parameter, no return value
public void BeginInvokeEx(Action actionFunction)
{
Exec exec = new Exec(actionFunction);
exec.BeginInvoke(new AsyncCallback(EndInvokeEx), exec);
}
private void EndInvokeEx(IAsyncResult iar)
{
Exec exec = (Exec)iar.AsyncState;
exec.EndInvoke(iar);
}
#endregion
#region one input parameter, no return value
public void BeginInvokeEx<T>(Action<T> actionFunction,T T1)
{
ExecWithParams<T> exec = new ExecWithParams<T>(actionFunction);
exec.BeginInvoke(T1, new AsyncCallback(EndInvokeEx1), exec);
}
private void EndInvokeEx1(IAsyncResult iar)
{
ExecWithParams<T> exec = (ExecWithParams<T>)iar.AsyncState;
exec.EndInvoke(iar);
}
#endregion
}
}
When I complied the code, it throws me an exception like "The type or namespace name 'T' could not be found (are you missing a using directive or an assembly reference?)", the exception is appeared at below code :
ExecWithParams<T> exec = (ExecWithParams<T>)iar.AsyncState;
I know that we must refer T parameter the specific type like int or string or anything else.
But who can help me without modifying the T parameter? is there any good method to deal with it?
thx.
Make EndInvokeEx1 generic:
private void EndInvokeEx1<T>(IAsyncResult iar)
{
ExecWithParams<T> exec = (ExecWithParams<T>)iar.AsyncState;
exec.EndInvoke(iar);
}
And change your BeginInvokeEx accordingly:
public void BeginInvokeEx<T>(Action<T> actionFunction,T T1)
{
ExecWithParams<T> exec = new ExecWithParams<T>(actionFunction);
exec.BeginInvoke(T1, new AsyncCallback(EndInvokeEx1<T>), exec);
}
You haven't defined what T is. You need to either have a generic parameter on the method EndInvokeEx1 such that it becomes EndInvokeEx<T>, where you can then pass T to the cast. Otherwise, it will need to be defined in the class declaration, so your class declaration becomes public class AsyncLibrary<T>. Otherwise, the compiler won't know what T is meant to represent.
The Action<T> delegate return void. Is there any other built-in delegate which returns non void value?
Yes. Func<> returns the type specified as the final generic type parameter, such that Func<int> returns an int and Func<int, string> accepts an integer and returns a string. Examples:
Func<int> getOne = () => 1;
Func<int, string> convertIntToString = i => i.ToString();
Action<string> printToScreen = s => Console.WriteLine(s);
// use them
printToScreen(convertIntToString(getOne()));
Sure, the Func Delegates return T.
Func<TResult> is "TResult method()"
Func<TInput, TResult> is "TResult method(TInput param)"
All the way down to
Func<T1, T2, T3, T4, TResult>
http://msdn.microsoft.com/en-us/library/bb534960.aspx
http://msdn.microsoft.com/en-us/library/bb534303.aspx
Also, for the sake of completeness, there is Predicate which returns bool.
Predicate<T> is "bool method(T param)"
http://msdn.microsoft.com/en-us/library/bfcke1bz.aspx
This question already has answers here:
Func<T> with out parameter
(4 answers)
Closed 9 years ago.
Func<a, out b, bool>, just don't compile, how to declare that i want the second parameter be an out one?
I want to use it like this:
public class Foo()
{
public Func<a, out b, bool> DetectMethod;
}
Actually, Func is just a simple delegate declared in the .NET Framework. Actually, there are several Func delegates declared there:
delegate TResult Func<TResult>()
delegate TResult Func<T, TResult>(T obj)
delegate TResult Func<T1, T2, TResult>(T1 obj1, T2 obj2)
delegate TResult Func<T1, T2, T3, TResult>(T1 obj1, T2 obj2, T3 obj3)
delegate TResult Func<T1, T2, T3, T4, TResult>(T1 obj1, T2 obj2, T3 obj3, T4 obj4)
delegate TResult Func<T1, T2, ... , T16, TResult>(T1 obj1, T2 obj2, ..., T16 obj16)
So the only thing you can do is declare your custom delegate:
delegate bool MyFunc<T1, T2>(T1 a, out T2 b)
You need to make your own delegate type, like this:
delegate bool MyFunc(Type1 a, out Type2 b);
You might want to rethink your design. Do you really need to complicate your code by adding an out parameter?
You can wrap the bool return type and the second out type in their own class (or .NET 4.0 Tuple) and use that as a return type:
public Func<Type1, Tuple<Type2, bool>> DetectMethod;
Of course when you want to use the delegates to reference try-parse methods, you are on the right track and you'll need to define a new delegate as others already described.
I have written a helper class which uses the Action - delegate as method parameter.
Like this:
public void SomeMethod(Action<T> methodToExecute, T argument);
According to the MSDN you can declare max. 4 arguments on an action delegate: Action<T1,T2,T3,T4>.
Now I'd like to call a method which needs 5! arguments. How could I do this?
The best solution would be something where I could use a dynamic number of method arguments.
Thanks
Declare the action delegate you need, there's nothing magic about it:
public delegate void Action<T1, T2, T3, T4, T5>(T1 p1, T2 p2, T3 p3, T4 p4, T5 p5);