Lambda Expressions

A function can be sent as a parameter to another sub procedure or function.

For example:

    <Extension>
    Public Iterator Function Filter(aBookEnum As IEnumerable(Of Book), selector As Func(Of Book, Boolean)) As IEnumerable(Of Book)
        For Each thisBook As Book In aBookEnum
            If selector(thisBook) Then
                Yield thisBook
            End If
        Next
    End Function

The selector parameter is defined with the Func type.  This means that a function must be passed in with a return type of boolean and takes a parameter of type Book.

So when we call the Filter extension method, can do the following:

Let's say you have a function that meets the criteria for the parameter:

    Function GetBookOne(myBook As Book) As Boolean
        Return myBook.Title = "Book One"
    End Function

You could send this function as a parameter like this:

Dim MySelector As Func(Of Book, Boolean) = AddressOf GetBookOne
MyBookShelf.Filter(MySelector)

Or you could use a lambda expression and seriously streamline your code like this:

MyBookShelf.Filter(Function(x) x.Title = "Book One")

So, "x" represents the Book input parameter.