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Invoke Function with async method delegate in C# [duplicate]

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."));
}

C# pass methods as parameters

I'm working with an C API in C#. In C Methods are passed as parameters and I'm trying to accomplish the same thing in C#.
in C I would call the functions the following way:
LL_SetStatusCb(OnStatusRcv);
LL_SetScanCb(scanCb);
LL_Scan();
Note that the used methods are defined in the following way:
void OnStatusRcv(ll_status_t status)
void scanCb(ll_scan_result_t *result)
In C# the methods are defined in the same way but I don't know how I can pass those methods.

C# equivalent of function pointers are delegates. You can use Func and Action to pass methods as parameters. Func delegate represents method which takes N arguments and returns value, Action delegate represents void method.
Consider this
void (* myFunction)(int parameter)
in C# would be
Action<int>

Please try this code:
create ll_scan_result_t and ll_status_t classes.
class Program
{
delegate void ActionRef<T>(ref T item);
static void Main(string[] args)
{
ll_status_t _status = new ll_status_t();
LL_SetStatusCb(_status, OnStatusRcv);
ll_scan_result_t _scan = new ll_scan_result_t();
LL_SetScanCb(ref _scan);
}
static void LL_SetScanCb(ref ll_scan_result_t status, ActionRef<ll_scan_result_t> getCachedValue)
{
//... do something
}
static void LL_SetStatusCb(ll_status_t result, Action<ll_status_t> getCachedValue)
{
//... do something
}
static void OnStatusRcv(ref ll_scan_result_t sresult)
{
//... do something
}
static void scanCb(ll_status_t s)
{
//... do something
}
}

Use the Func Delegate like below
public class myClass
{
public bool TestMethod(string input)
{
return true;
}
public bool Method1(Func<string, bool> methodName)
{
return true;
}
public void newMthod()
{
Method1(TestMethod);
}
}

In C#, the equivalent to C/C++ function pointers are delegates. A delegate is a type that represents references to methods with a particular parameter list and return type. When you instantiate a delegate, you can associate its instance with any method that has a compatible signature and return type. You can call the method through the delegate instance.
Here's an example. First, declare a delegate:
public delegate void Del(string message);
Now, Del is a delegate type which can be used to call to any method that returns void and accepts an argument of type string. Now, let's create some method matching the signature and return type of Del:
public static void DelegateMethod(string message)
{
Console.WriteLine(message);
}
Now, let's create an instance of Del and associate it with DelegateMethod, like this:
Del handler = DelegateMethod;
If you want to call DelegateMethod, you can do it by:
handler("Hello World");
Notice that since Del is a type, you can do something like this:
public static void SomeMethod(Del callback, string callbackParams)
{
callback(callbackParams);
}
Which can be used as:
SomeMethod(handler, "Hello World");
With that said, there are othes ways of working with delegates. You can use Func and Action delegates. Func is a delegate that points to a method that accepts one or more arguments and returns a value, that is, it doesn't return void. Action is a delegate that points to a method which in turn accepts one or more arguments but returns no value (returns void). In other words, you should use Action when your delegate points to a method that returns void.
Here's an example of using an Action delegate:
static void Main(string[] args)
{
Action<string> action = new Action<string>(Display);
action("Hello!!!");
Console.Read(); //Prevents from closing the command line right away.
}
static void Display(string message)
{
Console.WriteLine(message);
}
Therefore, something like
void (* funcPtr)(int) = &someFuncWithAnIntArg;
(*funcPtr)(10);
Is equivalent in C# to
Action<int> funcPtr = new Action<int>(someFuncWithAnIntArg);
funcPtr(10);
And now for a Func delegate:
static void Main(string[] args)
{
Func<int, double> func = new Func<int, double>(CalculateHra);
Console.WriteLine(func(50000));
Console.Read();
}
static double CalculateHra(int basic)
{
return (double)(basic * .4);
}
The syntax for a Func delegate accepting an argument and returning a value is like this Func<TArgument, TOutput> where TArgument is the type of the argument and TOutput is the type of returned value. There are many more types of Func (browse the left tree index) and Action (also browse the left tree index) delegates.
And last, but not least, we have the Predicate delegates which is typically used to search items in a collection or a set of data. Let's define some boilerplate code to explain:
class Customer
{
public int Id { get; set; }
public string FirstName { get; set; }
}
Then, let's try it in:
static void Main(string[] args)
{
List<Customer> customers = new List<Customer>();
customers.Add(new Customer { Id = 1, FirstName = "Stack" });
customers.Add(new Customer { Id = 2, FirstName = "Overflow" });
Predicate<Customer> pred = x => x.Id == 1;
Customer customer = customers.Find(pred);
Console.WriteLine(customer.FirstName);
Console.Read();
}
The last code snippet will print "Stack". What happened is that the Predicate delegate named prep was used as a search criteria to search in the list customers. Basically, this delegate was run on every element x of the list, and when x.Id == 1 it returns true, false otherwise. The x element where the predicate returned true is returned as the result of the Find method.

