Delegates and pointers

G

Guest

I am trying to use delegates for the first time and I am probably missing
something. I have a legacy DLL and I have used DllImport to get to the
functions that it contains...the normal function calls work fine, but I can't
get callbacks to work. The delegate declaration etc is below.

public delegate void callbackEvent();

callbackEvent Event = delegate()
{
MessageBox.Show("Callback Event Received");
}


The function in the DLL to register the callback is

myAPIregister(Event);

So I am expecting that myAPIregister(Event) will provide a pointer to the
Event delegate. When the callback event occurs it should simply display the
MessageBox with the text. The parameter in myAPIregister(parameter) is
expecting a pointer to the callback handler.

I have done a lot of reading about delegates over the past few days to get
this far but I appear to be missing something. The Compiler complains that
the argument is invalid.

Thanks for any help and guidance.
 
D

Dave

callbackEvent Event = delegate()
{
MessageBox.Show("Callback Event Received");
}
Click to expand...

The above syntax is incorrect.
Treat your callback as a method:

delegate void callbackEvent();

void MyCallBack()
{
MessageBox.Show("Callback Event Received");
}

void ConsumeDelegate()
{
// Create a pointer to MyCallBack using the callbackEvent signature:
callbackEvent cb = new callbackEvent(MyCallBack);

// Use the method pointer in your interop call:
myAPIregister(cb);

// > The parameter in myAPIregister(parameter) is
// > expecting a pointer to the callback handler.
//
// .NET Interop will automattically Marshal the delegate you pass to the external method as a pointer to the method
}
 
G

Guest

Thanks Dave,

I was confused on how to use delegates, but I think I understand how they
are used now. I have used your code as the basis for correcting what I had
done. I still have one final issue that perhaps you may know how to fix.
Here is the code:

delegate void callbackEvent();

void MyCallback()
{
MessageBox.Show("Callback Event Received");
}

private void ptRegisterLinkStateCallback_Click(object sender, EventArgs e)
{
callbackEvent cb = new callbackEvent(MyCallback);
status = ptAPI.ptRegisterLinkStateCallback(ptHostLinkStateChange, cb);
MessageBox.Show("Status = " + status.ToString());
}

So when the "ptRegisterLink...." button is clicked a pointer, cb, is created
to MyCallback. The "ptAPI.ptRegister..." function call is to an external DLL
using DLLImport and I am expecting the 'cb' argument to be the pointer to the
MyCallback method. The code in the DLL is:

public class ptAPI
{
[DllImport("patsapi.dll")]
public static extern int ptRegisterLinkStateCallback(int callbackID, int
mtdAddress);
}

The compiler complains that there are invalid arguments and that argument 2
'cannot convert from "...Form1.callbackEvent to int". Clearly I still have
something wrong with my code.

Any ideas as to why this is wrong? Thanks for your help with this, much
appreciated.

Wayne
 
D

Dave

No problem.

The declaration for your interop call (external method) should also declare the paramter type as the delegate your passing it:
[DllImport("patsapi.dll")]
public static extern int ptRegisterLinkStateCallback(int callbackID, callbackEvent CallBack);
Click to expand...

This, of course, means that you have to declare the delegate in your library and not the consuming program.

--
Dave Sexton
[email protected]
-----------------------------------------------------------------------
Wayne Weeks said:
Thanks Dave,

I was confused on how to use delegates, but I think I understand how they
are used now. I have used your code as the basis for correcting what I had
done. I still have one final issue that perhaps you may know how to fix.
Here is the code:

delegate void callbackEvent();

void MyCallback()
{
MessageBox.Show("Callback Event Received");
}

private void ptRegisterLinkStateCallback_Click(object sender, EventArgs e)
{
callbackEvent cb = new callbackEvent(MyCallback);
status = ptAPI.ptRegisterLinkStateCallback(ptHostLinkStateChange, cb);
MessageBox.Show("Status = " + status.ToString());
}

So when the "ptRegisterLink...." button is clicked a pointer, cb, is created
to MyCallback. The "ptAPI.ptRegister..." function call is to an external DLL
using DLLImport and I am expecting the 'cb' argument to be the pointer to the
MyCallback method. The code in the DLL is:

public class ptAPI
{
[DllImport("patsapi.dll")]
public static extern int ptRegisterLinkStateCallback(int callbackID, int
mtdAddress);
}

The compiler complains that there are invalid arguments and that argument 2
'cannot convert from "...Form1.callbackEvent to int". Clearly I still have
something wrong with my code.

Any ideas as to why this is wrong? Thanks for your help with this, much
appreciated.

