I have several methods all with the same parameter types and return values but different names and blocks. I want to pass the name of the method to run to another method that will invoke the passed method.
public int Method1(string)
{
// Do something
return myInt;
}
public int Method2(string)
{
// Do something different
return myInt;
}
public bool RunTheMethod([Method Name passed in here] myMethodName)
{
// Do stuff
int i = myMethodName("My String");
// Do more stuff
return true;
}
public bool Test()
{
return RunTheMethod(Method1);
}
This code does not work but this is what I am trying to do. What I don't understand is how to write the RunTheMethod code since I need to define the parameter.
You can use the Func delegate in .NET 3.5 as the parameter in your RunTheMethod method. The Func delegate allows you to specify a method that takes a number of parameters of a specific type and returns a single argument of a specific type. Here is an example that should work:
public class Class1
{
public int Method1(string input)
{
//... do something
return 0;
}
public int Method2(string input)
{
//... do something different
return 1;
}
public bool RunTheMethod(Func<string, int> myMethodName)
{
//... do stuff
int i = myMethodName("My String");
//... do more stuff
return true;
}
public bool Test()
{
return RunTheMethod(Method1);
}
}
You need to use a delegate. In this case all your methods take a string parameter and return an int - this is most simply represented by the Func<string, int> delegate1. So your code can become correct with as simple a change as this:
public bool RunTheMethod(Func<string, int> myMethodName)
{
// ... do stuff
int i = myMethodName("My String");
// ... do more stuff
return true;
}
Delegates have a lot more power than this, admittedly. For example, with C# you can create a delegate from a lambda expression, so you could invoke your method this way:
RunTheMethod(x => x.Length);
That will create an anonymous function like this:
// The <> in the name make it "unspeakable" - you can't refer to this method directly
// in your own code.
private static int <>_HiddenMethod_<>(string x)
{
return x.Length;
}
and then pass that delegate to the RunTheMethod method.
You can use delegates for event subscriptions, asynchronous execution, callbacks - all kinds of things. It's well worth reading up on them, particularly if you want to use LINQ. I have an article which is mostly about the differences between delegates and events, but you may find it useful anyway.
1 This is just based on the generic Func<T, TResult> delegate type in the framework; you could easily declare your own:
public delegate int MyDelegateType(string value)
and then make the parameter be of type MyDelegateType instead.
From OP's example:
public static int Method1(string mystring)
{
return 1;
}
public static int Method2(string mystring)
{
return 2;
}
You can try Action Delegate! And then call your method using
public bool RunTheMethod(Action myMethodName)
{
myMethodName(); // note: the return value got discarded
return true;
}
RunTheMethod(() => Method1("MyString1"));
Or
public static object InvokeMethod(Delegate method, params object[] args)
{
return method.DynamicInvoke(args);
}
Then simply call method
Console.WriteLine(InvokeMethod(new Func<string,int>(Method1), "MyString1"));
Console.WriteLine(InvokeMethod(new Func<string, int>(Method2), "MyString2"));
In order to provide a clear and complete answer, I'm going to start from the very beginning before showing three possible solutions.
A brief introduction
All .NET languages (such as C#, F#, and Visual Basic) run on top of the Common Language Runtime (CLR), which is a VM that runs code in the Common Intermediate Language (CIL), which is way higher level than machine code. It follows that methods aren't Assembly subroutines, nor are they values, unlike functional languages and JavaScript; rather, they're symbols that CLR recognizes. Not being values, they cannot be passed as a parameter. That's why there's a special tool in .NET. That is, delegates.
What's a delegate?
A delegate represents a handle to a method (the term handle is to be preferred over pointer as the latter would be an implementation detail). Since a method is not a value, there has to be a special class in .NET, namely Delegate, which wraps up any method. What makes it special is that, like very few classes, it needs to be implemented by the CLR itself and couldn't be simply written as a class in a .NET language.
Three different solutions, the same underlying concept
The type–unsafe way
Using the Delegate special class directly.
Example:
static void MyMethod()
{
Console.WriteLine("I was called by the Delegate special class!");
}
static void CallAnyMethod(Delegate yourMethod)
{
yourMethod.DynamicInvoke(new object[] { /*Array of arguments to pass*/ });
}
static void Main()
{
CallAnyMethod(MyMethod);
}
The drawback here is your code being type–unsafe, allowing arguments to be passed dynamically, with no constraints.
