FluentValidationExtensions.cs

public static class FluentValidationExtensions
{
    public static void RegisterValidatorsInAssembly(this IMvcBuilder builder, Assembly assembly)
    {
        builder.RegisterValidatorsInAssemblyList(assembly);
    }

    public static void RegisterValidatorsInAssemblyList(this IMvcBuilder builder, params Assembly[] assemblies)
    {
        var assemblyList = assemblies?.ToList() ?? new List<Assembly>();
        if (!assemblyList.Any())
        {
            return;
        }

        // register the validators from the respective assemblies
        assemblyList.ForEach(x=>builder.Services.AddValidatorsFromAssembly(x));
        
        // register the custom validator factory, to get `IValidator` instances to validate requests
        builder.Services.AddSingleton<ICustomValidatorFactory, CustomValidatorFactory>();
    }
}