Develop Reference

How to create a generic timeout object for various code blocks?

I have a series of code blocks that are taking too long. I don't need any finesse when it fails. In fact, I want to throw an exception when these blocks take too long, and just fall out through our standard error handling. I would prefer to NOT create methods out of each block (which are the only suggestions I've seen so far), as it would require a major rewrite of the code base.
Here's what I would LIKE to create, if possible.
public void MyMethod( ... )
{
...
using (MyTimeoutObject mto = new MyTimeoutObject(new TimeSpan(0,0,30)))
{
// Everything in here must complete within the timespan
// or mto will throw an exception. When the using block
// disposes of mto, then the timer is disabled and
// disaster is averted.
}
...
}
I've created a simple object to do this using the Timer class. (NOTE for those that like to copy/paste: THIS CODE DOES NOT WORK!!)
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Timers;
public class MyTimeoutObject : IDisposable
{
private Timer timer = null;
public MyTimeoutObject (TimeSpan ts)
{
timer = new Timer();
timer.Elapsed += timer_Elapsed;
timer.Interval = ts.TotalMilliseconds;
timer.Start();
}
void timer_Elapsed(object sender, ElapsedEventArgs e)
{
throw new TimeoutException("A code block has timed out.");
}
public void Dispose()
{
if (timer != null)
{
timer.Stop();
}
}
}
It does not work because the System.Timers.Timer class captures, absorbs and ignores any exceptions thrown within, which -- as I've discovered -- defeats my design. Any other way of creating this class/functionality without a total redesign?
This seemed so simple two hours ago, but is causing me much headache.

OK, I've spent some time on this one and I think I have a solution that will work for you without having to change your code all that much.
The following is how you would use the Timebox class that I created.
public void MyMethod( ... ) {
// some stuff
// instead of this
// using(...){ /* your code here */ }
// you can use this
var timebox = new Timebox(TimeSpan.FromSeconds(1));
timebox.Execute(() =>
{
/* your code here */
});
// some more stuff
}
Here's how Timebox works.
A Timebox object is created with a given Timespan
When Execute is called, the Timebox creates a child AppDomain to hold a TimeboxRuntime object reference, and returns a proxy to it
The TimeboxRuntime object in the child AppDomain takes an Action as input to execute within the child domain
Timebox then creates a task to call the TimeboxRuntime proxy
The task is started (and the action execution starts), and the "main" thread waits for for as long as the given TimeSpan
After the given TimeSpan (or when the task completes), the child AppDomain is unloaded whether the Action was completed or not.
A TimeoutException is thrown if action times out, otherwise if action throws an exception, it is caught by the child AppDomain and returned for the calling AppDomain to throw
A downside is that your program will need elevated enough permissions to create an AppDomain.
Here is a sample program which demonstrates how it works (I believe you can copy-paste this, if you include the correct usings). I also created this gist if you are interested.
