I have the following function
public static T Translate<T>(T entity)
{
....
}
Now if T is en IEnumerable<> I want to have a different behaviour so I made a second function
public static IEnumerable<T> Translate<T>(IEnumerable<T> entities)
{
....
}
When I invoke it like this
IEnumerable<string> test = new List<string>().AsEnumerable();
Translate(test);
However when I invoke it like this
Func<IEnumerable<string>> func = () => new List<string>().AsEnumerable();
Translate(func.Invoke())
It goes to the first one.
Why does this happen and what is the best construction to solve this?
UPDATE
I build a new example with the problem
static void Main(string[] args)
{
Func<IEnumerable<string>> stringFunction = () => new List<string>().AsEnumerable();
InvokeFunction(ExtendFunction(stringFunction));
}
private static T Convert<T>(T text) where T : class
{
return null;
}
private static IEnumerable<T> Convert<T>(IEnumerable<T> text)
{
return null;
}
private static Func<T> ExtendFunction<T>(Func<T> func) where T : class
{
return () => Convert(func.Invoke());
}
private static T InvokeFunction<T>(Func<T> func)
{
return func.Invoke();
}
The first function gets invoken now when I expect the second to be invoked.
You need to either add a second overload of ExtendFunction:
private static Func<IEnumerable<T>> ExtendFunction<T> (Func<IEnumerable<T>> func) where T : class
{
return () => Convert(func.Invoke());
}
Or make the first overload invoke Convert method dynamically:
private static Func<T> ExtendFunction<T> (Func<T> func) where T : class
{
return () => Convert((dynamic)func.Invoke());
}
The reason is that your ExtendFunction method chooses Convert method at compile time. You can avoid that be either adding a second overload of ExtendFunction which chooses the Convert method you need, or by moving the choice of Convert method to run time.
In C#, the closest to specialization is to use a more-specific overload; however this works only when the type is know at compile time.
In your case the type is decided at run time because of IEnumerable<T>, but the compiler cannot guarantee that it will be IEnumerable<T>.
If you add this line in your main method you'll get the second function called.
Convert(text: new List<string>().AsEnumerable());
this is because the type is know at compile time
so try your Main in this way and look the differences
static void Main(string[] args)
{
Func<IEnumerable<string>> stringFunction = () => new List<string>().AsEnumerable();
InvokeFunction(ExtendFunction(stringFunction));//first function invoked
Convert(text: new List<string>().AsEnumerable());//second function invoked
}
I had the same problem a few weeks ago. You can solve this by specifying the type explicitely on calling the method but that's not really worth it, because it means everybody using your methods have to know about this fact.
We solved our problem by actually giving the method another name. In your case, that would be a second method named:
public static IEnumerable<T> TranslateAll<T>(IEnumerable<T> entities)
Related
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 started using lambda expression and I use it often now but only the simple ones :-). Sometime I really get confused understanding lambda expressions in our existing code base. Tried hard to understand the code below but still not able to decipher completely :-(. I think because of the use of Func delegate I am not able to understand. I know that Func delegate is used when the delegate returns some thing. But in this case no clue.
Code snippet:
public class PrintProvider
{
private readonly IInstructionSheetViews _instructionSheetViews;
public PrintProvider(IInstructionSheetViews instructionSheetViews)
{
_instructionSheetViews = instructionSheetViews;
}
public void AddReport()
{
// Some implementation code goes here
var printViews = _instructionSheetViews.PrintViews;
// Some implementation code goes here
}
}
public class InstructionSheetViews : IInstructionSheetViews
{
private readonly IInstructionSheetFactory _factory;
private IEnumerable<IReport> _instructionSheetView;
private List<Func<IInstructionSheetFactory, IReport>> _instructionSheetViewList;
public InstructionSheetViews(IInstructionSheetFactory factory)
{
_factory = factory;
}
public IEnumerable<IReport> PrintViews
{
get
{
if (_instructionSheetView == null)
{
Init();
_instructionSheetView = _instructionSheetViewList.Select(x => x(_factory));
}
return _instructionSheetView;
}
}
private void Init()
{
_instructionSheetViewList = new List<Func<IInstructionSheetFactory, IReport>>();
_instructionSheetViewList.Add(x => x.BuildCommonData());
_instructionSheetViewList.Add(x => x.BuildSpecificData());
}
}
In the above code snippet, AddReport method calls "_instructionSheetViews.PrintViews" and this method inturn calls "Init()".
Q1. What is exactly getting added to "_instructionSheetViewList" here -
_instructionSheetViewList.Add(x => x.BuildCommonData());.
