public interface ICustomerAccountCalculation
{
    Decimal Balance(int customerId);
}

public interface ICustomerAccountCalculation
{
    Decimal Balance(Customer customer);
}

decimal balance = from c in Customers
                  where c.CustomerId == customerId
                  select c.Balance;

// caller with the object already loaded
productService.process(product);
// caller without the object loaded
productService.process(new Product(productId));

// service method
void process(Product product) {

    // Ensure product is loaded, only if we need more fields.
    // We could extract this to a helper method.
    if (!product.isLoaded()) {
        product = database.getProduct(product.getId());
    }

    ...
}