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What is the dependency inversion principle and why is it important?

Answer*

Dependency Inversion Principle (DIP) says that 

i) High level modules should not depend upon low-level modules. Both should depend upon abstractions.

ii) Abstractions should never depend upon details. Details should depend upon abstractions.

Example: 
    
        public interface ICustomer
		{
			string GetCustomerNameById(int id);
		}

		public class Customer : ICustomer
		{
			//ctor
			public Customer(){}

			public string GetCustomerNameById(int id)
			{
				return "Dummy Customer Name";
			}
		}

		public class CustomerFactory
		{
			public static ICustomer GetCustomerData()
			{
				return new Customer();
			}
		}

		public class CustomerBLL
		{
			ICustomer _customer;
			public CustomerBLL()
			{
				_customer = CustomerFactory.GetCustomerData();
			}

			public string GetCustomerNameById(int id)
			{
				return _customer.GetCustomerNameById(id);
			}
		}

		public class Program
		{
			static void Main()
			{
				CustomerBLL customerBLL = new CustomerBLL();
				int customerId = 25;
				string customerName = customerBLL.GetCustomerNameById(customerId);


				Console.WriteLine(customerName);
				Console.ReadKey();
			}
		}

Note: Class should depend on abstractions like interface or abstract classes, not specific details (implementation of interface).

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What is the dependency inversion principle and why is it important?

Answer*