gistfile1.cs

[DllImport("kernel32.dll", CharSet = CharSet.Auto)]
private static extern bool FreeLibrary(IntPtr hModule);

[DllImport("kernel32.dll", CharSet = CharSet.Ansi)]
private static extern IntPtr GetProcAddress(IntPtr hModule, string lpProcName); 

// Load the DLL file
IntPtr Handle = LoadLibrary(fileName);
if (Handle == IntPtr.Zero)
{
     int errorCode = Marshal.GetLastWin32Error();
     throw new Exception(string.Format("Failed to load library (ErrorCode: {0})",errorCode));
}

// load & invoke the desired method from DLL
IntPtr funcaddr = GetProcAddress(Handle,functionName);
YourFunctionDelegate function = Marshal.GetDelegateForFunctionPointer(funcaddr,typeof(YourFunctionDelegate )) as YourFunctionDelegate ;
function.Invoke(...parameters here...);

// When done, free the DLL
if(Handle != IntPtr.Zero)
        FreeLibrary(Handle);