Intermittent NullReferenceException in MediaDevice.GetDevices()

[blazej-kosmowski]

Describe the bug
Some users intermittently experience a NullReferenceException when calling MediaDevice.GetDevices(). This exception does not occur for all users, and the cause is not entirely clear.

To Reproduce

1.	Call `MediaDevice.GetDevices()` in a system with portable devices connected.
2.	For certain users, a `NullReferenceException` is thrown.

Expected behavior
The method should return a valid list of MediaDevice objects or an empty list if no devices are available.

Device
Various devices and operating systems (details unknown).

Version:
1.10.0.0.

Additional context
It is possible that deviceManager is not properly initialized, or deviceIds may contain null values. Adding null checks for both deviceManager and deviceIds in the GetDevices() method could resolve this issue. I can prepare a PR for this case if you would like.

[zhuxb711]

I met the same issue, here is the stacktrace from my sentry:

The exception throw from the ArrayWhereSelectIterator, I think this is the opt one for LINQ pattern: Where().Select(). From the screenshot above, the exception is from the Where() rather than Select(). From the stacktrace, NRE actually triggered by ListWhereSelectIterator.ToList() which make sense because ToList() will force enumerate the LINQ.

So: we have to find a LINQ pattern like this: Where(<Lambda>).Select(<Lambda>).ToList()

In MediaDevice.UpdateDeviceList(List<MediaDevice> deviceList, string[] deviceIdList) only the following line match the requirement above:

MediaDevices/Src/MediaDevicesShare/MediaDevice.cs

Line 154 in df11581

var remove = deviceList.Where(d => !idList.Contains(d.DeviceId)).Select(d => d.DeviceId).ToList();

Since the exception throw from Where(), let's check idList.Contains(d.DeviceId)

• idList must not null otherwise we would not reach this line.
• Contains() accept null as input, so it would not throw NRE in this situation
• d.DeviceId might throw NRE if d is null

In short: I have to suspect that there is an null value within idList and that value trigger the NRE in d.DeviceId. However, I could not find a way to make that happen from the code.

After read the code in MediaDevice.GetDevices(), I think it's unnecessary to cache the enumerate result inside. In most situation, developer will only call once MediaDevice.GetDevices(), and enumerate the device costs less. Should let the developer decide whether to cache the result. Maintain a cache inside is not the right choice for the library like this.

By the way, the code in MediaDevice.UpdateDeviceList(List<MediaDevice> deviceList, string[] deviceIdList) is bad, please rewrite it using LINQ.ExceptBy() and do not use ToList() unless necessary, or better, remove the cache totally🙂