public class Program
{
public static void Main()
{
try
{
var timebox = new Timebox(TimeSpan.FromSeconds(1));
timebox.Execute(() =>
{
// do your thing
for (var i = 0; i < 1000; i++)
{
Console.WriteLine(i);
}
});
Console.WriteLine("Didn't Time Out");
}
catch (TimeoutException e)
{
Console.WriteLine("Timed Out");
// handle it
}
catch(Exception e)
{
Console.WriteLine("Another exception was thrown in your timeboxed function");
// handle it
}
Console.WriteLine("Program Finished");
Console.ReadLine();
}
}
public class Timebox
{
private readonly TimeSpan _ts;
public Timebox(TimeSpan ts)
{
_ts = ts;
}
public void Execute(Action func)
{
AppDomain childDomain = null;
try
{
// Construct and initialize settings for a second AppDomain. Perhaps some of
// this is unnecessary but perhaps not.
var domainSetup = new AppDomainSetup()
{
ApplicationBase = AppDomain.CurrentDomain.SetupInformation.ApplicationBase,
ConfigurationFile = AppDomain.CurrentDomain.SetupInformation.ConfigurationFile,
ApplicationName = AppDomain.CurrentDomain.SetupInformation.ApplicationName,
LoaderOptimization = LoaderOptimization.MultiDomainHost
};
// Create the child AppDomain
childDomain = AppDomain.CreateDomain("Timebox Domain", null, domainSetup);
// Create an instance of the timebox runtime child AppDomain
var timeboxRuntime = (ITimeboxRuntime)childDomain.CreateInstanceAndUnwrap(
typeof(TimeboxRuntime).Assembly.FullName, typeof(TimeboxRuntime).FullName);
// Start the runtime, by passing it the function we're timboxing
Exception ex = null;
var timeoutOccurred = true;
var task = new Task(() =>
{
ex = timeboxRuntime.Run(func);
timeoutOccurred = false;
});
// start task, and wait for the alloted timespan. If the method doesn't finish
// by then, then we kill the childDomain and throw a TimeoutException
task.Start();
task.Wait(_ts);
// if the timeout occurred then we throw the exception for the caller to handle.
if(timeoutOccurred)
{
throw new TimeoutException("The child domain timed out");
}
// If no timeout occurred, then throw whatever exception was thrown
// by our child AppDomain, so that calling code "sees" the exception
// thrown by the code that it passes in.
if(ex != null)
{
throw ex;
}
}
finally
{
// kill the child domain whether or not the function has completed
if(childDomain != null) AppDomain.Unload(childDomain);
}
}
// don't strictly need this, but I prefer having an interface point to the proxy
private interface ITimeboxRuntime
{
Exception Run(Action action);
}
// Need to derive from MarshalByRefObject... proxy is returned across AppDomain boundary.
private class TimeboxRuntime : MarshalByRefObject, ITimeboxRuntime
{
public Exception Run(Action action)
{
try
{
// Nike: just do it!
action();
}
catch(Exception e)
{
// return the exception to be thrown in the calling AppDomain
return e;
}
return null;
}
}
}
EDIT:
The reason I went with an AppDomain instead of Threads or Tasks only, is because there is no bullet proof way for terminating Threads or Tasks for arbitrary code [1][2][3]. An AppDomain, for your requirements, seemed like the best approach to me.