What I can guess is, it adds a method that returns a "IReport". But "_instructionSheetViewList" contains a list of "Func". So, Ideally isn't it that it should contain a method that takes input as "IInstructionSheetFactory" and return "IReport"?
Q2. How does this statement works. Basically the control flow.
_instructionSheetViewList.Select(x => x(_factory));
Can someone please explain me?
Thanks in advance.
So, Ideally isn't it that it should contain a method that takes input as "IInstructionSheetFactory" and return "IReport"?
It does. _instructionSheetViewList.Add(x => x.BuildCommonData()); is basically equivalent to this:
_instructionSheetViewList.Add(anonymousMethod12345);
/*...*/
public static IReport anonymousMethod12345 (IInstructionSheetFactory x)
{
return x.BuildCommonData();
}
_instructionSheetViewList.Add(x => x.BuildCommonData());
is equivalent to -
private IReport GetReport(IInstructionSheetFactory x)
{
return x.BuildCommonData();
}
And when you do this -
_instructionSheetViewList.Select(x => x(_factory));
it actually calls a method with Func as a input parameter to that method
which inturn calls the method referenced by Func delegate with _factory as input parameter
private IReport DoSomething(Func<IInstructionSheetFactory, IReport> x)
{
return x(_factory);
}
Hope this helps.
This has been asked before (I think), but looking through the previous answers I still haven't been able to figure out what I need.
Lets say I have a private method like:
private void GenericMethod<T, U>(T obj, U parm1)
Which I can use like so:
GenericMethod("test", 1);
GenericMethod(42, "hello world!");
GenericMethod(1.2345, "etc.");
How can I then pass my GenericMethod to another method so that I can then call it in that method in a similar way? e.g.:
AnotherMethod("something", GenericMethod);
...
public void AnotherMethod(string parm1, Action<what goes here?> method)
{
method("test", 1);
method(42, "hello world!");
method(1.2345, "etc.");
}
I just can't get my head around this! What do I need to specify as the generic parameters to Action in AnotherMethod?!
You need to pass into AnotherMethod not a single delegate of some specific type, but a thing which contructs delegates. I think this can only be done using reflection or dynamic types:
void Run ()
{
AnotherMethod("something", (t, u) => GenericMethod(t, u));
}
void GenericMethod<T, U> (T obj, U parm1)
{
Console.WriteLine("{0}, {1}", typeof(T).Name, typeof(U).Name);
}
void AnotherMethod(string parm1, Action<dynamic, dynamic> method)
{
method("test", 1);
method(42, "hello world!");
method(1.2345, "etc.");
}
Note that (t, u) => GenericMethod(t, u) cannot be replaced with just GenericMethod.
Consider using an intermediate class (or implement an interface):
class GenericMethodHolder {
public void GenericMethod<T, U>(T obj, U parm1) {...};
}
public void AnotherMethod(string parm1, GenericMethodHolder holder)
{
holder.GenericMethod("test", 1);
holder.GenericMethod(42, "hello world!");
holder.GenericMethod(1.2345, "etc.");
}
Just wanted to post another solution that I found usefull for a while, especially with caching. When I do a Get from Cache, and the cache item doesnt exist, I call the provided function to get the data, cache it and return. But, this can be used in your specific way too.
Your other method would need a signature similar to this, where getItemCallback is the GenericMethod() you want to be executed. Example has been cleared down for brevity.
public static T AnotherMethod<T>(string key, Func<T> _genericMethod ) where T : class
{
result = _genericMethod(); //looks like default constructor but the passed in params are in tact.
//... do some work here
return otherData as T;
}
Then you would call your AnotherMethod() as follows
var result = (Model)AnotherMethod("some string",() => GenericMethod(param1,param2));
I know its about several months later but maybe it will help the next person, as I forgot how to do this and couldn't find any similar answer on SO.
I have multiple calls to methods in a 3rd party library.
These methods have a wide variety of different signatures / parameter combinations.
There are specific errors that the 3rd party library generates that I would like to catch and handle, and since the resolution is the same I would like it to take place in a single handler.
So I want to be able to have a method that essentially takes in as parameters a function (delegate) and its arguments, and invokes it inside some try/catch logic.
I don't know if its just not possible or I'm not getting the syntax right, but I can't figure out how to handle the fact that the signature for each method being passed in is different.
Any suggestions?
