How do I Add multiple projects to a solution using PowerShell? (I must be doing something fundamentall wrong here)

I am learning powershell while doing this (what I thought would have been) a small and simple assignment. I can't really figure out where I am going wrong so hoping someone would point me in the right direction. I am just trying to add a number of projects to a solution called "All.sln". I have the following code:

[string]$slnName = "All"
[string]$baseDir = "SOMEDIR_OF_PROJECTS"
[string]$slnPath = (Join-Path $baseDir 'All.sln')

$dteObj = New-Object -ComObject "VisualStudio.DTE.10.0"
$solution = $dteObj.Solution
$solution.Create($baseDir, $slnName)
Write-Output "Solution Path: " $slnPath    

(ls $baseDir -Recurse *.vbproj) | % { 

     Write-Output $_.FullName
     $solution = $dteObj.Solution
     $solution.AddFromFile($_.FullName, $false) 
     $solution.SaveAs( $slnPath ) 
 }

Write-Output ( $slnPath )
$dteObj.Solution.SaveAs( $slnPath ) 
$dteObj.Quit()

I keep getting all sorts of different errors so my thinking is that I am definitely doing something wrong here but it is not obvious to me what I am doing wrong.

I sometimes get this error (usually on the first project file I try to add):

WARNING: System.Runtime.InteropServices.COMException (0x80010001): Call was rejected by callee. (Exception from HRESULT: 0x80
010001 (RPC_E_CALL_REJECTED))
   at System.Runtime.InteropServices.ComTypes.ITypeInfo.GetTypeAttr(IntPtr& ppTypeAttr)
   at System.Management.Automation.ComTypeInfo.GetTypeAttr(ITypeInfo typeinfo)
   at System.Management.Automation.ComTypeInfo.Initialize()
   at System.Management.Automation.ComTypeInfo.GetDispatchTypeInfo(Object comObject)
   at System.Management.Automation.PSObject.GetMappedAdapter(Object obj, TypeTable typeTable)
   at System.Management.Automation.PSObject.get_InternalAdapterSet()
   at System.Management.Automation.PSObject.get_InternalTypeNames()
   at System.Management.Automation.CmdletParameterBinderController.BindPipelineParametersPrivate(PSObject inputToOperateOn)
   at System.Management.Automation.CmdletParameterBinderController.BindPipelineParameters(PSObject inputToOperateOn)
   at System.Management.Automation.CommandProcessor.Read()
   at System.Management.Automation.CommandProcessor.ProcessRecord()
   at System.Management.Automation.CommandProcessorBase.DoExecute()
   at System.Management.Automation.EnumerableOps.WriteEnumerableToPipe(IEnumerator enumerator, Pipe pipe, ExecutionContext co
ntext, Boolean dispose)
   at System.Dynamic.UpdateDelegates.UpdateAndExecuteVoid3[T0,T1,T2](CallSite site, T0 arg0, T1 arg1, T2 arg2)
   at System.Management.Automation.Interpreter.ActionCallInstruction`5.Invoke(Object arg0, Object arg1, Object arg2, Object a
rg3, Object arg4)
   at System.Management.Automation.Interpreter.CallInstruction.InvokeInstance(Object instance, Object[] args)
   at System.Management.Automation.Interpreter.DynamicInstructionN.Run(InterpretedFrame frame)
   at System.Management.Automation.Interpreter.EnterTryCatchFinallyInstruction.Run(InterpretedFrame frame)

or sometimes I get (Null??):

You cannot call a method on a null-valued expression.
At C:\DevHome\TFS\VCore\Scripts\BuildScripts\CreateMasterSolution.ps1:25 char:32
+                                $solution.AddFromFile($_.FullName)
+                                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull

or sometimes I get (MethodNotFound??):

Method invocation failed because [System.__ComObject] does not contain a method named 'SaveAs'.
At C:\DevHome\VCore\VCore\Scripts\BuildScripts\CreateMasterSolution.ps1:27 char:32
+                                $solution.SaveAs( $slnPath )
+                                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (SaveAs:String) [], RuntimeException
    + FullyQualifiedErrorId : MethodNotFound

or sometimes I get (MethodNotFound??):

Method invocation failed because [System.__ComObject] does not contain a method named 'AddFromFile'.
At C:\DevHome\TFS\VCore\Scripts\BuildScripts\CreateMasterSolution.ps1:18 char:32
+                                $solution.AddFromFile($_.FullName)
+                                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (AddFromFile:String) [], RuntimeException
    + FullyQualifiedErrorId : MethodNotFound

or sometimes I get:

Call was rejected by callee. (Exception from HRESULT: 0x80010001 (RPC_E_CALL_REJECTED))
At C:\DevHome\TFS\VCore\Scripts\BuildScripts\CreateMasterSolution.ps1:18 char:32
+                                $solution.AddFromFile($_.FullName)
+                                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : OperationStopped: (:) [], COMException
    + FullyQualifiedErrorId : System.Runtime.InteropServices.COMException

Without knowing more about VisualStudio and its "solutions", I'll just give you some general PowerShell advice:

1. Never assume you've successfully managed to get a handle to an object - you create your $dteObj, but then steam on, assuming the instantiation was successful. You need to wrap a try...catch around it, e.g.:

   try {
       $dteObj = New-Object -ComObject "VisualStudio.DTE.10.0";
       blah
       blah
   }
   catch [Exception] {
       error handling
   }
   

2. Just 'cause an object has an attribute, don't assume it's set. I'd recommend checking the Solution attribute isn't null, e.g.:

   if ( $dteObj.Solution -ne $null ) {
       do something
   } else {
       handle error state
   }
   

3. If a bit of code doesn't cause an exception to be thrown, check it using if ($?) {}, e.g.:

   $solution.SaveAs( $slnPath );
   if ( ! $? ) {
       save failed
   } else {
       save ok
   }