单元测试同时支持 NUnit/MSTest

using System;
#if !NUNIT    
using Microsoft.VisualStudio.TestTools.UnitTesting;     
using Category = Microsoft.VisualStudio.TestTools.UnitTesting.DescriptionAttribute;     
#else     
using NUnit.Framework;     
using TestClass = NUnit.Framework.TestFixtureAttribute;     
using TestMethod = NUnit.Framework.TestAttribute;     
using TestInitialize = NUnit.Framework.SetUpAttribute;     
using TestCleanup = NUnit.Framework.TearDownAttribute;     
using TestContext = System.Object;     
using ClassCleanup = NUnit.Framework.TestFixtureTearDownAttribute;     
using ClassInitialize = NUnit.Framework.TestFixtureSetUpAttribute;     
#endif