Wayne

Dave said:
The above syntax is incorrect.
Treat your callback as a method:

delegate void callbackEvent();

void MyCallBack()
{
MessageBox.Show("Callback Event Received");
}

void ConsumeDelegate()
{
// Create a pointer to MyCallBack using the callbackEvent signature:
callbackEvent cb = new callbackEvent(MyCallBack);

// Use the method pointer in your interop call:
myAPIregister(cb);

// > The parameter in myAPIregister(parameter) is
// > expecting a pointer to the callback handler.
//
// .NET Interop will automattically Marshal the delegate you pass to the external method as a pointer to the method
}
Click to expand...
Click to expand...
 
G

Guest

OK, thankyou, yes that makes sense, need to make the parameter types the
same. Just one final issue and I hope I can move on with the fun part of
this application. As you note, I added the declaration of the delegate to
the library code as follows:

public class ptAPI
{
delegate void callbackEvent();

[DllImport("patsapi.dll")]
public static extern int ptRegisterLinkStateCallback(int callbackID,
callbackEvent Callback);
}

However when I try to recompile the library the compiler complains that
there is "Inconsistent accessibility", parameter type ptAPI.callbackEvent is
less accessibile than ptAPI.ptRegisterLinkStateCallback. Am I missing a
switch for the compiler? Something wrong with declaration of the delegate in
the library. This delegate construct is a tricky thing to grasp....thanks
again, hopefully I can get past this soon.

Wayne



Dave said:
No problem.

The declaration for your interop call (external method) should also declare the paramter type as the delegate your passing it:
[DllImport("patsapi.dll")]
public static extern int ptRegisterLinkStateCallback(int callbackID, callbackEvent CallBack);
Click to expand...

This, of course, means that you have to declare the delegate in your library and not the consuming program.

--
Dave Sexton
[email protected]
-----------------------------------------------------------------------
Wayne Weeks said:
Thanks Dave,

I was confused on how to use delegates, but I think I understand how they
are used now. I have used your code as the basis for correcting what I had
done. I still have one final issue that perhaps you may know how to fix.
Here is the code:

delegate void callbackEvent();

void MyCallback()
{
MessageBox.Show("Callback Event Received");
}

private void ptRegisterLinkStateCallback_Click(object sender, EventArgs e)
{
callbackEvent cb = new callbackEvent(MyCallback);
status = ptAPI.ptRegisterLinkStateCallback(ptHostLinkStateChange, cb);
MessageBox.Show("Status = " + status.ToString());
}

So when the "ptRegisterLink...." button is clicked a pointer, cb, is created
to MyCallback. The "ptAPI.ptRegister..." function call is to an external DLL
using DLLImport and I am expecting the 'cb' argument to be the pointer to the
MyCallback method. The code in the DLL is:

public class ptAPI
{
[DllImport("patsapi.dll")]
public static extern int ptRegisterLinkStateCallback(int callbackID, int
mtdAddress);
}

The compiler complains that there are invalid arguments and that argument 2
'cannot convert from "...Form1.callbackEvent to int". Clearly I still have
something wrong with my code.

Any ideas as to why this is wrong? Thanks for your help with this, much
appreciated.

Wayne

Dave said:
callbackEvent Event = delegate()
{
MessageBox.Show("Callback Event Received");
}

The above syntax is incorrect.
Treat your callback as a method:

delegate void callbackEvent();

void MyCallBack()
{
MessageBox.Show("Callback Event Received");
}

void ConsumeDelegate()
{
// Create a pointer to MyCallBack using the callbackEvent signature:
callbackEvent cb = new callbackEvent(MyCallBack);

// Use the method pointer in your interop call:
myAPIregister(cb);

// > The parameter in myAPIregister(parameter) is
// > expecting a pointer to the callback handler.
//
// .NET Interop will automattically Marshal the delegate you pass to the external method as a pointer to the method
}

--
Dave Sexton
[email protected]
-----------------------------------------------------------------------
I am trying to use delegates for the first time and I am probably missing
something. I have a legacy DLL and I have used DllImport to get to the
functions that it contains...the normal function calls work fine, but I can't
get callbacks to work. The delegate declaration etc is below.

public delegate void callbackEvent();

callbackEvent Event = delegate()
{
MessageBox.Show("Callback Event Received");
}


The function in the DLL to register the callback is

myAPIregister(Event);

So I am expecting that myAPIregister(Event) will provide a pointer to the
Event delegate. When the callback event occurs it should simply display the
MessageBox with the text. The parameter in myAPIregister(parameter) is
expecting a pointer to the callback handler.

