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POCorrect use of SafeHandles in this P/Invoke use case
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  1. POCorrect use of SafeHandles in this P/Invoke use case
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    copied!<p>Working in C# with a native Dll, that uses opaque handles and internal reference counting, I have the following P/Invoke signatures (all decorated with DllImport attribute)</p> <pre><code>[DllImport("somedll.dll"] public extern IntPtr getHandleOfA(IntPtr handleToB, int index); //(1) public extern IntPtr makeNewHandleOfA(); //(2) public extern void addRefHandleToA(IntPtr handleToA); //(3) public extern void releaseHandleToA(IntPtr handleToA); //(4) public extern void doSomethingWithHandle(IntPtr handleToA) //(5) </code></pre> <p>The meanings of these calls are are follows:</p> <ol> <li><p>Get a pointer/handle to an opaque type A from an existing handle B. The internal reference count of the returned handle is unaffected.</p></li> <li><p>Create a new handle of A. The internal reference count is pre-incremented, and the handle should be released by the client with function 4, otherwise a leak will occur.</p></li> <li><p>Tell the dll to internally increase the reference count of a handle A. This allows us to be sure that the dll will not internally release a handle that we have acquired through function 1.</p></li> <li><p>Tell the dll to decrease the ref count of a handle. Should be called if we have increased the ref count of a handle, or acquired it through function 2.</p></li> <li><p>Perform some operation with the handle</p></li> </ol> <p>I would like to replace the IntPtr with my own subclass of SafeHandle. When I acquire handles by creating new ones, the procedure is obvious; the handle's ref count is pre-incremented inside the dll, so I just override the Release function of SafeHandle, and call releaseHandleToA(handle). Using this new class 'MySafeHandle', I can change the P/Incvoke signatures above like so:</p> <pre><code>public extern MySafeHandleA getHandleOfA(MySafeHandleB handleToB, int index); //(1) public extern MySafeHandleA makeNewHandleOfA(); //(2) public extern void addRefHandleToA(MySafeHandleA handleToA); //(3) public extern void releaseHandleToA(MySafeHandleA handleToA); //(4) public extern void doSomethingWithHandle(MySafeHandleA handleToA) //(5) </code></pre> <p>There is an error here though: in function 1, the handle acquired has not had its refcount increased, so trying to release the handle would be an error.</p> <p>So, perhaps I should always ensure getHandleOfA calls are paired with an immediate addRefHandleToA, like this:</p> <pre><code>[DllImport("somedll.dll"] private extern MySafeHandleA getHandleOfA(MySafeHandleB handleToB, int index); //(1) [DllImport("somedll.dll"] private extern void addRefHandleToA(MySafeHandleA handleToA); //(3) public MySafeHandleA _getHandleOfA(MySafeHandleB handleToB, int index) { var safehandle = getHandleOfA(handleToB, index); addRefHandleToA(safeHandle); return safeHandle; } </code></pre> <p>Is this safe?</p> <p>EDIT: Well, no it is clearly not safe, as addRefHandleToA(safeHandle); could fail. Is there a way I can make it safe? </p>
 

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