C# Signature:

    [DllImport("winspool.drv", CharSet=CharSet.Auto, SetLastError=true)]
    public static extern bool SetDefaultPrinter(string Name);

VB Signature:

Declare Function SetDefaultPrinter Lib "winspool.drv" Alias "SetDefaultPrinterA" (ByVal pszPrinter As String) As Boolean

User-Defined Types:

None.

Notes:

Tips & Tricks:

Optionally call SendMessageTimeout() to notify running programs of the change:

    {
    pinvokeResult = SendMessageTimeout(HWND_BROADCAST, WM_SETTINGCHANGE, IntPtr.Zero,
        IntPtr.Zero, SendMessageTimeoutFlags.SMTO_NORMAL, 1000, out innerPinvokeResult);
    }

Sample Code:

    // Locals
    bool result;
    IntPtr pinvokeResult;
    IntPtr HWND_BROADCAST;
    IntPtr innerPinvokeResult;

    // Set default printer
    result = SetDefaultPrinter(defaultPrinterName);

    // Notify running programs?
    if (result == true && notifyRunningPrograms == true)
    {
        // Tell all open programs that this change occurred.
        // Allow each program 1 second to handle this message.
        HWND_BROADCAST = new IntPtr(0xffff);
        pinvokeResult = SendMessageTimeout(HWND_BROADCAST, WM_SETTINGCHANGE, IntPtr.Zero,
            IntPtr.Zero, SendMessageTimeoutFlags.SMTO_NORMAL, 1000, out innerPinvokeResult);
    }

    // Return result
    return result;

Alternative Managed API:

Turns out that there are some clever and powerful tools in Visual Studio 2005 and 2008 for working with the Management Classes (although couldn’t find it in the “Standard Edition” installed on this machine, definitely exists in my laptop’s Visual Studio). You can use Server Explorer to navigate a treeview to your management class (in this example, the Printers node), right-click on it and select “Generate Managed Class”. This will automagically create a strongly-typed C#.Net class representing the object model of the WMI Printers Class, with all its properties and methods.

I tried this in a little test solution and now I have a magical method SetDefaultPrinter which you can use natively. There’s also methods for pretty much everything you can do to a Windows printer.