Here's an async implementation of timeouts:
...
private readonly semaphore = new SemaphoreSlim(1,1);
...
// total time allowed here is 100ms
var tokenSource = new CancellationTokenSource(100);
try{
await WorkMethod(parameters, tokenSource.Token); // work
} catch (OperationCancelledException ocx){
// gracefully handle cancellations:
label.Text = "Operation timed out";
}
...
public async Task WorkMethod(object prm, CancellationToken ct){
try{
await sem.WaitAsync(ct); // equivalent to lock(object){...}
// synchronized work,
// call tokenSource.Token.ThrowIfCancellationRequested() or
// check tokenSource.IsCancellationRequested in long-running blocks
// and pass ct to other tasks, such as async HTTP or stream operations
} finally {
sem.Release();
}
}
NOT that I advise it, but you could pass the tokenSource instead of its Token into WorkMethod and periodically do tokenSource.CancelAfter(200) to add more time if you're certain you're not at a spot that can be dead-locked (waiting on an HTTP call) but I think that would be an esoteric approach to multithreading.
Instead your threads should be as fast as possible (minimum IO) and one thread can serialize the resources (producer) while others process a queue (consumers) if you need to deal with IO multithreading (say file compression, downloads etc) and avoid deadlock possibility altogether.

I really liked the visual idea of a using statement. However, that is not a viable solution. Why? Well, a sub-thread (the object/thread/timer within the using statement) cannot disrupt the main thread and inject an exception, thus causing it to stop what it was doing and jump to the nearest try/catch. That's what it all boils down to. The more I sat and worked with this, the more that came to light.
In short, it can't be done the way I wanted to do it.
However, I've taken Pieter's approach and mangled my code a bit. It does introduce some readability issues, but I've tried to mitigate them with comments and such.
public void MyMethod( ... )
{
...
// Placeholder for thread to kill if the action times out.
Thread threadToKill = null;
Action wrappedAction = () =>
{
// Take note of the action's thread. We may need to kill it later.
threadToKill = Thread.CurrentThread;
...
/* DO STUFF HERE */
...
};
// Now, execute the action. We'll deal with the action timeouts below.
IAsyncResult result = wrappedAction.BeginInvoke(null, null);
// Set the timeout to 10 minutes.
if (result.AsyncWaitHandle.WaitOne(10 * 60 * 1000))
{
// Everything was successful. Just clean up the invoke and get out.
wrappedAction.EndInvoke(result);
}
else
{
// We have timed out. We need to abort the thread!!
// Don't let it continue to try to do work. Something may be stuck.
threadToKill.Abort();
throw new TimeoutException("This code block timed out");
}
...
}
Since I'm doing this in three or four places per major section, this does get harder to read over. However, it works quite well.

C# how cancel an executing method

I have a delegate method to run a heavy process in my app (I must use MS Framework 3.5):
private delegate void delRunJob(string strBox, string strJob);
Execution:
private void run()
{
string strBox = "G4P";
string strJob = "Test";
delRunJob delegateRunJob = new delRunJob(runJobThread);
delegateRunJob.Invoke(strBox, strJob);
}
In some part of the method runJobThread
I call to an external program (SAP - Remote Function Calls) to retrieve data. The execution of that line can take 1-30 mins.
private void runJobThread(string strBox, string strJob)
{
// CODE ...
sapLocFunction.Call(); // When this line is running I cannot cancel the process
// CODE ...
}
I want to allow the user cancel whole process.
How can achieve this? I tried some methods; but I fall in the same point; when this specific line is running I cannot stop the process.

Instead of using the delegate mechanism you have to study the async and await mechanism. When you understand this mechanism you can move to cancellationtoken.
An example doing both things can be found here :
http://blogs.msdn.com/b/dotnet/archive/2012/06/06/async-in-4-5-enabling-progress-and-cancellation-in-async-apis.aspx

Well; I find out a complicated, but effective, way to solve my problem:
a.) I created a "Helper application" to show a notification icon when the process is running (To ensure to don't interfere with the normal execution of the main app):
private void callHelper(bool blnClose = false)
{
if (blnClose)
fw.processKill("SDM Helper");
else
Process.Start(fw.appGetPath + "SDM Helper.exe");
}
b.) I created a Thread that call only the heavy process line.
c.) While the Thread is alive I check for external file named "cancel" (The "Helper application" do that; when the user click an option to cancel the process the Helper create the file).
d.) If exists the file; dispose all objects and break the while cycle.
e.) The method sapLocFunction.Call() will raise an exception but I expect errors.
private void runJobThread(string strBox, string strJob)
{
// CODE ...
Thread thrSapCall = new Thread(() =>
{
try { sapLocFunction.Call(); }
catch { /* Do nothing */ }
});
thrSapCall.Start();
while (thrSapCall.IsAlive)
{
Thread.Sleep(1000);
try
{
if (fw.fileExists(fw.appGetPath + "\\cancel"))
{
sapLocFunction = null;
sapLocTable = null;
sapConn.Logoff();
sapConn = null;
canceled = true;
break;
}
}
finally { /* Do nothing */ }
}
thrSapCall = null;
// CODE ...
}
Works like a charm!

I think you would have to resort to the method described here. Read the post to see why this is a long way from ideal.
Perhaps this might work...
private void runJobThread(string strBox, string strJob, CancellationToken token)
{
Thread t = Thread.CurrentThread;
using (token.Register(t.Abort))
{
// CODE ...
sapLocFunction.Call(); // When this line is running I cannot cancel the process
// CODE ...
}
}

