VB Signature:

  <Runtime.InteropServices.DllImport("kernel32.dll", CharSet:=Runtime.InteropServices.CharSet.Auto, CallingConvention:=Runtime.InteropServices.CallingConvention.StdCall)> _
  Public Shared Function SetPriorityClass( _
    ByVal hProcess As IntPtr, _
    ByVal dwPriorityClass As IntPtr _
    ) As Boolean
  End Function

C# Signature:

[DllImport("kernel32.dll", CharSet=CharSet.Auto, SetLastError=true)]
public static extern bool SetPriorityClass(IntPtr handle, PriorityClass priorityClass);

   public enum PriorityClass : uint
   {
       ABOVE_NORMAL_PRIORITY_CLASS = 0x8000,
       BELOW_NORMAL_PRIORITY_CLASS = 0x4000,
       HIGH_PRIORITY_CLASS = 0x80,
       IDLE_PRIORITY_CLASS = 0x40,
       NORMAL_PRIORITY_CLASS = 0x20,
       PROCESS_MODE_BACKGROUND_BEGIN = 0x100000,// 'Windows Vista/2008 and higher
       PROCESS_MODE_BACKGROUND_END = 0x200000,//   'Windows Vista/2008 and higher
       REALTIME_PRIORITY_CLASS = 0x100
   }

User-Defined Types:

  Public Enum PriorityClass
    ABOVE_NORMAL_PRIORITY_CLASS = &H8000
    BELOW_NORMAL_PRIORITY_CLASS = &H4000
    HIGH_PRIORITY_CLASS = &H80
    IDLE_PRIORITY_CLASS = &H40
    NORMAL_PRIORITY_CLASS = &H20
    PROCESS_MODE_BACKGROUND_BEGIN = &H100000 'Windows Vista/2008 and higher
    PROCESS_MODE_BACKGROUND_END = &H200000   'Windows Vista/2008 and higher
    REALTIME_PRIORITY_CLASS = &H100
  End Enum

Notes:

VB no warnings with option strict = on.

Tips & Tricks:

Please add some!

Sample Code:

  SetPriorityClass(mhHwnd, CType(20, System.IntPtr))    '20 = high !!

  SetPriorityClass(Process.GetCurrentProcess.Handle, PriorityClass.PROCESS_MODE_BACKGROUND_BEGIN)

Alternative Managed API:

System.Diagnostics.Process.PriorityClass (Does not provide PROCESS_MODE_BACKGROUND options)