C# methods with variable paramterlist as parameter

Hi I'm currently trying to pass methods (with no return value) as parameters to another method (so that they can be called from within the methods).
The problem I'm currently having is, that I'm using Action in the parameterlist and thus need to exactly define which parameters this method takes.
The question thus is: Is there any way to omit this? Thus that I don't have to define which parameters exactly the method has in the parameterdeclaration?
Codeexample:
public void A(int myint)
{
Console.WriteLine(myint.ToString());
}
public void B(int myint1, int myint2)
{
Console.WriteLine((myint1 + myint2).ToString());
}
public void myQuestionMethod(Action<int> parameterMethod)
{
//....Dosomething special by creating the parameters within and calling the given methods
}
myQuestionMethod(A);
myQuestionMethod(B);
Thus Aciton parameterMethod can that be replaced by something else that allows me to give methods as parameters who have differing parameters?
Edit:
I forgot to mention that the TYPE of the parameters is also not fixated.
Thus a function C could exist with (int param1, String param2)

No. There is no way to do this with the Action delegate (that's why there are 16 overloads).
You could opt, if the variables are all of the same type and have the same meaning, to create an array of integers:
public void A(params int[] myint)
{
}
public void myQuestionMethod(Action<int[]> parameterMethod)
{
//....Dosomething special by creating the parameters within and calling the given methods
}

Depending on how big your methods are, you could go for just Action and use anonymous methods rather than explicitly defining the functions
public void myQuestionMethod(Action parameterMethod)
{
//
}
...
myQuestionMethod(() => Console.WriteLine(myInt.ToString()));
myQuestionMethod(() => Console.WriteLine((myInt1 + myInt2).ToString()));

One solution would be to use reflection. Of course don't use it unless you do not have any other choice (specifying a method using its name should be avoided if possible):
public class Foo
{
public void A(int myint)
{
Console.WriteLine(myint.ToString());
}
public void B(int myint1, int myint2)
{
Console.WriteLine((myint1 + myint2).ToString());
}
public void myQuestionMethod(string parameterMethodName, params object[] parameters)
{
var method = this.GetType().GetMethod(parameterMethodName, BindingFlags.Instance | BindingFlags.Public);
method.Invoke(this, parameters);
}
}
public class Test
{
public static void Main()
{
var foo = new Foo();
foo.myQuestionMethod("B", 1, 2);
Console.Read();
}
}

Getting rid of function's argument

It is better to explain my intention with code. So right now I have the following code:
class A, IInterfaceUsedByB
{
}
class B
{
void func(A someObject)
{
func2(someObject, 1);
func3(someObject, "string");
func4(someObject, new MyObject());
var anotherObject = someObject.NewInstance;
func2(anotherObject, 1);
.......................
}
func2(A someObject, int val);
func3(A someObject, string val);
func4(A someObject, C val);
}
Where func2, func3, func4 do need references to someObject. I want to change this to
void func(A someObject)
{
with(someObject,
() =>
{
func2(1);
func3("string");
func4(new MyObject());
}
);
var anotherObject = someObject.NewInstance;
with(anotherObject,
() =>
{
func2(1);
.......................
}
);
}
Or even better to
void func(A someObject)
{
func2(1);
func3("string");
func4(new MyObject());
}
So that I don't have to drag this someObject around, but I should still be able to use it inside func2,3,4. In other words it's like setting some context object for a particular code block. I can use any of the three languages (C#, F# or IronPython) for this.
UPDATE In the ideal solution class B would be independent of A. func* functions only depend on a small interface of A consisting of 2 methods.
UPDATE2 The someObject can be different for different function calls.
UPDATE3 I don't think that there's a way to do this only with language primitives like classes, interfaces or extension methods. I think that the solution might be possible with more hackish tools, like attributes, reflection or F#'s computation workflow feature. Or maybe some dynamic features of IronPython. Essentially what I'm trying to do is to create a small embedded DSL.

Partial function application might work for you here.
// object for example first param
var objectInstance = new object();
// partial function application
var func2Partial = new Action<int>((i) => func2(objectInstance, i));
func2Partial(5);
If func2 returns something then you'll need to use Func<> rather than Action<>
Alternatively, as per my comment below an extension method may be an option also.
internal static class IInterfaceUsedByBExtender
{
internal static void Func2(this IInterfaceUsedByB obj, int value)
{
}
}
// caller
someObject.Func2(5);
Another option may be the pipe operator in F#. I have v. basic F# knowledge but I think you can define a function with two params and use the pipe operator to fill the value for the first param. So something like:
let printnums x y = printfn "%A - %A" x y;;
Can be called by
4 |> printnums 5;;
Which outputs:
4 - 5
With func2 you'd have something like
let func2 obj, num = ...;
someObject |> func2 1;;
More examples of this: stackoverflow: Piping F# Parameters
I hope this helps!

