Program.cs

using System;
using System.IO;
using System.Linq;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.Text;

return Run(args);

static int Run(string[] args)
{
    //args = new[] { "C:\\Users\\Max\\source\\repos\\CodeAsPreProcessed\\Example\\VisibleComponent.cs", "OPT3" };

    if (args.Length < 1)
    {
        Console.Error.WriteLine("Usage: CodeAsPreProcessed <path-to-code-file> [DEFINE1 DEFINE2 ...]");
        return 1;
    }

    var inputPath = args[0];
    var defines = args.Skip(1).ToArray();

    if (!File.Exists(inputPath))
    {
        Console.Error.WriteLine($"Input file '{inputPath}' does not exist.");
        return 1;
    }

    var code = File.ReadAllText(inputPath);

    var parseOptions = new CSharpParseOptions(
        preprocessorSymbols: defines
    );

    var tree = CSharpSyntaxTree.ParseText(code, parseOptions);
    var root = tree.GetRoot();

    // This is the expanded source with preprocessor directives and
    // disabled regions removed.
    var cleanedRoot = new PreprocessorStripRewriter().Visit(root);
    var processed = cleanedRoot.ToFullString();

    var directory = Path.GetDirectoryName(inputPath) ?? "";
    var fileNameWithoutExtension = Path.GetFileNameWithoutExtension(inputPath);
    var extension = Path.GetExtension(inputPath);

    var suffix = "_PREPROCESSED";
    if (defines.Length > 0)
    {
        suffix += "_" + string.Join("_", defines);
    }

    var outputFileName = fileNameWithoutExtension + suffix + ".cs";
    var outputPath = Path.Combine(directory, outputFileName);

    File.WriteAllText(outputPath, processed);

    return 0;
}

sealed class PreprocessorStripRewriter : CSharpSyntaxRewriter
{
    public override SyntaxToken VisitToken(SyntaxToken token)
    {
        // First visit into child structure so we don't lose changes.
        var visited = base.VisitToken(token);

        var leading = FilterTrivia(visited.LeadingTrivia);
        var trailing = FilterTrivia(visited.TrailingTrivia);

        if (leading != visited.LeadingTrivia)
        {
            visited = visited.WithLeadingTrivia(leading);
        }

        if (trailing != visited.TrailingTrivia)
        {
            visited = visited.WithTrailingTrivia(trailing);
        }

        return visited;
    }

    private static SyntaxTriviaList FilterTrivia(SyntaxTriviaList triviaList)
    {
        var kept = new System.Collections.Generic.List<SyntaxTrivia>(triviaList.Count);

        foreach (var trivia in triviaList)
        {
            // Drop all directives (#if, #else, #endif, #define, #region, etc.)
            if (trivia.IsDirective)
            {
                continue;
            }

            // Drop disabled text (code from inactive branches).
            if (trivia.Kind() == SyntaxKind.DisabledTextTrivia)
            {
                continue;
            }

            kept.Add(trivia);
        }

        return SyntaxFactory.TriviaList(kept);
    }
}