The custom way
Besides the Delegate special class, the concept of delegates spreads to custom delegates, which are declarations of methods preceded by the delegate keyword. They are type–checked the same way as “normal” method invocations, making for type-safe code.
Example:
delegate void PrintDelegate(string prompt);
static void PrintSomewhere(PrintDelegate print, string prompt)
{
print(prompt);
}
static void PrintOnConsole(string prompt)
{
Console.WriteLine(prompt);
}
static void PrintOnScreen(string prompt)
{
MessageBox.Show(prompt);
}
static void Main()
{
PrintSomewhere(PrintOnConsole, "Press a key to get a message");
Console.Read();
PrintSomewhere(PrintOnScreen, "Hello world");
}
The standard library's way
Alternatively, you can stick with a delegate that's part of the .NET Standard:
Action wraps up a parameterless void method;
Action<T1> wraps up a void method with one parameter of type T1;
Action<T1, T2> wraps up a void method with two parameters of types T1 and T2, respectively,
and so forth;
Func<TR> wraps up a parameterless function with TR return type;
Func<T1, TR> wraps up a function with TR return type and with one parameter of type T1;
Func<T1, T2, TR> wraps up a function with TR return type and with two parameters of types T1 and T2, respectively;
and so forth.
However, bear in mind that by using predefined delegates like these, parameter names won't be self-describing, nor is the name of the delegate type meaningful as to what instances are supposed to do. Therefore, refrain from using them in contexts where their purpose is not absolutely self-evident.
The latter solution is the one most people posted. I'm also mentioning it in my answer for the sake of completeness.
The solution involves Delegates, which are used to store methods to call. Define a method taking a delegate as an argument,
public static T Runner<T>(Func<T> funcToRun)
{
// Do stuff before running function as normal
return funcToRun();
}
Then pass the delegate on the call site:
var returnValue = Runner(() => GetUser(99));
You should use a Func<string, int> delegate, that represents a function taking a string argument and returning an int value:
public bool RunTheMethod(Func<string, int> myMethod)
{
// Do stuff
myMethod.Invoke("My String");
// Do stuff
return true;
}
Then invoke it this way:
public bool Test()
{
return RunTheMethod(Method1);
}
While the accepted answer is absolutely correct, I would like to provide an additional method.
I ended up here after doing my own searching for a solution to a similar question.
I am building a plugin driven framework, and as part of it I wanted people to be able to add menu items to the applications menu to a generic list without exposing an actual Menu object because the framework may deploy on other platforms that don't have Menu UI objects. Adding general info about the menu is easy enough, but allowing the plugin developer enough liberty to create the callback for when the menu is clicked was proving to be a pain. Until it dawned on me that I was trying to re-invent the wheel and normal menus call and trigger the callback from events!
So the solution, as simple as it sounds once you realize it, eluded me until now.
Just create separate classes for each of your current methods, inherited from a base if you must, and just add an event handler to each.
Here is an example Which can help you better to understand how to pass a function as a parameter.
Suppose you have Parent page and you want to open a child popup window. In the parent page there is a textbox that should be filled basing on child popup textbox.
Here you need to create a delegate.
Parent.cs
// declaration of delegates
public delegate void FillName(String FirstName);
Now create a function which will fill your textbox and function should map delegates
//parameters
public void Getname(String ThisName)
{
txtname.Text=ThisName;
}
Now on button click you need to open a Child popup window.
private void button1_Click(object sender, RoutedEventArgs e)
{
ChildPopUp p = new ChildPopUp (Getname) //pass function name in its constructor
p.Show();
}
IN ChildPopUp constructor you need to create parameter of 'delegate type' of parent //page
ChildPopUp.cs
public Parent.FillName obj;
public PopUp(Parent.FillName objTMP)//parameter as deligate type
{
obj = objTMP;
InitializeComponent();
}
private void OKButton_Click(object sender, RoutedEventArgs e)
{
obj(txtFirstName.Text);
// Getname() function will call automatically here
this.DialogResult = true;
}
If you want to pass Method as parameter, use:
using System;
public void Method1()
{
CallingMethod(CalledMethod);
}
public void CallingMethod(Action method)
{
method(); // This will call the method that has been passed as parameter
}
public void CalledMethod()
{
Console.WriteLine("This method is called by passing it as a parameter");
}
If the method passed needs to take one argument and return a value, Func is the best way to go. Here is an example.
