为什么“Win32_CDROMDrive”类的“SerialNumber”属性在与WMI分开请求时返回null？

Question

When acquiring the value of the property SerialNumber from the WMI class Win32_CDROMDrive like this SELECT SerialNumber FROM Win32_CDROMDrive it throughs a NullReferenceException unless i change the query to SELECT * FROM Win32_CDROMDrive . Then loop arround all the properties including the SerialNumber in-which in that case is not null.

And since the first method is faster than the second (not quite sure) I prefer to use it. So what is happening? Am I missing something? Note that it works perfectly fine with other properties and classes!

This is my code

string result = "";
var searcher = new ManagementObjectSearcher("SELECT SerialNumber FROM Win32_CDROMDrive");
ManagementObjectCollection collec = searcher.Get();
foreach (ManagementObject obj in collec)
{
    result = obj["SerialNumber"].ToString();
    break;
}
MessageBox.Show(result);

It won't work unless i change to:

var searcher = new ManagementObjectSearcher("SELECT * FROM Win32_CDROMDrive");

Update

The first method works with the other properties of the same class and the value can be extracted without an exception. It seems like the problem is with the SerialNumber property only!

Update 2

It seems like the problem is indeed with just SerialNumber as looping arround all the non-nulled-values of properties of the Win32_CDROMDrive will list the SerialNumber with a real value as the code below explains:

listView1.Items.Clear();
var searcher = new ManagementObjectSearcher("SELECT * FROM Win32_CDROMDrive");
foreach (ManagementObject mo in searcher.Get())
{
    foreach (PropertyData pd in mo.Properties)
    {
        if (pd.Value != null)
            listView1.Items.Add(pd.Name).SubItems.Add(pd.Value.ToString());
    }
}

However, if the query is changed to the specific wanted property method it will give the same error!

Update 3

I managed to get the value of this naughty property without looping arround all the remaining ones via a different class Win32_PhysicalMedia which contains less properties for all connected drives ( HDD, ODD, Floppy, ... ) including the SerialNumber property using this WQL query

SELECT * FROM Win32_PhysicalMedia

Or to be specific (to the CDROMDrive)

SELECT * FROM Win32_PhysicalMedia WHERE Tag Like '%CD%'

Or to be specific (to the SerialNumber of the CDROMDrive

SELECT SerialNumber FROM Win32_PhysicalMedia WHERE Tag Like '%CD%'

var searcher = new ManagementObjectSearcher("SELECT SerialNumber FROM Win32_PhysicalMedia WHERE TAG LIKE '%CD%'");
ManagementObjectCollection collec = searcher.Get();
foreach (ManagementObject obj in collec)
{
     Console.WriteLine(obj["SerialNumber"].ToString());
}
Console.Read();

But i can't consider this to be an answer as my question is why WQL doesn't allow specifing a record inside the SELECT statement to the (and only the) SerialNumber property of the CDROMDrive class?

Answer 1

I just tested on my PC and on my case it seems to be because SerialNumber property is null on the only instance I have. As it seems, WMI is no working properly when looking for a property that is NULL (that is on my case).

Anyway, you can use ORMi to work with WMI and do all the work using Linq.

Example:

WMIHelper helper = new WMIHelper("root\\CimV2");

var data = helper.Query("SELECT * FROM Win32_CDROMDrive").Where(p => p.SerialNum == "yourSerialNum");