参考：http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.marshal.releasecomobject.aspx
This method is used to explicitly control the lifetime of a COM object used from managed code. You should use this method to free the underlying COM object that holds references to resources in a timely manner or when objects must be freed in a specific order.
The runtime callable wrapper has a reference count that is incremented every time a COM interface pointer is mapped to it. The ReleaseComObject method decrements the reference count of a runtime callable wrapper. When the reference count reached zero, the runtime releases all its references on the unmanaged COM object, and throws a System..::.NullReferenceException if you attempt to use the object further. If the same COM interface is passed more than once from unmanaged to managed code, the reference count on the wrapper is incremented every time and calling ReleaseComObject returns the number of remaining references.
private void ReleaseCOMObject(Object obj)
{
try
{
if (obj != null)
{
System.Runtime.InteropServices.Marshal.ReleaseComObject(obj);
}
}
catch (Exception)
{
throw new Exception("Release COM object failed!");
}
finally
{
obj = null;
}
}