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There is a workaround solution in C#. I am not sure why the /device endpoint is not documented. But at least, I can read all devices under an account using this endpoint 'restapi/v1.0/account/~/device'.

Unfortunately, this endpoint is not supported in the .NET SDK (because it is not in the specs). But you can call the raw Get function as shown in the sample code below:

static private async Task read_account_devices()
{
    try
    {
        string endpoint = "restapi/v1.0/account/~/device";
        var response = await rcsdk.Get(endpoint);
        var body = await response.Content.ReadAsStringAsync();
        Console.WriteLine(body);
    }
    catch (Exception ex)
    {
        Console.WriteLine(ex);
    }
}