Question

The code below produces the error:

LINQ to Entities does not recognize the method System.String GenerateSubscriptionButton(Int32) method, and this method cannot be translated into a store expression.

How do I create correct custom methods in LINQ to Entities?

var model = _serviceRepository.GetProducts().Select(p => new ProductModel
{
    Id = p.Id,
    Name = p.Name,
    Credits = p.Credits,
    Months = p.Months,
    Price = p.Price,
    PayPalButton = GenerateSubscriptionButton(p.Id)
});        

private string GenerateSubscriptionButton(int id)
{
    return new PaymentProcessor.PayPalProcessor().CreateSubscriptionButton(id);
}

Answer 1

You can’t do that. How should the provider translate your method to SQL?

Remember: LINQ to Entities doesn’t actually execute the C# code of your queries. Instead, it interpretes the expressions and translates them to SQL.

In your conrete case, the solution would probably look something like this:

var model = _serviceRepository.GetProducts()
                              .Select(p => new ProductModel 
                                           { 
                                               Id = p.Id, 
                                               Name = p.Name, 
                                               Credits = p.Credits, 
                                               Months = p.Months, 
                                               Price = p.Price
                                           })
                              .ToList()
                              .Select(x =>
                                      {
                                          x.PayPalButton = GenerateSubscriptionButton(x.Id);
                                          return x;
                                      }); 

The call to ToList executes the query so far against the database and returns the result. From that point forward, the query actually is a LINQ to objects query where the code is not interpreted but executed.