A bit of dnspy exposes a cancel method on nco3.0.
private readonly static Type RfcConnection = typeof(RfcSessionManager).Assembly.GetType("SAP.Middleware.Connector.RfcConnection");
private readonly static Func<RfcDestination, object> GetConnection = typeof(RfcSessionManager).GetMethod("GetConnection", BindingFlags.Static | BindingFlags.NonPublic).CreateDelegate(typeof(Func<RfcDestination, object>)) as Func<RfcDestination, object>;
private readonly static MethodInfo Cancel = RfcConnection.GetMethod("Cancel", BindingFlags.Instance | BindingFlags.NonPublic);
object connection = null;
var completed = true;
using (var task = Task.Run(() => { connection = GetConnection(destination); rfcFunction.Invoke(destination); }))
{
try
{
completed = task.Wait(TimeSpan.FromSeconds(invokeTimeout));
if (!completed)
Cancel.Invoke(connection, null);
task.Wait();
}
catch(AggregateException e)
{
if (e.InnerException is RfcCommunicationCanceledException && !completed)
throw new TimeoutException($"SAP FM {functionName} on {destination} did not respond in {timeout} seconds.");
throw;
}
}

what does callback mean? [duplicate]

What's a callback and how is it implemented in C#?

I just met you,
And this is crazy,
But here's my number (delegate),
So if something happens (event),
Call me, maybe (callback)?

In computer programming, a callback is executable code that is passed as an argument to other code.
—Wikipedia: Callback (computer science)
C# has delegates for that purpose. They are heavily used with events, as an event can automatically invoke a number of attached delegates (event handlers).

A callback is a function that will be called when a process is done executing a specific task.
The usage of a callback is usually in asynchronous logic.
To create a callback in C#, you need to store a function address inside a variable. This is achieved using a delegate or the new lambda semantic Func or Action.
public delegate void WorkCompletedCallBack(string result);
public void DoWork(WorkCompletedCallBack callback)
{
callback("Hello world");
}
public void Test()
{
WorkCompletedCallBack callback = TestCallBack; // Notice that I am referencing a method without its parameter
DoWork(callback);
}
public void TestCallBack(string result)
{
Console.WriteLine(result);
}
In today C#, this could be done using lambda like:
public void DoWork(Action<string> callback)
{
callback("Hello world");
}
public void Test()
{
DoWork((result) => Console.WriteLine(result));
DoWork(Console.WriteLine); // This also works
}

Definition
A callback is executable code that
is passed as an argument to other code.
Implementation
// Parent can Read
public class Parent
{
public string Read(){ /*reads here*/ };
}
// Child need Info
public class Child
{
private string information;
// declare a Delegate
delegate string GetInfo();
// use an instance of the declared Delegate
public GetInfo GetMeInformation;
public void ObtainInfo()
{
// Child will use the Parent capabilities via the Delegate
information = GetMeInformation();
}
}
Usage
Parent Peter = new Parent();
Child Johny = new Child();
// Tell Johny from where to obtain info
Johny.GetMeInformation = Peter.Read;
Johny.ObtainInfo(); // here Johny 'asks' Peter to read
Links
more details for C#.

A callback is a function pointer that you pass in to another function. The function you are calling will 'callback' (execute) the other function when it has completed.
Check out this link.

If you referring to ASP.Net callbacks:
In the default model for ASP.NET Web
pages, the user interacts with a page
and clicks a button or performs some
other action that results in a
postback. The page and its controls
are re-created, the page code runs on
the server, and a new version of the
page is rendered to the browser.
However, in some situations, it is
useful to run server code from the
client without performing a postback.
If the client script in the page is
maintaining some state information
(for example, local variable values),
posting the page and getting a new
copy of it destroys that state.
Additionally, page postbacks introduce
processing overhead that can decrease
performance and force the user to wait
for the page to be processed and
re-created.
To avoid losing client state and not
incur the processing overhead of a
server roundtrip, you can code an
ASP.NET Web page so that it can
perform client callbacks. In a client
callback, a client-script function
sends a request to an ASP.NET Web
page. The Web page runs a modified
version of its normal life cycle. The
page is initiated and its controls and
other members are created, and then a
specially marked method is invoked.
The method performs the processing
that you have coded and then returns a
value to the browser that can be read
by another client script function.
Throughout this process, the page is
live in the browser.
Source: http://msdn.microsoft.com/en-us/library/ms178208.aspx
If you are referring to callbacks in code:
Callbacks are often delegates to methods that are called when the specific operation has completed or performs a sub-action. You'll often find them in asynchronous operations. It is a programming principle that you can find in almost every coding language.
More info here: http://msdn.microsoft.com/en-us/library/ms173172.aspx

