c#

.net

methods

delegates

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.

Solution 1

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);
    }
}

Solution 2

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.

Solution 3

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"));

Solution 4

In order to provide a clear and complete answer, I'm going to start from the very beginning before coming up with three possible solutions.


A brief introduction

All languages that run on top of the CLR (Common Language Runtime), such as C#, F#, and Visual Basic, work under a VM that runs higher level code than machine code. It follows that methods aren't Assembly subroutines, nor are they values, unlike JavaScript and most functional languages; rather, they're symbols that CLR recognizes. Thus, you cannot think to pass a method as a parameter, because methods don't produce any values themselves, as they're not expressions but statements, which are stored in the generated assemblies. At this point, you'll face delegates.


What's a delegate?

A delegate represents a handle to a method (the term handle should be preferred over pointer as the latter would be an implementationdetail). 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 implemented by one's own.

Look at the following 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);
}

Three different solutions, the same underlying concept

  • The typeunsafe way

    Using the Delegate special class directly the same way as the example above. The drawback here is your code being typeunsafe, 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 typechecked the same as method declarations, leading to flawlessly safe code.

    Here's an 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 use 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 describe what they have to be passed in, nor is the delegate name meaningful on what it's supposed to do. Therefore, be cautious about when using these delegates won't impact code selfdescribing benefits and refrain from using them in contexts where their purpose is not absolutely selfevident.

The latter solution is the one most people posted. I'm also mentioning it in my answer for the sake of completeness.

Solution 5

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));

Solution 6

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);
}

Solution 7

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.

Solution 8

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;
    }

Solution 9

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");
}

Solution 10

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

Solution 11

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 objects 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);
  }
}

Solution 12

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."));
}

Solution 13

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