Roslyn Source Generator optimized for build time only usage

BuildtimeOptimizedRoslynSourceGenerator.cs

using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using System;
using System.Collections.Generic;
using System.IO;
using System.Reflection;
using System.Text;
using Microsoft.CodeAnalysis.Text;

namespace SourceGenLib
{
    // Shows how to build a source generator that will run only at build time
    // but not at IDE time.
    // When running at IDE time, the Execute method can be called 5+ times per ~keystrokes/changes in the file.
    // If your source generator generates code that doesn't require the user to know about it
    // better to generate the code only at build time to save more valuable IDE time.
    [Generator]
    public class MySourceGenerator : ISourceGenerator
    {
        public void Execute(GeneratorExecutionContext context)
        {
            // Don't run if not in buildtime
            if (!IsBuildTime) return;

            // ...
        }

        public void Initialize(GeneratorInitializationContext context)
        {
            // Register only when running from VBCSCompiler/csc
            if (IsBuildTime)
            {
                // ...
                context.RegisterForSyntaxNotifications(() => new BuildableReceiver());
            }
        }

        // Entry assembly is null in VS IDE
        // At build time, it is `csc` or `VBCSCompiler`
        private static readonly bool IsBuildTime = Assembly.GetEntryAssembly() != null;

        // Receiver example
        internal class BuildableReceiver : ISyntaxReceiver
        {
            public List<ClassDeclarationSyntax> CandidateClasses { get; } = new List<ClassDeclarationSyntax>();

            public void OnVisitSyntaxNode(SyntaxNode syntaxNode)
            {
                if (syntaxNode is ClassDeclarationSyntax cds)
                {
                    CandidateClasses.Add(cds);
                }
            }
        }
    }
}