You could use Action and Func<T> to wrap the methods, and closures to pass arguments.
public TResult CallMethod<TResult>(Func<ThirdPartyClass, TResult> func)
{
try
{
return func(this.wrappedObject);
}
catch(ThirdPartyException e)
{
// Handle
}
}
public void CallMethod(Action<ThirdPartyClass> method)
{
this.CallMethod(() => { method(this.WrappedObject); return 0; });
}
You could then use this via:
var result = wrapper.CallMethod(thirdParty => thirdParty.Foo(bar, baz));
Edit: The above was assuming you were wrapping an instance of the third party library. Given (from your comments) that these are static methods, you can just use:
public static TResult CallMethod<TResult>(Func<TResult> func)
{
try
{
return func();
}
catch(ThirdPartyException e)
{
// Handle
}
}
public static void CallMethod(Action method)
{
CallMethod(() => { method(); return 0; });
}
And then call via:
var result = Wrapper.CallMethod(() => ThirdParty.Foo(bar, baz));
I have a series of functions that I want to have the following functionality.
When the function is called, add itself to a list of functions remembering the parameters and values
Allow the list of functions to be called at a later date
The different functions have a variety of different parameters and I'm struggling to think of an elegant way to do this. Any help would be appreciated.
I think this would meet your needs, however the functions are not "adding themselves".
public class Recorder
{
private IList<Action> _recording;
public Recorder()
{
_recording = new List<Action>();
}
public void CallAndRecord(Action action)
{
_recording.Add(action);
action();
}
public void Playback()
{
foreach(var action in _recording)
{
action();
}
}
}
//usage
var recorder = new Recorder();
//calls the functions the first time, and records the order, function, and args
recorder.CallAndRecord(()=>Foo(1,2,4));
recorder.CallAndRecord(()=>Bar(6));
recorder.CallAndRecord(()=>Foo2("hello"));
recorder.CallAndRecord(()=>Bar2(0,11,true));
//plays everything back
recorder.Playback();
One way to make the functions "add themselves" would be to use an AOP lib such as postsharp or linfu dynamic proxy, and add an interceptor which adds the function and args to the array. To do this would probably be more work than it would be worth IMO, as the above is much simpler and still achieves the desired functionality.
There's hardly an elegant solution to this. Since you said the methods would all have different signature, there's no way to store them in a single array as delegates. With that out of the way, next you can try is using reflection, storing each parameter value in object[], storing the method as MethodInfo, and then invoking it.
Edit: This is what I could come up with:
private Dictionary<MethodBase, object[]> methodCollection = new Dictionary<MethodBase, object[]>();
public void AddMethod(MethodBase method, params object[] arguments)
{
methodCollection.Add(method, arguments);
}
private void MyMethod(int p1, string p2, bool p3)
{
AddMethod(System.Reflection.MethodInfo.GetCurrentMethod(), new object[] { p1, p2, p3 });
}
private void MyOtherMethod()
{
AddMethod(System.Reflection.MethodInfo.GetCurrentMethod(), new object[] { });
}
Then just invoke with method.Invoke(method.ReflectedType, args)
Maybe you could some how use the Delegate.DynamicInvoke(Object[] obj) function. You could add each method to an object array, then loop through the array calling DynamicInvoke on each one.
I'm not sure I understand your question, but I think you could use array of pointers to a functions(in C# it is called delegates). So when function is called, put function pointer in a list. In this way you can call function from list. Here is some idea. Notice when you add new delegate pointer to a list (functionPointers), in second list myParameters you add new object of type Parameters which holds function parameters in public attribute called parameters. This means that delegate i in list functionPointers for parameters has i-th object in list myParameters. This is how you know which parameters, are for which function. Probably there are some betters solutions, but this is alternative.
delegate void NewDelegate();
class Parameter{
public ArrayList parameters;
}
ArrayList functionPointers=new ArrayList();
ArrayList<Parameter> myParameters=new ArrayList<Parameter>();
NewDelegate myDelegate;
void someFunction(int a, int b){
myDelegate+=someFunction;//you add this function to delegate because this function is called
functionPointers.add(myDelegate);//Add delegete to list
Parameter p=new Parameter();//Create new Parameter object
p.parameters.add(a);//Add function parameters
p.parameters.add(b);
myParameters.add(p);//add object p to myParameters list
}
You could consider using a list of actions or functions
using System;
using System.Collections.Generic;
namespace ReplayConsole
{
class Program
{
private static IList<Action> _actions;
static void Main(string[] args)
{
_actions = new List<Action>
{
() => {
//do thing
},
() => {
//do thing
},
() => {
//do thing
},
() => {
//do thing
},
};
foreach (var action in _actions)
{
action();
}
}
}
if you want to store parameters as well and have you could use a Func and store and use it in much the same way
You could also look at Tasks
EDIT:
looking at the answers that popped up while I was writing mine this solution is very similar to Brook's