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POC#-PInvoke: What does UnmanagedFunctionPointer do?
 

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  1. POC#-PInvoke: What does UnmanagedFunctionPointer do?
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    <p>First, why do I ask this question? Because the Windows CE platform I am stuck with does not support this attribute. So I need to do everything to my delegate (that is getting called from C++) that the attribute normally does.</p> <p>In normal Windows I have</p> <pre><code>[UnmanagedFunctionPointer(CallingConvention.Cdecl)] public delegate UInt32 UnmanagedAuthenticationEvent(UInt32 authType, UInt32 numberToDisplay, IntPtr btAddress, ref UInt32 numberOfDigits) </code></pre> <p>And it works fine. When called from C++ the <strong>return value</strong> and the <strong>ref UInt32 numberOfDigits</strong> value are both properly received from the C# layer. Now I remove the attribute and use the Windows CE required approach of passing an unmanaged function pointer to C++ via PInvoke:</p> <pre><code>_authenticationEvent = new UnmanagedAuthenticationEvent(OnReceiveUnmanagedAuthenticationEvent); _unmanagedAuthenticationEvent = Marshal.GetFunctionPointerForDelegate(_authenticationEvent); CallRegisterForAuthenticationEvent(_uBTTransportPtr, _unmanagedAuthenticationEvent); </code></pre> <p>where 'CallRegisterForAuthenticationEvent(IntPtr, IntPtr)' is the PInvoke signature.</p> <p>I have declared the following class 'variables'</p> <pre><code>private IntPtr _unmanagedAuthenticationEvent; private Delegate _authenticationEvent; private UInt32 _numberOfDigits; // the ref UInt32 passed back to C++ private UInt32 _matchValue; // the return value </code></pre> <p>to protect my delegate function pointer AND the variables passed back to C++ from garbage collection. At first I didn't add the return value and ref UInt32 value and both were garbage in C++. After doing the above, the '_numberOfDigits' value is good but the return value is junk.</p> <p>The function I am now calling from C++ looks as follows (at least to start with):</p> <pre><code> #region OnReceiveUnmanagedAuthenticationEvent // This is the function called by the unmanaged code private UInt32 OnReceiveUnmanagedAuthenticationEvent(UInt32 authType, UInt32 numberToDisplay, IntPtr btAddress, ref UInt32 numberOfDigits) { byte[] byteAddress = new byte[6]; Marshal.Copy(btAddress, byteAddress, 0, 6); string btAddressString = Common.StaticHelper.BluetoothAddressFromByteArray(byteAddress); switch (authType) { case 2: //Auth_BT_PIN: if(OnPinRequestEvent != null) { string pin = ""; OnPinRequestEvent(ref pin, btAddressString); try { _matchValue = UInt32.Parse(pin); _numberOfDigits = (uint)pin.Length; // protect value from being GCed numberOfDigits = _numberOfDigits; return _matchValue; // The case I am viewing returns here } catch(ArgumentNullException e) { Console.WriteLine("Application entered a bad PIN value. Details " + e.Message); } catch(FormatException e) { Console.WriteLine("Application entered a bad PIN value. Details " + e.Message); } catch(OverflowException e) { Console.WriteLine("Application entered a bad PIN value. Details " + e.Message); } } break; </code></pre> <p><strong>What am I missing that the return value is junk?</strong> I suppose I could return the value as a ref value in the parameters and solve my problem but I want to know how UnmanagedFunctionPointer does it so I can address other issues that might come up in this painful Windows CE implementation.</p>
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    1. USBrian Reinhold
    1. USBrian Reinhold
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    1. COThere's no reasonable way to deal with this, the stack gets imbalanced when the calling convention is wrong. You *must* alter your C code and declare the function pointer __stdcall.
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    2. COMy understanding is that Windows CE has only one calling convention and that is Winapi which, I guess it CDECL. Is there a way I can declare the calling convention without using UnmanagedFunctionPointer attribute?
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      1. USBrian Reinhold
    3. COMaybe Windows CE doesnt need this declaration at all since it only supports one calling convention.
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      1. USBrian Reinhold
 

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