[DllImport("Psapi.dll")]
static extern bool EmptyWorkingSet(IntPtr hProcess);

public static void ClearSpace(System.IntPtr ptr)
{
EmptyWorkingSet(ptr);
GC.WaitForPendingFinalizers();
GC.Collect();
}

...
ClearSpace(EmptyWorkingSet(System.Diagnostics.Process.GetCurrentProcess().Handle);
...