释放临时COM对象

Question

使用COM对象考虑以下C#代码.

MyComObject o = new MyComObject;
try
{
 var baz = o.Foo.Bar.Baz;
 try
 { 
  // do something with baz
 }
 finally
 {
  Marshal.ReleaseComObject(baz);
 }
}
finally
{
 Marshal.ReleaseComObject(o);
}

这将释放COM对象o和baz,而不是临时对象returnd的o.Foo和o.Foo.Bar.当这些对象拥有大量非托管内存或其他资源时,这可能会导致问题.

一个明显但丑陋的解决方案是,使用try-finally和更加混乱代码Marshal.ReleaseComObject.请参阅 C#+ COM Interop,确定性版本

作为一种解决方法,我创建了一个帮助类

class TemporaryComObjects: IDisposable
{
 public C T<C>(C comObject)
 {
  m_objects.Add(comObject);
  return comObject;
 }
 public void Dispose()
 {
  foreach (object o in m_objects)
   Marshal.ReleaseComObject(o);
 }
}

用法:

using (TemporaryComObjects t = new TemporaryComObjects())
{
 MyComObject o = t.T(new MyComObject);
 var baz = t.T(t.T(t.T(o.Foo).Bar).Baz);
 // do something with baz
}

我的问题:此代码是否存在潜在问题？有人有更优雅的解决方案吗？

Answer 1

我最大的抱怨就是这个名字T; Add可能更具说服力.我还添加where T : class了泛型方法,但"流畅的API"似乎可用.我也倾向于将代码扁平化.我还可以看到一些使用ExpressionAPI来遍历整个树并捕获所有中间步骤的方法,但这不会是微不足道的 - 但是想象一下:

using(var com = new SomeWrapper()) {
    var baz = com.Add(() => new MyComObject().Foo.Bar.Baz);
}

这是一个表达式树,我们自动获得中介.

(也可以,你可以Clear()或null列表中Dispose())

像这样:

static class ComExample {
    static void Main()
    {
        using (var wrapper = new ReleaseWrapper())
        {
            var baz = wrapper.Add(
                () => new Foo().Bar.Baz);
            Console.WriteLine(baz.Name);
        }
    }
}

class ReleaseWrapper : IDisposable
{
    List<object> objects = new List<object>();
    public T Add<T>(Expression<Func<T>> func)
    {
        return (T)Walk(func.Body);
    }
    object Walk(Expression expr)
    {
        object obj = WalkImpl(expr);
        if (obj != null && Marshal.IsComObject(obj) && !objects.Contains(obj)) 
        {
            objects.Add(obj);
        }
        return obj;
    }
    object[] Walk(IEnumerable<Expression> args)
    {
        if (args == null) return null;
        return args.Select(arg => Walk(arg)).ToArray();
    }
    object WalkImpl(Expression expr)
    {
        switch (expr.NodeType)
        {
            case ExpressionType.Constant:
                return ((ConstantExpression)expr).Value;
            case ExpressionType.New:
                NewExpression ne = (NewExpression)expr;
                return ne.Constructor.Invoke(Walk(ne.Arguments));
            case ExpressionType.MemberAccess:
                MemberExpression me = (MemberExpression)expr;
                object target = Walk(me.Expression);
                switch (me.Member.MemberType)
                {
                    case MemberTypes.Field:
                        return ((FieldInfo)me.Member).GetValue(target);
                    case MemberTypes.Property:
                        return ((PropertyInfo)me.Member).GetValue(target, null);
                    default:
                        throw new NotSupportedException();

                }
            case ExpressionType.Call:
                MethodCallExpression mce = (MethodCallExpression)expr;
                return mce.Method.Invoke(Walk(mce.Object), Walk(mce.Arguments));
            default:
                throw new NotSupportedException();
        }
    }
    public void Dispose()
    {
        foreach(object obj in objects) {
            Marshal.ReleaseComObject(obj);
            Debug.WriteLine("Released: " + obj);
        }
        objects.Clear();
    }
}