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I found such sentences in the Users Manual of PointCloud:
"Frame Object Lifetime
Frame objects in the Point Cloud system are a limited resource. Be sure to call the Free() method to release the INPPointCloudFrame interface as quickly as possible."

But I didn't find any methods called Free() in the NPPointCloudFrame object in C#.

Then how can I release a NPPointCloudFrame object?

There may be a misprint in the user manual. I believe you would want to call Release() and not Free().

Give that a try and let me know if that solves your problem.