Question

what are the benefits of implementing an interface in C# 3.5 ?

Answer 1

You’ll be able to pass your object to a method (or satisfy a type constraint) that expects the interface as an argument. C# does not support “duck typing.” Just by writing the methods defined by the interface, the object will not automatically be “compatible” with the interface type:

public void PrintCollection<T>(IEnumerable<T> collection) {
    foreach (var x in collection)
       Console.WriteLine(x);
}

If List<T> did not implement the IEnumerable<T> interface, you wouldn’t be able to pass it as an argument to PrintCollection method (even if it had a GetEnumerator method).

Basically, an interface declares a contract. Implementing an interface enforces your class to be bound to the contract (by providing the appropriate members). Consequently, everything that relies on that contract (a method that relies on the functionality specified by the interface to be provided by your object) can work with your object too.