【Using Mono DLLs in a Unity Project】
The path to the Unity DLLs will typically be：
一个生成dll的例子如下：
mcs -r:/Applications/Unity/Unity.app/Contents/Frameworks/Managed/UnityEngine.dll -target:library ClassesForDLL.cs
Once compiled, the DLL file can simply be dragged into the Unity project like any other asset. The DLL asset has a foldout triangle which can be used to reveal the separate classes inside the library. Classes that derive from MonoBehaviour can be dragged onto Game Objects like ordinary scripts. Non-MonoBehaviour classes can be used directly from other scripts in the usual way.
创建DLL：In MonoDevelop, you do this by selecting File > New > Solution and then choosing C# > Library.
引用DLL：In MonoDevelop, you should open the contextual menu For Referencesin the Solution Browser and choose Edit References. Then, choose the option .Net Assembly tab > File System > select file.
参考：http://docs.unity3d.com/Manual/UsingDLL.html