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Wednesday, February 07, 2007

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Perusing Powershell Part 2: Error or No Output?

In yesterday's post on PS, I provided the source for my implementation of Get-SQLServer. I realized after I made the post that there was a significant bug in the ProcessRecord method. If you specify a service instance (default or named), the cmdlet makes no effort to actually validate that such a SQL server instance exists. So if you ask for a instance that doesn't exist, Get-SQLServer will happily write an invalid Server object to the pipeline. So I changed it to actually validate that the specified instance exists. I connect to the specified machine (local machine if not specified) using ManagedComputer and look in it's ServerInstances collection for the specified SQL instance.

The question is, what should you do if the specified SQL instance doesn't exist on the specified machine? One the one hand, you could write an error indicating that the SQL instance doesn't exist. Or, you could simply write nothing to the output pipeline, which may cause an error down the line.

Which is the right approach?

At first, I wrote an error when I couldn't find the instance, but decided that wasn't the right approach. It isn't really an error unless you attempt to act on that instance, right? So I thought the more PS friendly approach would be to write nothing and let the down stream cmdlets deal with it. I do write a debug message if the specified instance doesn't exist, so the scripter isn't completely in the dark.

So here's the new and improved ProcessRecord method of my Get-SQLServer cmdlet:

protected override void ProcessRecord() 
{ 
    //Make sure both -Name and -Default aren't specified 
    if (!string.IsNullOrEmpty(_Name) && _Default.IsPresent) 
    { 
        WriteError(new ErrorRecord( 
            new ArgumentException( 
                "Default and Name parameters can't both be specified"), 
            "DefaultAndName", 
            ErrorCategory.InvalidArgument, 
            null)); 

        return; 
    } 

    //If the machine name is not specified, assume the local machine  
    //(via the "." value) 
    string machine = string.IsNullOrEmpty(_MachineName) ? "." : _MachineName; 

    //Connect to the specified machine via the SMO WMI ManagedComputer object 
    SmoWmi.ManagedComputer mc = new SmoWmi.ManagedComputer(machine); 

    if (string.IsNullOrEmpty(_Name) && !_Default.IsPresent) 
    { 
        //If neither Name or Default are specified, write all the  
        //server instances on specified machine 
        foreach (SmoWmi.ServerInstance si in mc.ServerInstances) 
            WriteServerObject(si); 

        return; 
    } 

    string instanceName = _Default.IsPresent ? "MSSQLSERVER" : _Name; 

    if (mc.ServerInstances.Contains(instanceName)) 
        WriteServerObject(mc.ServerInstances[instanceName]); 
    else 
        WriteDebug("The specified SQL instance does not exist"); 
} 

//Helper method to create a SMO Server object from a  
//SMO WMI ServerInstance object and write it to the pipeline 
private void WriteServerObject(SmoWmi.ServerInstance si) 
{ 
    if (si.Name == "MSSQLSERVER") 
        WriteObject(new Smo.Server(si.Parent.Name)); 
    else 
        WriteObject(new Smo.Server(si.Parent.Name + "\\" + si.Name)); 
} 