Could C# delegate point to another class/object's method? - c#

Here, I wish to gain a delegate like function pointer, to point to a class method that's in another class (named Inner), and then pass it to a static function, like below:
public class Inner
{
public int M = 3;
public void F() { Console.WriteLine("f"); }
public void G() { Console.WriteLine("g"); }
}
class Program
{
public static void Caller(Action a)
{
a();
}
static void Main(string[] args)
{
var i = new Inner();
var f = i.F;
var g = i.G;
f();//error
g();//error
Program.Caller(f);
Console.WriteLine("Hello World!");
}
}
I'm from c/c++, and in c/c++, function pointer like this is very straight forward, but this C# code fail to compile. I googled and found almost all delegate explanations talks about delegate that points to class method inside itself.
My question is, how to fix the code to make it work?

Coding Seb's answer highlight's the cause of the issue, but doesn't really explain why.
It is because i.F is a method group, and not a specific method pointer. For example, imagine Inner was defined as so:
public class Inner
{
public void F() { Console.WriteLine("f"); }
public void F(string name) { Console.WriteLine(name); }
}
Which method would i.F refer to? F() or F(string)?
For that reason, you need to define the variable type explicitly, or cast the pointer:
Action f = i.F;
Or:
var f = (Action)i.F;

You can not set methods group in implicit variables in C# so if you just change 2 var in Action it's working
public class Inner
{
public int M = 3;
public void F() { Console.WriteLine("f"); }
public void G() { Console.WriteLine("g"); }
}
class Program
{
public static void Caller(Action a)
{
a();
}
static void Main(string[] args)
{
var i = new Inner();
Action f = i.F;
Action g = i.G;
f();
g();
Program.Caller(f);
Console.WriteLine("Hello World!");
}
}

Related

How to call a non static method in the main?

class Program
{
static void Main(string[] args)
{
//I want to call the bird method here
}
public void bird(Program program)
{
birdSpeech();
}
public void birdSpeech()
{
Console.WriteLine("Chirp Chirp");
}
}
How do I call bird in the main, I also tried to call the method from a object of the class but that didn't work
Does it even make sense to do this (I try to avoid static methods as much as possible).
If a method can be made static without changing the code, you should make it static. To answer your question though, you can create an instance of Program in your Main function and call the non-static methods through it.
class Program
{
static void Main(string[] args)
{
var p = new Program();
p.bird();
}
public void bird()
{
birdSpeech();
}
public void birdSpeech()
{
Console.WriteLine("Chirp Chirp");
}
}
Could do something like this
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Hello World!");
var p = new Program();
p.bird(p);
}
public void bird(Program program)
{
birdSpeech();
}
public void birdSpeech()
{
Console.WriteLine("Chirp Chirp");
}
}

