ProjectionBuilder.cs

public class ProjectionBuilder
{
    private readonly ProductDbContext _dbContext;

    public ProjectionBuilder(ProductDbContext dbContext)
    {
        _dbContext = dbContext;
    }

    public void ReceiveEvent(IEvent evnt)
    {
        switch (evnt)
        {
            case ProductShipped shipProduct:
                Apply(shipProduct);
                break;
            case ProductReceived receiveProduct:
                Apply(receiveProduct);
                break;
        }
    }

    public Product GetProduct(string sku)
    {
        var product = _dbContext.Products.SingleOrDefault(x => x.Sku == sku);
        if (product == null)
        {
            product = new Product
            {
                Sku = sku
            };
            _dbContext.Products.Add(product);
        }

        return product;
    }

    private void Apply(ProductShipped shipProduct)
    {
        var product = GetProduct(shipProduct.Sku);
        product.Shipped += shipProduct.Quantity;
        _dbContext.SaveChanges();
    }

    private void Apply(ProductReceived productReceived)
    {
        var state = GetProduct(productReceived.Sku);
        state.Received += productReceived.Quantity;
        _dbContext.SaveChanges();
    }
}