Dedication to LightStriker:
Sample Code:
class CallBackExample
{
public delegate void MyNumber();
public static void CallMeBack()
{
Console.WriteLine("He/She is calling you. Pick your phone!:)");
Console.Read();
}
public static void MetYourCrush(MyNumber number)
{
int j;
Console.WriteLine("is she/he interested 0/1?:");
var i = Console.ReadLine();
if (int.TryParse(i, out j))
{
var interested = (j == 0) ? false : true;
if (interested)//event
{
//call his/her number
number();
}
else
{
Console.WriteLine("Nothing happened! :(");
Console.Read();
}
}
}
static void Main(string[] args)
{
MyNumber number = Program.CallMeBack;
Console.WriteLine("You have just met your crush and given your number");
MetYourCrush(number);
Console.Read();
Console.Read();
}
}
Code Explanation:
I created the code to implement the funny explanation provided by LightStriker in the above one of the replies. We are passing delegate (number) to a method (MetYourCrush). If the Interested (event) occurs in the method (MetYourCrush) then it will call the delegate (number) which was holding the reference of CallMeBack method. So, the CallMeBack method will be called. Basically, we are passing delegate to call the callback method.
Please let me know if you have any questions.

Probably not the dictionary definition, but a callback usually refers to a function, which is external to a particular object, being stored and then called upon a specific event.
An example might be when a UI button is created, it stores a reference to a function which performs an action. The action is handled by a different part of the code but when the button is pressed, the callback is called and this invokes the action to perform.
C#, rather than use the term 'callback' uses 'events' and 'delegates' and you can find out more about delegates here.

callback work steps:
1) we have to implement ICallbackEventHandler Interface
2) Register the client script :
String cbReference = Page.ClientScript.GetCallbackEventReference(this, "arg", "ReceiveServerData", "context");
String callbackScript = "function UseCallBack(arg, context)" + "{ " + cbReference + ";}";
Page.ClientScript.RegisterClientScriptBlock(this.GetType(), "UseCallBack", callbackScript, true);
1) from UI call Onclient click call javascript function for EX:- builpopup(p1,p2,p3...)
var finalfield= p1,p2,p3;
UseCallBack(finalfield, ""); data from the client passed to server side by using UseCallBack
2) public void RaiseCallbackEvent(string eventArgument) In eventArgument we get the passed data
//do some server side operation and passed to "callbackResult"
3) GetCallbackResult() // using this method data will be passed to client(ReceiveServerData() function) side
callbackResult
4) Get the data at client side:
ReceiveServerData(text) , in text server response , we wil get.