public int Method1(string)
{
// Do something
return 6;
}
public int Method2(string)
{
// Do something different
return 5;
}
public bool RunTheMethod(Func<string, int> myMethodName)
{
// Do stuff
int i = myMethodName("My String");
Console.WriteLine(i); // This is just in place of the "Do more stuff"
return true;
}
public bool Test()
{
return RunTheMethod(Method1);
}
Read the docs here
However, if your method that is passed as a parameter does not return anything, you can also use Action. It supports up to 16 paramaters for the passed method. Here is an example.
public int MethodToBeCalled(string name, int age)
{
Console.WriteLine(name + "'s age is" + age);
}
public bool RunTheMethod(Action<string, int> myMethodName)
{
// Do stuff
myMethodName("bob", 32); // Expected output: "bob's age is 32"
return true;
}
public bool Test()
{
return RunTheMethod(MethodToBeCalled);
}
Read the documentation here
Here is an example without a parameter:
http://en.csharp-online.net/CSharp_FAQ:_How_call_a_method_using_a_name_string
with params:
http://www.daniweb.com/forums/thread98148.html#
you basically pass in an array of objects along with name of method. you then use both with the Invoke method.
params Object[] parameters
class PersonDB
{
string[] list = { "John", "Sam", "Dave" };
public void Process(ProcessPersonDelegate f)
{
foreach(string s in list) f(s);
}
}
The second class is Client, which will use the storage class. It has a Main method that creates an instance of PersonDB, and it calls that object’s Process method with a method that is defined in the Client class.
class Client
{
static void Main()
{
PersonDB p = new PersonDB();
p.Process(PrintName);
}
static void PrintName(string name)
{
System.Console.WriteLine(name);
}
}
I don't know who might need this, but in case you're unsure how to send a lambda with a delegate, when the function using the delegate doesn't need to insert any params in there you just need the return value.
SO you can also do this:
public int DoStuff(string stuff)
{
Console.WriteLine(stuff);
}
public static bool MethodWithDelegate(Func<int> delegate)
{
///do stuff
int i = delegate();
return i!=0;
}
public static void Main(String[] args)
{
var answer = MethodWithDelegate(()=> DoStuff("On This random string that the MethodWithDelegate doesn't know about."));
}
I'm programming in C# about 2 months, and I've been noticing to the fact that the only way a method will get a parameter by reference is when the method sets as a static method, and I don't understand the reason behind this situation. As example, in C++ we could just send parameters by reference to any kind of method we wanted to.
Thanks ahead :)
I'm adding a very simple code for those who asked for:
public void f(ref int num)
{
num++;
}
private static void Main(string[] args)
{
int a = 0;
f(ref a);
Console.WriteLine(a);
Console.Read();
}
the error I get:
An object reference is required for the non-static field, method, or property
now, I know that in C# almost everything that not int, string, double... and other built-in vars are send automatically by ref. I wonder why the method must be static in case of sending those vars by ref.
the only way a method will get a parameter by reference is when the method sets as a static method
That's incorrect. C# has a concept of Reference Types and Value Types, where both are passed by value, but that value differs. For reference types, the value that gets passed is the reference itself, where for value types it's a copy of the value.
If you want a value type by reference, you can use the ref modifier in the methods signature:
public void M(ref int x)
Same goes for reference type, only for the fact that passing them by ref will pass the current reference pointing to the data structure, and won't create a copy of that reference.
You are not able to use f because it's an instance method and the Main is a static one so you cannot call it from here without having an instance of the class that contains f (I guess Program).
You need to either create an instance of Program:
this is exaclty the same as what the error message says:
An object reference is required for the non-static field, method, or property
so you create an object reference to Program:
class Program
{
public void f(ref int num)
{
num++;
}
private static void Main(string[] args)
{
int a = 0;
var p = new Program();
p.f(ref a);
Console.WriteLine(a);
Console.Read();
}
}
or make the method static to be able to use it:
class Program
{
public static void f(ref int num)
{
num++;
}
private static void Main(string[] args)
{
int a = 0;
f(ref a); // is the same as: Program.f(ref a)
Console.WriteLine(a);
Console.Read();
}
}
The ref keyword has nothing to do with it. It's all about how you call the method - via an object reference or as a static one without an object reference. Neither is better then the other. It's a design decision and each serves different puposes.
