class UpdateProductHandler
{
    private readonly ISession session;
    public UpdateProductHandler(ISession session)
    {
        this.session = session;
    }

    public async Task Update(int id, ProductIdentity productIdentity)
    {
        var product = session.Get(id);
        product.Update(productIdentity);
    }
}

class GetProductHandler
{
    private readonly ISession session;
    public GetProductHandler(ISession session)
    {
        this.session = session;
    }

    public async Task<Product> Get(int id)
    {
        var product = session.Get(id);
        if (product == null)
            throw new Exception("Entity not found");
        return Task.FromResult(product);
    }
}