C#中的编译开关

c#不支持宏定义，只支持编译开关。

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我想加上#define xxx就编译a代码，否则编译b代码，像这样的：
#if xxx
a
#else
b
#endif

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//#define Timing
#define Channel

上面两个，任意选一个，就说明开关打开了

#if Timing     [#define Timing]
phy.StartInfo.FileName = "DrawTiming.exe";
#elif Channel
phy.StartInfo.FileName = "DrawChannel.exe";
#else
phy.StartInfo.FileName = "DrawPhysical.exe";
#endif