Marshal.ReleaseComObject 释放COM对象
.NET的垃圾收集机制不负责回收COM对象，因此要手动对COM对象进行回收，可采用.NET提供的Marshal对象的ReleaseComObject和FinalReleaseComObject来解决。
其中，ReleaseComObject
Decrements the reference count of the supplied runtime callable wrapper.
FinalReleaseComObject：
Releases all references to a runtime callable wrapper (RCW) by setting the reference count of the supplied RCW to 0.
具体参见以下链接：
微软Marshal.ReleaseComObject 方法的来龙去脉
ESRI Developer Network--How to release COM references