If all those functions operate on one object (sameObject), it looks like you should introduce a class and make them class methods. I don't know what type someObject is of, but here is an example in C#:
class Workflow<T>
where T: class
{
private readonly T _someObject;
public Workflow(T someObject)
{
_someObject = someObject;
}
public void func()
{
func2(1);
func3("string");
func4(new MyObject());
}
private void func2(int a)
{
// use _someObject here
}
private void func3(string s)
{
// ...
}
}
// usage
var flow = new Workflow(someObject);
flow.f();

class A, IInterfaceUsedByB
{
}
class B<T> where T : A
{
private T m_SomeObject;
public B(T someobject)
{
m_Someobject = someobject;
}
void func()
{
func2(1);
func3("string");
func4(new MyObject());
}
func2(int val) { //Do whatever with m_SomeObject }
func3(string val) { //Do whatever with m_SomeObject }
func4(C val){ //Do whatever with m_SomeObject }
}

You could write a function that takes a A and returns a func2 function.
For example, if the func2 return type is void (if not, then use Func < int, ReturnType>):
Action<int> func2Gen(A someObject)
{
return i => Console.WriteLine(i+someObject.ToString());
}
Then, whereever suited, you can declare
var func2=func2Gen(someObject);
and call it:
func2(1);

Function in C# to call a class/method as in php

I'm taking my first steps in c # & asp.net and I'm enjoying it much.
Now, I have a question...
Is there a function in C# to call a class/method as in php?
For example:
$class = array(
"foo", // class name
"bar" // method name
);
$params = array(
"one",
"two"
);
call_user_func_array($class, $params); //execute Foo->bar("one","two");

Nope, there is nothing built in to do exactly that. You could build a method that does something similar using reflection, but it seems like a solution looking for a problem.
void Main()
{
CallUserFuncArray("UserQuery+Foo", "Bar", "One", "Two");
}
// Define other methods and classes here
public class Foo
{
public static void Bar(string a, string b){}
}
public void CallUserFuncArray(string className, string methodName, params object[] args)
{
Type.GetType(className).GetMethod(methodName).Invoke(null, args);
}

As others have noted, there are multiple ways to simulate this, but no "baked-in" functionality in C#. The most flexible way is with reflection, but you can do it in a much simpler (and easier to deal with) way if you know the list of methods you'll be calling beforehand.
class Foo
{
public static string FooA(int p1, int p2)
{
return "FooA:" + p1 + p2;
}
public static string FooB(int p1, int p2) { return "FooB:" + p1 + p2; }
public static string FooC(int p1, int p2) { return "FooC:" + p1 + p2; }
}
class Bar
{
//You can use Func<int, int, object> instead of a delegate type,
//but this way is a little easier to read.
public delegate string Del(int p1, int p2);
public static string DoStuff()
{
var classes = new Dictionary<string, Dictionary<string, Del>>();
classes.Add("Foo", new Dictionary<string, Del>());
classes["Foo"].Add("FooA", Foo.FooA);
classes["Foo"].Add("FooB", Foo.FooB);
classes["Foo"].Add("FooC", Foo.FooC);
//...snip...
return classes["Foo"]["FooA"](5, 7);
}
}
Which, by the way, does work.
If you don't know which methods you want to make available this way, I suggest you reconsider whaever you're trying to do. The only reason I can think of for using strings to choose your execution path would be if you were planning to get those strings from the user. Not only is it a huge no-no to expose inner details of your application like this, but it comes dangerously close to eval-type functionality. There's a reason C# doesn't have an eval method, and it isn't because the designers forgot to put it in.

As #StriplingWarrior said, there's no built-in equivalent of call_user_func_array, but you can do something like it with Reflection.
The problem is that Reflection code can get very complicated very quickly, and can be brittle and error-prone if you're not VERY careful.
For example the following function does what you want:
public static void CallUserFuncArray(string[] func, params string[] args)
{
var type = Type.GetType(func[0]);
if (type == null)
{
throw new ArgumentException("The specified Class could not be found");
}
var method = type.GetMethod(func[1], BindingFlags.Static | BindingFlags.Public);
if (method== null)
{
throw new ArgumentException("The specified Method could not be found");
}
method.Invoke(null, args);
}
You call it like this:
var func = new [] { "Foo", "Bar" };
var args = new [] { "one", "two" };
CallUserFuncArray(func, args);
The problems though are many.
The code only works if Bar is a public static method.
There's a whole new layer of complexity if you need to call an instance method on an object.
The code will explode if the parameters in the args array aren't just right for the target method.
There's no support here for calling methods that expect anything other than string parameters. It's possible to query the type of the arguments expected by the method and convert the types before calling 'Invoke', but you're getting even more messy.
There are many more edge cases that blow out the complexity of this code even more if you need to cater for them.
To paraphrase Carl Franklin (of dotNetRocks fame):
I had a problem I needed to solve, so I used Reflection. Now I have two problems.
I you find yourself need to do this sort of thing thenyou probably need to rethink your overall design.