.NET interoperability

The .NET BCL is built in an object oriented way, so the ability to work with existing classes is essential for the interoperability. Many (in fact almost all) of the classes are also mutable, so the eager evaluation and the support for side-effects are two key features when working with any .NET library. The following example demonstrates working with the mutable generic ResizeArray type from the BCL (ResizeArray is an alias for a type System.Collections. Generic.List to avoid a confusion with the F# list type):

> let list = new ResizeArray<_>()
list.Add("hello")
list.Add("world")
Seq.to_list list;;
val it : string list = ["hello"; "world"]

As you can see, we used the underscore symbol when creating an instance of the generic type, because the type inference algorithm in F# can deduce the type argument from the code (in this example it infers that the type argument is string, because the Add method is called with a string as an argument). After creating an instance we used Add method to modify the list and add two new items. Finally, we used a Seq.to_list function to convert the collection to the F# list. As a fully compatible .NET language, F# also provides a way for declaring its own classes (called object types in F#), which are compiled into CLR classes or interfaces and therefore the types can be accessed from any other .NET language as well as used to extend classes written in other .NET languages. This is an important feature that allows accessing complex .NET libraries like Windows Forms or ASP.NET from F#.