As you can see in the code below, I have declared an Action<> object as a variable.
Would anybody please let me know why this action method delegate behaves like a static method?
Why does it return true in the following code?
Code:
public static void Main(string[] args)
{
Action<string> actionMethod = s => { Console.WriteLine("My Name is " + s); };
Console.WriteLine(actionMethod.Method.IsStatic);
Console.Read();
}
Output:
This is most likely because there are no closures, for example:
int age = 25;
Action<string> withClosure = s => Console.WriteLine("My name is {0} and I am {1} years old", s, age);
Action<string> withoutClosure = s => Console.WriteLine("My name is {0}", s);
Console.WriteLine(withClosure.Method.IsStatic);
Console.WriteLine(withoutClosure.Method.IsStatic);
This will output false for withClosure and true for withoutClosure.
When you use a lambda expression, the compiler creates a little class to contain your method, this would compile to something like the following (the actual implementation most likely varies slightly):
private class <Main>b__0
{
public int age;
public void withClosure(string s)
{
Console.WriteLine("My name is {0} and I am {1} years old", s, age)
}
}
private static class <Main>b__1
{
public static void withoutClosure(string s)
{
Console.WriteLine("My name is {0}", s)
}
}
public static void Main()
{
var b__0 = new <Main>b__0();
b__0.age = 25;
Action<string> withClosure = b__0.withClosure;
Action<string> withoutClosure = <Main>b__1.withoutClosure;
Console.WriteLine(withClosure.Method.IsStatic);
Console.WriteLine(withoutClosure.Method.IsStatic);
}
You can see the resulting Action<string> instances actually point to methods on these generated classes.
The "action method" is static only as a side effect of the implementation. This is a case of an anonymous method with no captured variables. Since there are no captured variables, the method has no additional lifetime requirements beyond those for local variables in general. If it did reference other local variables, its lifetime extends to the lifetime of those other variables (see sec. L.1.7, Local variables, and sec. N.15.5.1, Captured outer variables, in the C# 5.0 specification).
Note that the C# specification only talks about anonymous methods being converted to "expression trees", not "anonymous classes". While the expression tree could be represented as additional C# classes, for example, in the Microsoft compiler, this implementation is not required (as acknowledged by sec. M.5.3 in the C# 5.0 specification). Therefore, it is undefined whether the anonymous function is static or not. Moreover, section K.6 leaves much open as to the details of expression trees.
Delegate caching behavior was changed in Roslyn. Previously, as stated, any lambda expression which didn't capture variables was compiled into a static method at the call site. Roslyn changed this behavior. Now, any lambda, which captures variables or not, is transformed into a display class:
Given this example:
public class C
{
public void M()
{
var x = 5;
Action<int> action = y => Console.WriteLine(y);
}
}
Native compiler output:
public class C
{
[CompilerGenerated]
private static Action<int> CS$<>9__CachedAnonymousMethodDelegate1;
public void M()
{
if (C.CS$<>9__CachedAnonymousMethodDelegate1 == null)
{
C.CS$<>9__CachedAnonymousMethodDelegate1 = new Action<int>(C.<M>b__0);
}
Action<int> arg_1D_0 = C.CS$<>9__CachedAnonymousMethodDelegate1;
}
[CompilerGenerated]
private static void <M>b__0(int y)
{
Console.WriteLine(y);
}
}
Roslyn:
public class C
{
[CompilerGenerated]
private sealed class <>c__DisplayClass0
{
public static readonly C.<>c__DisplayClass0 CS$<>9__inst;
public static Action<int> CS$<>9__CachedAnonymousMethodDelegate2;
static <>c__DisplayClass0()
{
// Note: this type is marked as 'beforefieldinit'.
C.<>c__DisplayClass0.CS$<>9__inst = new C.<>c__DisplayClass0();
}
internal void <M>b__1(int y)
{
Console.WriteLine(y);
}
}
public void M()
{
Action<int> arg_22_0;
if (arg_22_0 = C.
