Projection

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Projection is the abstraction of taking one shape of data and creating a different shape of it.  For instance, you could take a collection of one type, filter it and/or sort it, and then project a collection of a new type.

The Select extension method is what you use to implement projection on a collection.  You can pass a lambda expression to the Select extension method that takes one object from the source collection, and projects one object into the new projection.

Let's say that we have a collection of integers, and we want a new collection of strings of x's, where the string lengths are determined by the source collection of integers.  The following code shows how to do this:

Dim lengths() As Integer = { 1, 3, 5, 7, 9, 7, 5, 3, 1 }
Dim stringCollection As IEnumerable(Of String) = _
    lengths.Select(Function (i) "".PadRight(i, "x"C))
 
For Each s In stringCollection
    Console.WriteLine(s)
Next
 

This code produces the following output:

x
xxx
xxxxx
xxxxxxx
xxxxxxxxx
xxxxxxx
xxxxx
xxx
x
 

Or we could take an array of doubles, and project a collection of integers, casting each double to int:

Dim dbls() As Double = { 1.5, 3.2, 5.2, 7.3, 9.7, 7.5, 5.0, 3.2, 1.9 }
Dim ints As IEnumerable(Of Integer) = _
    dbls.Select(Function(d) CInt(d))
For Each i In ints
    Console.WriteLine(i)
Next
 

Or we can take a collection of elements from an XML tree, and project a collection of strings.  The strings projection contains the value of each element:

Dim xmlDoc = _
    <Root>
        <Child>abc</Child>
        <Child>def</Child>
        <Child>ghi</Child>
    </Root>
Dim values As IEnumerable(Of String) = _
    xmlDoc.Elements().Select(Function(e) CStr(e))
For Each s In values
    Console.WriteLine(s)
Next
 

But in the real world, our projections will typically be much more involved than this.  For instance, we will often take a collection of some class, and project a collection of a different class.  Or we may project a collection of anonymous types (introduced in the next topic).

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