Using the #if pre-processor directive to do different things for x64 builds

I'd just about finished writing an answer for a question on Stack Overflow when the poster deleted it, so I thought I'd drop the answer here instead.

The question asked was essentially:

How can I swap between two different structs depending on whether my application has been compiled as x86 or x64

Here's my answer:

You could use the #if pre-processor directive to achieve this. Here's a standalone example of using this:

using System;

namespace IfTest
{
    class Program
    {
        static void Main(string[] args)
        {
            dynamic context;
#if (BUILDARCH64)
            context = new Context64();
#else
            context = new Context();
#endif
            Console.Write(context.ToString());
        }
    }

    public struct Context
    {
    }
    public struct Context64
    {
    }
}

In your 64 bit build you'll need to set a Conditional compilation symbol, which you can do by opening the Properties for your project by double clicking on the *Properties* node in Solution Explorer, going to the *Build* tab and entering `BUILDARCH64` against the appropriate build Configuration.