A callback is a function passed as an argument to another function. This technique allows a function to invoke the parameter function argument and even to pass a value back to the caller. A callback function can be designed to run before/after the function has finished and can pass a value.
It is a kind of construct where you call a long running function and ask him to call you back once it has finished with can return a parameter result to the caller.
It's like someone calls you in the middle of your work asking for status and you say "you know what give me 5 min and i will call you back" and at the end you call him to update. If you are a function the caller just added and passed another function that you invoked at the end. This can simpley be written in C# as:
public void VinodSrivastav(Action statusUpdate){
//i am still here working..working
//i have finished, calling you
statusUpdate();
}
//invokes
stackoverflow.VinodSrivastav((cam) => {
Console.Write("Is it finished");
});
The one simple example is the iterator function where the return will be multiple times, one can argue that we have yield for it:
public void IntreationLoop(int min, int max,Action<int> Callback)
{
for(int i = min;i<= max;i++)
Callback(i);
}
//call
IntreationLoop(5,50,(x) => { Console.Write(x); }); //will print 5-50 numbers
In the code above the function return type is void but it has an Action<int> callback which is called and sends each item from the loop to the caller.
The same thing can be done with if..else or try..catch block as:
public void TryCatch(Action tryFor,Action catchIt)
{
try{
tryFor();
}
catch(Exception ex)
{
Console.WriteLine($"[{ex.HResult}] {ex.Message}");
catchIt();
}
}
And call it as:
TryCatch(()=>{
int r = 44;
Console.WriteLine("Throwing Exception");
throw new Exception("something is wrong here");
}, ()=>{
Console.WriteLine("It was a mistake, will not try again");
});
In 2022 we have Func & Action doing the same, please see the demo code below which shows how this can be be used:
void Main()
{
var demo = new CallbackDemo();
demo.DoWork(()=> { Console.WriteLine("I have finished the work"); });
demo.DoWork((r)=> { Console.WriteLine($"I have finished the work here is the result {r}"); });
demo.DoWork(()=> { Console.WriteLine($"This is passed with func"); return 5;});
demo.DoWork((f)=> { Console.WriteLine($"This is passed with func and result is {f}"); return 10;});
}
// Define other methods and classes here
public class CallbackDemo
{
public void DoWork(Action actionNoParameter)
{
int a = 5;
int b = 10;
//i will do th maths and call you back
int result = a + b;
//callback
actionNoParameter(); //execute
Console.WriteLine($"[The Actual Result is {result}]");
}
public void DoWork(Action<int> actionWithParameter)
{
int a = 5;
int b = 10;
//i will do th maths and call you back
int result = a + b;
//callback
actionWithParameter(result); //execute
Console.WriteLine($"[The Actual Result is {result}]");
}
public void DoWork(Func<int> funcWithReturn)
{
int a = 5;
int b = 10;
//i will do th maths and call you back
int result = a + b;
//callback
int c = funcWithReturn(); //execute
result += c;
Console.WriteLine($"[The Actual Result is {result}]");
}
public void DoWork(Func<int,int> funcWithParameter)
{
int a = 5;
int b = 10;
//i will do th maths and call you back
int result = a + b;
//callback
result += funcWithParameter(result); //execute
Console.WriteLine($"[The Actual Result is {result}]");
}
}

Catching an exception thrown in an asynchronous callback

I have a method that takes a callback argument to execute asynchronously, but the catch block doesn't seem to be catching any exceptions thrown by the synchronous call (this.Submit refers to a synchronous method).
public void Submit(FileInfo file, AnswerHandler callback)
{
SubmitFileDelegate submitDelegate = new SubmitFileDelegate(this.Submit);
submitDelegate.BeginInvoke(file, (IAsyncResult ar) =>
{
string result = submitDelegate.EndInvoke(ar);
callback(result);
}, null);
}
Is there a way to catch the exception thrown by the new thread and send it to the original thread? Also, is this the "proper" way to handle async exceptions? I wrote my code so it could be called like this (assuming the exception issue is fixed):
try
{
target.Submit(file, (response) =>
{
// do stuff
});
}
catch (Exception ex)
{
// catch stuff
}
but is there a more proper or elegant way to do this?

If you're targeting .NET 4.0, you can utilize the new Task Parallel Library, and observe the Task object's Exception property.
public Task Submit(FileInfo file)
{
return Task.Factory.StartNew(() => DoSomething(file));
}
private void DoSomething(FileInfo file)
{
throw new Exception();
}
Then use it like this:
Submit(myFileInfo).ContinueWith(task =>
{
// Check task.Exception for any exceptions.
// Do stuff with task.Result
});
where DoSomething is the method you'd like call asynchronously, and the delegate you pass to ContinueWith is your callback.
More information about exception handling in TPL can be found here: http://msdn.microsoft.com/en-us/library/dd997415.aspx

This is not a 'best practice' solution, but I think it's a simple one that should work.
Instead of having the delegate defined as
private delegate string SubmitFileDelegate(FileInfo file);
define it as
private delegate SubmitFileResult SubmitFileDelegate(FileInfo file);
and define the SubmitFileResult as follows:
public class SubmitFileResult
{
public string Result;
public Exception Exception;
}
Then, the method that actually does the file submission (not shown in the question) should be defined like this:
private static SubmitFileResult Submit(FileInfo file)
{
try
{
var submissionResult = ComplexSubmitFileMethod();
return new SubmitFileResult { Result = submissionResult };
}
catch (Exception ex)
{
return new SubmitFileResult {Exception = ex, Result = "ERROR"};
}
}
This way, you'll examine the result object, see if it has the Result or the Exception field set, and act accordingly.

