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Overriding Methods from Non-F# Libraries
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When overriding methods from non-F# libraries, the method definition must be in the tuple style, that is, surrounded by brackets and separated by commas. If you need to use a method like this as a value, then you will need to create an F# function from the method. The following sample shows a class that implements the interface System.Net.ICredentials. Its single method, GetCredential, has two parameters. Just after the interface has been implemented, the example demonstrates using it as a value in the method GetCredentialList. #light type CredentialsFactory() = class interface System.Net.ICredentials with member x.GetCredential(uri, authType) = new System.Net.NetworkCredential("rob", "whatever", "F# credentials") member x.GetCredentialList uri authTypes = let y = (x :> System.Net.ICredentials) let getCredential s = y.GetCredential(uri, s) List.map getCredential authTypes end I discuss the relationship between F# signatures and C# signatures in 13.
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Defining Delegates
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Delegates are the mechanism both C# and Visual Basic use to treat their methods as values. A delegate basically acts as a .NET object that wraps the method and provides an invoke method so it can be called. There is rarely a need to define delegates in F# because it can treat a function as a value without the need for any wrapper. However, sometimes they are useful to define delegates to expose F# functionality to other .NET languages in a friendlier manner and to define callbacks for directly calling C code from F#. To define a delegate, you use the keyword delegate followed directly by the keyword of and then the type of the delegate s signature, which follows the standard F# type annotation notation. The next example shows the definition of a delegate, MyDelegate, which takes an int and returns unit. You then create a new instance of this delegate and apply it to a list of integers. As you ve already seen in 3, there are much shorter ways of implementing this functionality in F#. #light type MyDelegate = delegate of int -> unit let inst = new MyDelegate (fun i -> print_int i) let ints = [1 ; 2 ; 3 ] ints |> List.iter (fun i -> inst.Invoke(i)) The results of this example, when compiled and executed, are as follows: 123
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Structs
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You define structs in a similar manner to classes. The keyword class is replaced with struct. The main difference between a class and struct is the area of memory where the object will be allocated. When used as a local variable or parameter, a struct is allocated on the stack, while a class is allocated on the managed heap. Because structs are allocated on the stack, they are not garbage collected but are automatically deallocated when a function exits. It is generally slightly faster accessing their fields and slightly slower passing them to methods, but these differences do tend to be quite small. Because they are allocated on the stack, it is generally best to create structs with a small number of fields to avoid stack overflow. You can t use inheritance when implementing structs, so this means structs can t define virtual methods or abstract methods. The next example defines a struct representing an IP address. Note the only difference from defining a class is that the keyword struct is used.
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CHAPTER 5 I OBJECT-ORIENTED PROGRAMMING
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type IpAddress = struct val first : byte val second : byte val third : byte val fourth : byte new(first, second, third, fourth) = { first = first; second = second; third = third; fourth = fourth } override x.ToString() = Printf.sprintf "%O.%O.%O.%O" x.first x.second x.third x.fourth member x.GetBytes() = x.first, x.second, x.third, x.fourth end So, when should you use a classes, and when should you use a struct A good rule of thumb is to avoid structs, using them only when really necessary, for example, when interoperating with unmanaged C/C++ code (for more details on this, see 13).
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