Send static method + parameter as parameter

I'd like to do something like this, but it's not possible.(Cann't convert from 'void' to 'System.Action').
class Program
{
public static void Main()
{
int n = 2;
ClassB cb = new ClassB();
cb.SetMethod(ClassA.MethodA(n)); //Cann't convert 'void' to 'System.Action<int>'
}
}
public class ClassA
{
public static void MethodA(int a)
{
//code
}
}
public class ClassB
{
Delegate del;
public void SetMethod(Action<int> action)
{
del = new Delegate(action);
}
public void ButtonClick()
{
del.Invoke();
}
}
public delegate void Delegate(int n);
I can send the argument "n", as second argument in the "setMethod" method, but I would have to store a variable to after pass to "del.Invoke(PARAM)". I'd like to use "del.Invoke()".
You seem to have a misunderstanding of delegates. Delegates represent methods, not method calls. If you supply arguments to a method, it becomes a method call. So here:
cb.setMethod(ClassA.methodA(n));
ClassA.methodA(n) is a method call, and you can't assign that to a delegate.
Basically, you can't pass the parameter at this stage. You have to pass the parameter when you invoke the delegate. e.g.
del.Invoke(5);
But you said you want to always write del.Invoke(), with no arguments. Well, then you should not use an Action<int>, you should just use Action, which does not accept any parameters.
class Program
{
public static void Main()
{
int n = 2;
ClassB cb = new ClassB();
cb.setMethod(() => ClassA.methodA(n));
}
}
public class ClassA
{
public static void methodA(int a)
{
//code
}
}
public class ClassB
{
Delegate del;
public void setMethod(Action action)
{
del = new Delegate(action);
}
public void ButtonClick()
{
del.Invoke();
}
}
public delegate void Delegate();
cb.setMethod(new Action(ClassA.methodA));
It isn't clear whether you want to capture the integer at the call site (e.g. as a closure), or whether you intend passing in a parameter explicitly to the delegate.
Here's the former case, where the value is captured:
public static void Main()
{
var n = 2;
var cb = new ClassB();
cb.setMethod(() => ClassA.methodA(n));
}
The delegate is thus unaware of the captured variable, and is just defined as:
public delegate void Delegate();
If however you do intend passing the int at invoke time, then the value for the int needs to be passed in the ButtonClick:
public static void Main()
{
var cb = new ClassB();
cb.setMethod(ClassA.methodA);
}
public class ClassB
{
Delegate del;
public void setMethod(Action<int> action)
{
del = new Delegate(action);
}
public void ButtonClick()
{
var n = 2;
del.Invoke(n);
}
}
public delegate void Delegate(int n);
Edit - Re Do you think there's a better way
There's no real reason to explicitly require a delegate. Action and Func (and Action<int>, depending on the above) are already delegates. As an improvement, you should check that the action is assigned before invoking it. The null conditional operator will simplify this as _action?.Invoke(). But you can go one step further, and prevent the action from ever being unassigned by requiring it in the constructor:
public class ClassB
{
// Can be readonly if it is assigned only ever once, in the ctor.
private readonly Action _action;
public ClassB(Action action)
{
Contract.Assert(action != null);
_action = action;
}
public void ButtonClick()
{
_action(); // i.e. no need for Invoke or null check.
}
}

A way to store a method in a variable?

Hello I am new to programming.
Basically I need to store a method A from class A in variable A from class B inside a method from class B but I cannot seem to find a way.
To give an example:
Class A
public void methodA()
{
*method*
}
Class B
Delegate variableA; //I believe using Delegate is wrong
public void methodB();
{
variableA = ClassA.methodA();
}
Then in Class B there would be another method that will utilize the variable with the stored method.
public void methodC();
{
variableA;
}
This isn't the exact code I have but this is basically the gist of it. Any help is appreciated :)
Edit: Thanks for the help everyone!
ClassA definition:
public class ClassA
{
public void MethodA()
{
Console.WriteLine("Hello World!");
}
}
ClassB definition:
public class ClassB
{
private Action VariableA { get; set; }
public void MethodB(ClassA classA)
{
VariableA = classA.MethodA;
}
public void MethodC()
{
VariableA();
}
}
Program definition:
static void Main(string[] args)
{
ClassA classA = new ClassA();
ClassB classB = new ClassB();
classB.MethodB(classA);
classB.MethodC();
Console.ReadLine();
}
Here is an example:
public class Test
{
private Action<int> hiddenMethod = new Action<int>((i) =>
{
Console.WriteLine(i);
});
public void PublicMethod(int i)
{
hiddenMethod(i);
}
}
And using it:
class Program
{
static void Main(string[] args)
{
Test t = new Test();
t.PublicMethod(21);
Console.Read();
}
}
Use reflection:
public class A
{
public void MethodA()
{
Console.WriteLine("MethodA");
}
public static void StaticMethodA()
{
Console.WriteLine("StaticMethodA");
}
}
public class B
{
MethodInfo mv = typeof(A).GetMethod("MethodA");
MethodInfo smv = typeof(A).GetMethod("StaticMethodA");
public void CheckA(bool useStatic)
{
if (useStatic) smv.Invoke(null, null);
else mv.Invoke(new A(), null);
}
}
class MainClass
{
[STAThread]
public static void Main(string[] args)
{
var b = new B();
b.CheckA(true);
b.CheckA(false);
}
}
See details in MSDN.

