Change Order Number generator

By default, the orders numbers are generated by an instance of EPiServer.Commerce.Order.IOrderNumberGenerator.

The default number generation is:

 public string GenerateOrderNumber(IOrderGroup orderGroup)
 {
         int num = new Random().Next(100, 999);
         return $"PO{orderGroup.OrderLink.OrderGroupId}{num}";
 }

So, to change this, first we need to inject a custom implementation and remove the default one. On an initialization module that depends on EPiServer.Web.InitializationModule have the following:

(IServiceConfigurationProvider)services.RemoveAll<IOrderNumberGenerator>();
(IServiceConfigurationProvider)services.AddSingleton<IOrderNumberGenerator, MyCustomOrderNumberGenerator>();

And my custom order generator, can be like this:

 public class MyCustomOrderNumberGenerator: IOrderNumberGenerator
    {
        private readonly string prefix;
        private readonly string version;

        public MyCustomOrderNumberGenerator()
        {
            this.prefix = ConfigurationManager.AppSettings["OrderNumberGenerator:prefix"] ?? "PO";
            this.version = ConfigurationManager.AppSettings["OrderNumberGenerator:version"] ?? "00";
        }

        public string GenerateOrderNumber(IOrderGroup orderGroup)
        {
            return $"{prefix}{version}{new Random().Next(100, 999)} 
                                     {orderGroup.OrderLink.OrderGroupId}";
        }
    }