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POC# callback from C++ gives access violation
 

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  1. POC# callback from C++ gives access violation
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    <p>EDIT(Thank you ildjarn!): Solved by changing delegate(and Callback function signature to match) to</p> <pre><code>[UnmanagedFunctionPointer(CallingConvention.Cdecl)] public delegate void InstallStatusDel([MarshalAs(UnmanagedType.LPStr)]string Mesg, int Status); </code></pre> <p>Original post:</p> <p>I have a problem with a .net application written in C# that calls functions in a C dll. I've looked at other threads with similar questions but I must be missing something different. When I debug this in C# I am able to hit a breakpoint in InstallStatusCallback but when execution exits InstallStatusCallback there is an AccessViolationException. I've tried debugging with C and the access violation occurs before execution returns back from the callback. Thank you for any input.</p> <p>Project settings in C dll are set to use __cdecl by default. In C DLL land the following code is in place:</p> <pre><code>typedef void (__cdecl *StatusCallback)(const char* Mesg, int Status); __declspec(dllexport) int Install(void* thing1, void* thing2, void* thing3, StatusCallback Func); int Install(void* thing1, void* thing2, void* thing3, StatusCallback Func) { Func("msg", 3); return 0; } </code></pre> <p>In C# I have:</p> <pre><code>public partial class InstallerStatus : Form { [UnmanagedFunctionPointer(CallingConvention.Cdecl)] public delegate void InstallStatusDel(StringBuilder Mesg, int Status); public static extern int Install(IntPtr thing1, IntPtr thing2, IntPtr thing3, InstallStatusDel Func); [DllImport("myDll.dll", CallingConvention = CallingConvention.Cdecl)] private IntPtr mThing1; private IntPtr mThing2; private InstallStatusDel mInstallStatusFunc; private BackgroundWorker mInstallWorker; public InstallerStatus(IntPtr pThing1, IntPtr pThing2) { InitializeComponent(); mThing1 = pThing1; mThing2 = pThing2; mInstallStatusFunc = InstallStatusCallback; mProgressBar.Minimum = 0; mProgressBar.Maximum = 100; mProgressBar.Value = 0; mInstallWorker = new BackgroundWorker(); mInstallWorker.DoWork += new DoWorkEventHandler(InstallWork); mInstallWorker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(InstallWork_Completed); } private void InstallWork(object sender, DoWorkEventArgs e) { Install(mThing1, mThing2, IntPtr.Zero, mInstallStatusFunc); } private void InstallWork_Completed(object sender, RunWorkerCompletedEventArgs e) { Close(); } private void InstallStatusCallback(StringBuilder PartName, int Status) { } private void InstallLoad_Shown(object sender, EventArgs e) { mInstallWorker.RunWorkerAsync(); } } </code></pre>
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    1. USmicheal blunning
    1. USmicheal blunning
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    1. COWhy is `InstallStatusDel`'s first parameter a `StringBuilder` rather than a `string`? `StringBuilder` is for _mutable_ strings. In any case, you may want to add `[MarshalAs(UnmanagedType.LPStr)]` to said parameter.
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      1. USildjarn
    2. COThat was the problem! Thank you!
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      1. USmicheal blunning
 

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