I have done a lot of reading about delegates over the past few days to get
this far but I appear to be missing something. The Compiler complains that
the argument is invalid.

Thanks for any help and guidance.
Click to expand...
Click to expand...
Click to expand...
 
D

Dave

you haven't specified an access modifier on the delegate declaration. This error has nothing to do with the fact that your using a
delegate :) My sample code didn't specify it because it was intended for use within class-scope.

Try:

public delegate void callbackEvent();



--
Dave Sexton
[email protected]
-----------------------------------------------------------------------
Wayne Weeks said:
OK, thankyou, yes that makes sense, need to make the parameter types the
same. Just one final issue and I hope I can move on with the fun part of
this application. As you note, I added the declaration of the delegate to
the library code as follows:

public class ptAPI
{
delegate void callbackEvent();

[DllImport("patsapi.dll")]
public static extern int ptRegisterLinkStateCallback(int callbackID,
callbackEvent Callback);
}

However when I try to recompile the library the compiler complains that
there is "Inconsistent accessibility", parameter type ptAPI.callbackEvent is
less accessibile than ptAPI.ptRegisterLinkStateCallback. Am I missing a
switch for the compiler? Something wrong with declaration of the delegate in
the library. This delegate construct is a tricky thing to grasp....thanks
again, hopefully I can get past this soon.

Wayne



Dave said:
No problem.

The declaration for your interop call (external method) should also declare the paramter type as the delegate your passing it:
[DllImport("patsapi.dll")]
public static extern int ptRegisterLinkStateCallback(int callbackID, callbackEvent CallBack);
Click to expand...

This, of course, means that you have to declare the delegate in your library and not the consuming program.

--
Dave Sexton
[email protected]
-----------------------------------------------------------------------
Wayne Weeks said:
Thanks Dave,

I was confused on how to use delegates, but I think I understand how they
are used now. I have used your code as the basis for correcting what I had
done. I still have one final issue that perhaps you may know how to fix.
Here is the code:

delegate void callbackEvent();

void MyCallback()
{
MessageBox.Show("Callback Event Received");
}

private void ptRegisterLinkStateCallback_Click(object sender, EventArgs e)
{
callbackEvent cb = new callbackEvent(MyCallback);
status = ptAPI.ptRegisterLinkStateCallback(ptHostLinkStateChange, cb);
MessageBox.Show("Status = " + status.ToString());
}

So when the "ptRegisterLink...." button is clicked a pointer, cb, is created
to MyCallback. The "ptAPI.ptRegister..." function call is to an external DLL
using DLLImport and I am expecting the 'cb' argument to be the pointer to the
MyCallback method. The code in the DLL is:

public class ptAPI
{
[DllImport("patsapi.dll")]
public static extern int ptRegisterLinkStateCallback(int callbackID, int
mtdAddress);
}

The compiler complains that there are invalid arguments and that argument 2
'cannot convert from "...Form1.callbackEvent to int". Clearly I still have
something wrong with my code.

Any ideas as to why this is wrong? Thanks for your help with this, much
appreciated.

Wayne

:

callbackEvent Event = delegate()
{
MessageBox.Show("Callback Event Received");
}

The above syntax is incorrect.
Treat your callback as a method:

delegate void callbackEvent();

void MyCallBack()
{
MessageBox.Show("Callback Event Received");
}

void ConsumeDelegate()
{
// Create a pointer to MyCallBack using the callbackEvent signature:
callbackEvent cb = new callbackEvent(MyCallBack);

// Use the method pointer in your interop call:
myAPIregister(cb);

// > The parameter in myAPIregister(parameter) is
// > expecting a pointer to the callback handler.
//
// .NET Interop will automattically Marshal the delegate you pass to the external method as a pointer to the method
}

--
Dave Sexton
[email protected]
-----------------------------------------------------------------------
I am trying to use delegates for the first time and I am probably missing
something. I have a legacy DLL and I have used DllImport to get to the
functions that it contains...the normal function calls work fine, but I can't
get callbacks to work. The delegate declaration etc is below.

public delegate void callbackEvent();

callbackEvent Event = delegate()
{
MessageBox.Show("Callback Event Received");
}


The function in the DLL to register the callback is

myAPIregister(Event);

So I am expecting that myAPIregister(Event) will provide a pointer to the
Event delegate. When the callback event occurs it should simply display the
MessageBox with the text. The parameter in myAPIregister(parameter) is
expecting a pointer to the callback handler.

I have done a lot of reading about delegates over the past few days to get
this far but I appear to be missing something. The Compiler complains that
the argument is invalid.