Hiding of member function does not work

I guess I have I miss-conception. I try to override the behavior of a class in a sub-class by replacing a public function. In the program below I expect "B" to be written to the console, but the program prints "A". Where do I think wrong? And how can I achieve it. (In my real case I cannot change class A).
class Program
{
class A { public void F() { Console.WriteLine("A"); } }
class B : A { public new void F() { Console.WriteLine("B"); } }
static void Main(string[] args)
{
A x;
x = new B();
x.F();
Console.ReadLine();
}
}
The desired behaviour can be obtained by using a virtual method and overriding it at follows; note the override keyword.
class Program
{
class A { public virtual void F() { Console.WriteLine("A"); } }
class B : A { public override void F() { Console.WriteLine("B"); } }
static void Main(string[] args)
{
A x;
x = new B();
x.F();
Console.ReadLine();
}
}
If you happen to have a Java background, note that this type of inheritance is the default behaviour in Java, but in C# it has to be explicitly declared.

Delegate Array in C#

I am experimenting with calling delegate functions from a delegate array. I've been able to create the array of delegates, but how do I call the delegate?
public delegate void pd();
public static class MyClass
{
static void p1()
{
//...
}
static void p2 ()
{
//...
}
//...
static pd[] delegates = new pd[] {
new pd( MyClass.p1 ),
new pd( MyClass.p2)
/* ... */
};
}
public class MainClass
{
static void Main()
{
// Call pd[0]
// Call pd[1]
}
}
EDIT: The reason for the array is that I need to call the delegate functions by an index as needed. They are not run in response to an event. I see a critical (stupid) error in my code as I had tried to execute the delegate function using the pd[] type rather than the name of the array (delegates).
If they're all the same type, why not just combine them into a single multicast delegate?
static pd delegateInstance = new pd(MyClass.p1) + new pd(MyClass.p2) ...;
...
pd();
public class MainClass
{
static void Main()
{
pd[0]();
pd[1]();
}
}
In .Net, any delegate is in fact actually a "multicast" delegate (it inherits from this built-in base class), and therefore contains an internal linked list which can contain any number of target delegates.
You can access this list by calling the method GetInvocationList() on the delegate itself. This method returns an array of Delegates...
The only restriction is that all the delegates inside of a given delegate's linked list must have the same signature, (be of the same delegate type). If you need your collection to be able to contain delegates of disparate types, then you need to construct your own list or collection class.
But if this is ok, then you can "call" the delegates in a given delegate's invocation list like this:
public delegate void MessageArrivedHandler(MessageBase msg);
public class MyClass
{
public event MessageArrivedHandler MessageArrivedClientHandler;
public void CallEachDelegate(MessageBase msg)
{
if (MessageArrivedClientHandler == null)
return;
Delegate[] clientList = MessageArrivedClientHandler.GetInvocationList();
foreach (Delegate d in clientList)
{
if (d is MessageArrivedHandler)
(d as MessageArrivedHandler)(msg);
}
}
}
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
pd[0]();
pd[1]();
}
public delegate void delegates();
static delegates[] pd = new delegates[]
{
new delegates(MyClass.p1),
new delegates(MyClass.p2)
};
public static class MyClass
{
public static void p1()
{
MessageBox.Show("1");
}
public static void p2()
{
MessageBox.Show("2");
}
}
}
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
pd[0](1);
pd[1](2);
}
public delegate void delegates(int par);
static delegates[] pd = new delegates[]
{
new delegates(MyClass.p1),
new delegates(MyClass.p2)
};
public static class MyClass
{
public static void p1(int par)
{
MessageBox.Show(par.ToString());
}
public static void p2(int par)
{
MessageBox.Show(par.ToString());
}
}
}

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