声明preprocesor符号（如调试）全球范围内为整个项目

我想这样的NUnit和VS测试之间切换：

I would like to switch between NUnit and VS Tests like this:

#if !NUNIT
using Microsoft.VisualStudio.TestTools.UnitTesting;
#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.String;
  using DeploymentItem = NUnit.Framework.DescriptionAttribute;
#endif

我的问题是，怎么可能我宣布NUNIT preprocesor符号在一个地方（ App.config中左右，将是巨大的），到NUnit的和VSTests轻松之间切换？因为当我使用的#define NUNIT ，它只能为文件，它是写的。

My question is, how may I declare NUNIT preprocesor symbol at one place (App.config or so, would be great), to switch between NUnit and VSTests easily? Because when I use #define NUNIT, it works only for the file, where it is written.