Thanks for any help and guidance.
Click to expand...
Click to expand...
Click to expand...
 
G

Guest

Dave, a big thank you....finally I can get the code to build and it seems to
work....thanks again for your support getting this resolved...now on to more
interesting development.

Dave said:
you haven't specified an access modifier on the delegate declaration. This error has nothing to do with the fact that your using a
delegate :) My sample code didn't specify it because it was intended for use within class-scope.

Try:

public delegate void callbackEvent();



--
Dave Sexton
[email protected]
-----------------------------------------------------------------------
Wayne Weeks said:
OK, thankyou, yes that makes sense, need to make the parameter types the
same. Just one final issue and I hope I can move on with the fun part of
this application. As you note, I added the declaration of the delegate to
the library code as follows:

public class ptAPI
{
delegate void callbackEvent();

[DllImport("patsapi.dll")]
public static extern int ptRegisterLinkStateCallback(int callbackID,
callbackEvent Callback);
}

However when I try to recompile the library the compiler complains that
there is "Inconsistent accessibility", parameter type ptAPI.callbackEvent is
less accessibile than ptAPI.ptRegisterLinkStateCallback. Am I missing a
switch for the compiler? Something wrong with declaration of the delegate in
the library. This delegate construct is a tricky thing to grasp....thanks
again, hopefully I can get past this soon.

Wayne



Dave said:
No problem.

The declaration for your interop call (external method) should also declare the paramter type as the delegate your passing it:

[DllImport("patsapi.dll")]
public static extern int ptRegisterLinkStateCallback(int callbackID, callbackEvent CallBack);

This, of course, means that you have to declare the delegate in your library and not the consuming program.

--
Dave Sexton
[email protected]
-----------------------------------------------------------------------
Thanks Dave,

I was confused on how to use delegates, but I think I understand how they
are used now. I have used your code as the basis for correcting what I had
done. I still have one final issue that perhaps you may know how to fix.
Here is the code:

delegate void callbackEvent();

void MyCallback()
{
MessageBox.Show("Callback Event Received");
}

private void ptRegisterLinkStateCallback_Click(object sender, EventArgs e)
{
callbackEvent cb = new callbackEvent(MyCallback);
status = ptAPI.ptRegisterLinkStateCallback(ptHostLinkStateChange, cb);
MessageBox.Show("Status = " + status.ToString());
}

So when the "ptRegisterLink...." button is clicked a pointer, cb, is created
to MyCallback. The "ptAPI.ptRegister..." function call is to an external DLL
using DLLImport and I am expecting the 'cb' argument to be the pointer to the
MyCallback method. The code in the DLL is:

public class ptAPI
{
[DllImport("patsapi.dll")]
public static extern int ptRegisterLinkStateCallback(int callbackID, int
mtdAddress);
}

The compiler complains that there are invalid arguments and that argument 2
'cannot convert from "...Form1.callbackEvent to int". Clearly I still have
something wrong with my code.

Any ideas as to why this is wrong? Thanks for your help with this, much
appreciated.

Wayne

:

callbackEvent Event = delegate()
{
MessageBox.Show("Callback Event Received");
}

The above syntax is incorrect.
Treat your callback as a method:

delegate void callbackEvent();

void MyCallBack()
{
MessageBox.Show("Callback Event Received");
}

void ConsumeDelegate()
{
// Create a pointer to MyCallBack using the callbackEvent signature:
callbackEvent cb = new callbackEvent(MyCallBack);

// Use the method pointer in your interop call:
myAPIregister(cb);

// > The parameter in myAPIregister(parameter) is
// > expecting a pointer to the callback handler.
//
// .NET Interop will automattically Marshal the delegate you pass to the external method as a pointer to the method
}

--
Dave Sexton
[email protected]
-----------------------------------------------------------------------
I am trying to use delegates for the first time and I am probably missing
something. I have a legacy DLL and I have used DllImport to get to the
functions that it contains...the normal function calls work fine, but I can't
get callbacks to work. The delegate declaration etc is below.

public delegate void callbackEvent();

callbackEvent Event = delegate()
{
MessageBox.Show("Callback Event Received");
}


The function in the DLL to register the callback is

myAPIregister(Event);

So I am expecting that myAPIregister(Event) will provide a pointer to the
Event delegate. When the callback event occurs it should simply display the
MessageBox with the text. The parameter in myAPIregister(parameter) is
expecting a pointer to the callback handler.

I have done a lot of reading about delegates over the past few days to get
this far but I appear to be missing something. The Compiler complains that
the argument is invalid.

Thanks for any help and guidance.
Click to expand...
Click to expand...
Click to expand...
 

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