In short, no.
When you call submitDelegate.BeginInvoke, it spawns the new thread, returns, and promptly exits your try/catch block (while the new thread runs in the background).
You could, however, catch all unhandled exceptions like this:
AppDomain.CurrentDomain.UnhandledException += new UnhandledExceptionEventHandler(YourException);
This will catch everything in the application domain, however (not just your async method).

C# parameter count mismatch when trying to add AsyncCallback into BeginInvoke()

I have main form (PrenosForm) and I am trying to run Form2 asynchronously.
It works without callback delegate:
this.BeginInvoke(cp, new object[] { datoteke, this.treeView1.SelectedNode.FullPath.ToString(), this, efekt }, null); //works 1.
Doesn't work with callback delegate (parameter count mismatch):
this.BeginInvoke(cp, new object[] { datoteke, this.treeView1.SelectedNode.FullPath.ToString(), this, efekt }, new AsyncCallback(callBackDelegate), null); //doesn't work parameter count mismatch 2.
Works with callback delegate if I do it like this:
cp.BeginInvoke(datoteke, this.treeView1.SelectedNode.FullPath.ToString(), this, efekt, new AsyncCallback(callBackDelegate), null); //works 3.
My question is why does one way work and the other doesn't? I'm new at this. Would anyone be so kind as to answer my question and point out my mistakes?
private delegate void copyDelegat(List<ListViewItem> datoteke, string path, PrenosForm forma, DragDropEffects efekt);
private delegate void callBackDelegat(IAsyncResult a);
public void doCopy(List<ListViewItem> datoteke, string path, PrenosForm forma, DragDropEffects efekt)
{
new Form2(datoteke, path, forma, efekt);
}
public void callBackFunc(IAsyncResult a)
{
AsyncResult res = a.AsyncState as AsyncResult;
copyDelegat delegat = res.AsyncDelegate as copyDelegat;
delegat.EndInvoke(a);
}
public void kopiraj(List<ListViewItem> datoteke, DragDropEffects efekt)
{
copyDelegat cp = new copyDelegat(doCopy);
callBackDelegat callBackDelegate = new callBackDelegat(callBackFunc);
this.BeginInvoke(cp, new object[] { datoteke, this.treeView1.SelectedNode.FullPath.ToString(), this, efekt }, new AsyncCallback(callBackDelegate), null); //doesn't work parameter count missmatch 2.
this.BeginInvoke(cp, new object[] { datoteke, this.treeView1.SelectedNode.FullPath.ToString(), this, efekt }, null); //works 1.
cp.BeginInvoke(datoteke, this.treeView1.SelectedNode.FullPath.ToString(), this, efekt, new AsyncCallback(callBackDelegate), null); //works 3.
}

It is because Control.BeginInvoke() has a completely different signature from SomeDelegate.BeginInvoke(). While their method names are the same, they are fundamentally different methods. And fundamentally work differently at runtime, there is no comparison.
Control.BeginInvoke() takes a delegate and an object[]. Cast in stone.
private delegate SomeDelegate(mumble, foo, bar) automatically creates a SomeDelegate.BeginInvoke() method. Whose signature takes those three arguments, plus two extra arguments, a callback and a state object.
A significant runtime difference is that Control.BeginInvoke() can call a delegate and if it bombs with an exception then the exception is raised on the UI thread. A delegate's BeginInvoke() method doesn't do this, it re-raises the exception in the callback that calls EndInvoke().
Very confuzzling, I know, maybe they shouldn't have used the same name.

Don't do this at all.
Showing multiple forms on different threads is an extremely bad idea and will end up causing lots of trouble.
Your second example doesn't work because Control.BeginInvoke doesn't support a callback parameter.
Your code is interpreted as calling a delegate with three parameters; an array, and AsyncCallback, and null.
Since your method doesn't take such parameters, an exception is thrown.
Also, calling Control.BeginInvoke won't run a function in the background; it will run it on the UI thread when it next reaches the message loop.