How to prevent C# compiler/CLR from optimizing away unused variables in DEBUG builds?

The lazy option would be.... use the value, ideally in a way that doesn't allow it to be held on the stack. For example:

 var tmp = SomeMethod();
 // your other code
 Debug.WriteLine(tmp);

the use of the value as an argument means it must be retained, but that line is automatically not compiled into release builds.

However! I must emphasize that locals are pretty-much always retained in an unoptimized/debug build, so I'm finding the scenario from the question hard to envisage.

If you are using Visual Studio, why not just add a breakpoint on the line following the line on which the calculation is being made, and then you can simply hover over the calculation to see the result in the intellisense/tooltip popup? I also think that you can add the calculation to the "watch" screen and view the result that way as well.

This is usually how I view the results of things I am debugging. Alternatively, you could just use the temp variable you created in some simple way to avoid getting the warning.

For example:

Console.Write(tempVariable);

Handy utility:

using static _globals;

static class _globals
{
    [MethodImpl(MethodImplOptions.NoInlining), DebuggerHidden]
    public static void Nop<T>(out T x) => x = default(T);
};

class Program
{
    static void Main()
    {
        int i;     // unreferenced variable

        /// ...

        Nop(out i);

        /// ...
    }
};