public class CustomCSharpMigrationCodeGenerator : CSharpMigrationsGenerator
    {
        public override string GenerateMigration(string migrationNamespace,
            string migrationName,
            IReadOnlyList<MigrationOperation> upOperations,
            IReadOnlyList<MigrationOperation> downOperations)
        {
            throw new Exception("Hello world");
            return base.GenerateMigration(migrationNamespace, migrationName, upOperations, downOperations);
        }

        public CustomCSharpMigrationCodeGenerator(MigrationsCodeGeneratorDependencies dependencies,
            CSharpMigrationsGeneratorDependencies csharpDependencies) : base(dependencies, csharpDependencies)
        {
            throw new Exception("Hello world");
        }
    }

public class MyDesignTimeServices : IDesignTimeServices
{
    public void ConfigureDesignTimeServices(IServiceCollection serviceCollection)
    {
        serviceCollection.AddSingleton<CSharpMigrationsGenerator, CustomCSharpMigrationCodeGenerator>();
    }
}