使用Wrapper对象正确清理excel互操作对象

Question

All of these questions:

• Excel 2007 Hangs When Closing via .NET
• How to properly clean up Excel interop objects in C#
• How to properly clean up interop objects in C#

struggle with the problem that C# does not release the Excel COM objects properly after using them. There are mainly two directions of working around this issue:

1. Kill the Excel process when Excel is not used anymore.
2. Take care to explicitly assign each COM object used to a variable first and to guarantee that eventually, Marshal.ReleaseComObject is executed on each.

Some have stated that 2 is too tedious and there is always some uncertainty whether you forget to stick to this rule at some places in the code. Still 1 seems dirty and error-prone to me, also I guess that in a restricted environment trying to kill a process could raise a security error.

So I've been thinking about solving 2 by creating another proxy object model which mimics the Excel object model (for me, it would suffice to implement the objects I actually need). The principle would look as follows:

• Each Excel Interop class has its proxy which wraps an object of that class.
• The proxy releases the COM object in its finalizer.
• The proxy mimics the interface of the Interop class.
• Any methods that originally returned a COM object are changed to return a proxy instead. The other methods simply delegate the implementation to the inner COM object.

Example:

public class Application
{
    private Microsoft.Office.Interop.Excel.Application innerApplication
        = new Microsoft.Office.Interop.Excel.Application innerApplication();

    ~Application()
    {
        Marshal.ReleaseCOMObject(innerApplication);
        innerApplication = null;
    }

    public Workbooks Workbooks
    {
        get { return new Workbooks(innerApplication.Workbooks); }
    }
}

public class Workbooks
{
    private Microsoft.Office.Interop.Excel.Workbooks innerWorkbooks;

    Workbooks(Microsoft.Office.Interop.Excel.Workbooks innerWorkbooks)
    {
        this.innerWorkbooks = innerWorkbooks;
    }

    ~Workbooks()
    {
        Marshal.ReleaseCOMObject(innerWorkbooks);
        innerWorkbooks = null;
    }
}

My questions to you are in particular:

• Who finds this a bad idea and why?
• Who finds this a gread idea? If so, why hasn't anybody implemented/published such a model yet? Is it only due to the effort, or am I missing a killing problem with that idea?
• Is it impossible/bad/error-prone to do the ReleaseCOMObject in the finalizer? (I've only seen proposals to put it in a Dispose() rather than in a finalizer - why?)
• If the approach makes sense, any suggestions to improve it?

Answer 1

Is it impossible/bad/dangerous to do the ReleaseCOMObject in the destructor? (I've only seen proposals to put it in a Dispose() rather than in a destructor - why?)

It is recommended not to put your clean up code in the finalizer because unlike the destructor in C++ it is not called deterministically. It might be called shortly after the object goes out of scope. It might take an hour. It might never be called. In general if you want to dispose unmanaged objects you should use the IDisposable pattern and not the finalizer.

This solution that you linked to attempts to work around that problem by explicitly calling the garbage collector and waiting for the finalizers to complete. This is really not recommended in general but for this particular situation some people consider it to be an acceptable solution due to the difficulty of keeping track of all the temporary unmanaged objects that get created. But explicitly cleaning up is the proper way of doing it. However given the difficulty of doing so, this "hack" may be acceptable. Note that this solution is probably better than the idea you proposed.

If instead you want to try to explicitly clean up, the "don't use two dots with COM objects" guideline will help you to remember to keep a reference to every object you create so that you can clean them up when you're done.

Answer 2

We use the LifetimeScope class that was described in the MSDN magazine. Using it properly cleans up objects and has worked great with our Excel exports. The code can be downloaded here and also contains the magazine article:

http://lifetimescope.codeplex.com/SourceControl/changeset/changes/1266

Answer 3

Look at my project MS Office for .NET . There is solved problem with referencich wrapper objects and native objects via native VB.NET late-binding ability.

Answer 4

For what it's worth, the Excel Refresh Service on codeplex uses this logic:

    public static void UsingCOM<T>(T reference, Action<T> doThis) where T : class
    {
        if (reference == null) return;
        try
        {
            doThis(reference);
        }
        finally
        {
            Marshal.ReleaseComObject(reference);
        }
    }

Answer 5

What I'd do:

class ScopedCleanup<T> : IDisposable where T : class
{
    readonly Action<T> cleanup;

    public ScopedCleanup(T o, Action<T> cleanup)
    {
        this.Object = o;
        this.cleanup = cleanup;
    }

    public T Object { get; private set; }

    #region IDisposable Members

    public void Dispose()
    {
        if (Object != null)
        {
            if(cleanup != null)
                cleanup(Object);
            Object = null;
            GC.SuppressFinalize(this);
        }
    }

    #endregion

    ~ScopedCleanup() { Dispose(); }
}

static ScopedCleanup<T> CleanupObject<T>(T o, Action<T> cleanup) where T : class
{
    return new ScopedCleanup<T>(o, cleanup);
}

static ScopedCleanup<ComType> CleanupComObject<ComType>(ComType comObject, Action<ComType> actionBeforeRelease) where ComType : class
{
    return
        CleanupObject(
            comObject,
            o =>
            {
                if(actionBeforeRelease != null)
                    actionBeforeRelease(o);
                Marshal.ReleaseComObject(o);
            }
        );
}

static ScopedCleanup<ComType> CleanupComObject<ComType>(ComType comObject) where ComType : class
{
    return CleanupComObject(comObject, null);
}

Usage case. Note the call to Quit, which seems to be necessary to make the process end:

using (var excel = CleanupComObject(new Excel.Application(), o => o.Quit()))
using (var workbooks = CleanupComObject(excel.Object.Workbooks))
    {
        ...
    }