<>c__DisplayClass0.CS$<>9__CachedAnonymousMethodDelegate2 == null)
{
C.<>c__DisplayClass0.CS$<>9__CachedAnonymousMethodDelegate2 =
new Action<int>(C.<>c__DisplayClass0.CS$<>9__inst.<M>b__1);
}
}
}
Delegate caching behavior changes in Roslyn talks about why this change was made.
As of C# 6, this will always default to instance methods now, and will never be static (so actionMethod.Method.IsStatic will always be false).
See here: Why has a lambda with no capture changed from a static in C# 5 to an instance method in C# 6?
and here: Difference in CSC and Roslyn compiler's static lambda expression evaluation?
The method has no closures and also references a static method itself (Console.WriteLine), so I would expect it to be static. The method will declare an enclosing anonymous type for a closure, but in this instance it is not required.
If I only register one method of one class to a delegate, I can use the delegate.Target, but when I subscripe more methods from different classes this does not work anymore. Is there another way to access the subscribers list of this delegate?
Here is the code: The foreach loop is being evaluated to null at runtime (it compiles)
public delegate void WriteMessage(string msg);
internal class Program
{
private static void Main(string[] args)
{
var myClass = new MyClass();
var writer = new WriteMessage(myClass.WriteMessage);
writer += SaySomething; //method in this class
writer += myClass.SayShit; //instance class
writer += AnotherClass.Say; //static class
foreach(string target in (string[])writer.Target)
{
Console.WriteLine(target);
}
Console.ReadLine();
}
private static void SaySomething(string msg)
{
Console.WriteLine("HI!!!!");
}
}
complete code:
http://pastebin.com/AzzRGMY9
Delegate[] list = delegate.GetInvocationList();
That will get you an array of Delegate objects, which you can use to get the list of Targets.
This is just additional information from the accepted answer, as I am looking the same information online.
If you want to invoke all registered methods upon receiving all the invocation list, you can use this code:
Delegate[] listAllRegisteredMethods = writer.GetInvocationList(); //writer is the variable based on the question example
foreach(Delegate c in listAllRegisteredMethods )
{
object[] p = { }; //Insert your parameters here inside the array if your delegate has parameters
c.DynamicInvoke(p); //Invoke it, if you have return values, assign it on a different variable
}
im kinda new to c#, so I came up with this problem. Question: why is func2 called?
oh, and one more thing. say I add a function to a delegate. In this function I call another delegate, however I want to make sure that every other function added to the first delegate is called before this function calls this delegate, is there any clean solution ( not really interested in getInvocationList).
Thanks guys, you're the best.
class Program
{
delegate void voidEvent();
voidEvent test;
private void func1()
{
Console.Write("func1");
test -= func2;
}
private void func2()
{
Console.WriteLine("func2");
}
static void Main(string[] args)
{
Program p = new Program();
p.test += p.func1;
p.test += p.func2;
p.test();
}
}
Every time you change a delegate (+= or -=), you're effectively creating an entire copy of the invocation list (the methods that will get called).
Because of that, when you call p.test();, you're going to invoke every delegate in the invocation list at that point in time. Changing this inside of one of those handlers will change it for the next call, but won't change the currently executing call.
Reed is of course correct. Here's another way to think about it.
class Number
{
public static Number test;
private int x;
public Number(int x) { this.x = x; }
public Number AddOne()
{
return new Number(x + 1);
}
public void DoIt()
{
Console.WriteLine(x);
test = test.AddOne();
Console.WriteLine(x);
}
public static void Main()
{
test = new Number(1);
test.DoIt();
}
}
Should that print 1, 1, or 1, 2? Why?
It should print 1, 1. When you say
test.DoIt();
that does not mean
Console.WriteLine(test.x);
test = test.AddOne();
Console.WriteLine(test.x);
! Rather, that means
Number temporary = test;
Console.WriteLine(temporary.x);
test = test.AddOne();
Console.WriteLine(temporary.x);
Changing the value of test does not change the value of this in DoIt.
You're doing exactly the same thing. Changing the value of test does not change the list of functions that you are invoking; you asked for a particular list of functions to be invoked and that list is going to be invoked. You don't get to change it halfway through, any more than you get to change the meaning of this halfway through a method call.