Delegates to LINQ [passing logic as a parameter] (part I)
it's normal to pass data to a function just thow in a parameter of the type of data you want to pass and ur set
public int add (int i,int u ){ return i+u;
}
now think in a different way, u now want to build a set of operations that the user can choose from now what you need is to build a function that would take the user's input and the operation as parameters and put them together...
for instance he has a set of .. customers if you will and he wants to have them filtered in predefined filters like valuable customers , local customers and new customer.
Data that we're using looks like this
public class Customer { public int Id{ get; set;}; public string Name{ get; set;}; public DateTime FirstPurchaseDate{ get; set;}; public string city{get; set;}; public decimal TotalPurchase {get; set;}}
we'll be passing that customer in a list of type customer
var customerList= new List<Customer>();
now for the logic, how to implement the three requested filter in a way they can be sent as a parameter to the search Method, well this is how it's done:
1-to do that we think of a way that we can reference a logic (a method) which is a delegate. So we'll build a delegate that can receive a Customer as a parameter and return weather it's valid customer according to the required criteria so the delegate's declaration should look something like this
public delegate bool evaluatetorDelegate(Customer customer);
2- we need to build a function that will actually do the evaluation but to do that it will have to have the same signature as the delegate; here we'll implement the three simple required functions
private const string LOCAL_CITY ="Alexandria"; //my city in Egypt :D
private const decimal VALUABLE_CUSTOMER_LIMIT = 10000.00;
public bool IsValuableCustomer(Customer customer){ return customer.TotalPurchase > VALUABLE_CUSTOMER_LIMIT;
}
public bool IsLocalCustomer(Customer customer){ return customer.City == LOCAL_CITY ;
}
public bool IsNewCustomer(Customer customer){ return customer.FirstPurchaseDate > DateTime.ToDay.AddMonth(-1);
}
3- now we need to implement the function that will actually apply the filtering; should also look something like this
public List<Customer> executeFilter(List<Customer> customers,
evaluatetorDelegate filter) { List<Customer> resultList = List<Customer>();
foreach (Customer customer in customers)
{ if (filter.Invoke(customer))
resultList.Add(customer);
}
return resultList;
}
4- now that the example is complete let's see how can we utilize this code
public static void main(string param){}{switch (param){ case 1:
evaluatetorDelegate d = IsLocalCustomer;
break;
case 2:
evaluatetorDelegate d = IsNewCustomer;
break;
case 3:
evaluatetorDelegate d = IsValuableCustomer;
break;
}
List<Customers> Customers= GetCustomersFromDB ()// :D
// and then we call the method
//then we apply the filtering we need
Customers = executeFilter (Customers,d);
// now the Customers contain only the customers which meet the criteria.
