This article was originally published on DNJ Online

Modifying an existing object

Let’s say we need to change the share status of our printer. In order to do that we need to find it in the repository, which we can do using a straightforward SQL query. For example, the following finds a printer with the name ‘Agfa’:

"SELECT * FROM Win32_Printer WHERE DeviceID = 'Agfa' 

To initialise the search we create a FindPrinters method. This returns a ManagementObjectSearcher:

private ManagementObjectSearcher 
                  FindPrinters( string printerName)  
{  
    SelectQuery selectQuery = new SelectQuery(); 
	//specify query  
	selectQuery.QueryString = String.Format( 
		"SELECT * FROM Win32_Printer 
		WHERE DeviceID = '{0}' 
		OR ShareName = '{1}'",  
		printerName, printerName);
	//create new object searcher  
	ManagementObjectSearcher managementObjectSearcher = 
		new ManagementObjectSearcher( 
		this.managementScope, selectQuery);  
	return managementObjectSearcher;  
} 

The ManagementObjectSearcher contains all the objects which satisfy the query. We can then use its Get method to find the appropriate printerObject, modify its properties and put it back into the repository:

//check if anything found  if (managementObjectSearcher.Get().Count == 0)  {  
	Console.WriteLine(String.Format(
		"No printers with the name '{0}' 
		 or shared name '{1}' found.", 
		 printerName, printerName));  }  else  {  
	//if found: go through all found objects set sharing  
	foreach (ManagementObject printerObject in 
				 managementObjectSearcher.Get())  
	{  
		// set sharing parameters  
		if (sharePrinter)  
		{ 
		   printerObject["Shared"] = true; 
		   printerObject["ShareName"] = printerName;  
		}  
		else  
		{ 
		   printerObject["Shared"] = false; 
		   printerObject["ShareName"] = "";  
		} 
		//put object back in "update only mode"  
		PutOptions options = new PutOptions();  
		options.Type = PutType.UpdateOnly;  
		printerObject.Put(options); 
	}
} 

Note that we use the options type ‘UpdateOnly’ which means ‘update an existing object only without creating a new one’.

Run a method

Let’s try renaming a printer using the RenamePrinter method of the Win32_Printer class. First we need to find an object on which to run the method, which we’ve already covered, and then we call the method. We start by specifying the method parameters, and then execute the method using InvokeMethod:

//specify method parameters  
object[] methodParameters = { newPrinterName };  
foreach (ManagementObject printerObject in 

			managementObjectSearcher.Get())  
{  
   ...  
   //call method with specified parameters 
   object methodResult = 
		 printerObject.InvokeMethod("RenamePrinter", 
	methodParameters);
   //check invokation results  
   Int32 numericResult = Convert.ToInt32(methodResult);  
   if (numericResult != 0)  
   {  
	  switch (numericResult)  
	  {  
		 case 5: 
			Console.WriteLine("Error: printer access denied");  
			break;  
		 case 1801: 

			Console.WriteLine("Error: invalid printer name");  
			break;  
		 default: 
			Console.WriteLine("Unknown error");  
			break ;  
	  }  
   } 
...  
} 

Note how we check the result: if 0 then the method was invoked successfully, otherwise we display the appropriate error message.

Deleting objects from repository

Again we start with an object search, and this time simply delete it with the Delete() method:

// if found: go through all found objects and delete them
foreach (ManagementObject printerObject in
  managementObjectSearcher.Get())
{
	printerObject.Delete();
}

So working with WMI from C#, or indeed Visual Basic .NET, is pretty straightforward, and such simplicity should make WMI very popular. It can be used, for example, in automated software testing, or when you need to know if an application has detected a new device, or to check if an application has modified a particular registry key.