C# Definition:

[StructLayout(LayoutKind.Sequential)]
struct PRINTER_DEFAULTS {
   public IntPtr pDatatype;
   public IntPtr pDevMode;
   public int DesiredAccess;
}

/// <summary>
/// The PRINTER_DEFAULTS structure specifies the default data type,
/// environment, initialization data, and access rights for a printer.
/// </summary>
/// <seealso href="http://msdn.microsoft.com/en-us/library/windows/desktop/dd162839(v=vs.85).aspx"/>
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
internal class PRINTER_DEFAULTS
{
   /// <summary>
   /// Pointer to a null-terminated string that specifies the
   /// default data type for a printer.
   /// </summary>
   public IntPtr pDatatype;

   /// <summary>
   /// Pointer to a DEVMODE structure that identifies the
   /// default environment and initialization data for a printer.
   /// </summary>
   public IntPtr pDevMode;

   /// <summary>
   /// Specifies desired access rights for a printer.
   /// The <see cref="OpenPrinter(string, out IntPtr, IntPtr)"/> function uses
   /// this member to set access rights to the printer. These rights can affect
   /// the operation of the SetPrinter and DeletePrinter functions.
   /// </summary>
   public ACCESS_MASK DesiredAccess;
}

VB Definition:

Structure PRINTER_DEFAULTS
   Public pDatatype As IntPtr
   Public pDevMode As IntPtr
   Public DesiredAccess As Integer
End Structure

User-Defined Field Types:

If you don't have an enum ACCESS_MASK defined, use

public int DesiredAccess;

instead.

Notes:

If you use the class-version, you only need one OpenPrinter declaration, because you can pass NULL or an object references.