How to use #if to decide which platform is being compiled for in C#

You can add any constants you want to the .csproj file. These can be put into conditional property groups like the one below.

 <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'Release|x64'">
    <DefineConstants>TRACE;X64</DefineConstants>
    ...
 </PropertyGroup>

For my Release x64 build, I have defined a X64 constant that I can use like this:

#if X64

#endif