[2008] Simple LINQ examples

[Atheist]

I've just begun learning about LINQ and I thought it'd be good to share some simple examples to get more people aware of this great functionallity.

LINQ stands for Language-Integrated Query, and can perform queries on anything that implements the IEnumerable interface.

Here's something pretty basic:

Dim values() As Integer = {43, 12, 0, 75, 433, 33, 76, 21, 5, 8, 320}
 
        Dim query As IEnumerable(Of Integer) = From i In values _
                                              Where i Mod 2 = 0 _
                                              Order By i _
                                              Select i

After this code has executed, 'query' will contain every integer from the 'values' array (From i In values) that can be evenly divided into 2 (Where i Mod 2 = 0). (Order By i) specifies that the returned values should be ordered numerically. And finally, the Select operator specifies what columns to include in the result, but seeing as we're dealing with simple Integers, we dont have much choice other than returning the integer value.

Now, take a look at this example:

Private Structure ShopItem
        Public Name As String
        Public BestBefore As Date
    End Structure
 
    Private ShopItemList As New List(Of ShopItem)

A simple structure holding the name and "Best before" date of an item in a shop, and a list of the structure.

Dim query As IEnumerable(Of String) = From item In ShopItemList _
                                              Where (item.BestBefore >= Now.Date) _
                                              Select item.Name

This query will return the name of each grocery that has passed the "Best before" date. Take a look at the Select statement in this one, instead of returning the entire structure to the query result, I'm only returning the string from the Name field.

*More to come*

[masfenix]

PART 2
LINQ TO SQL

Firstly I am gonna assume you have watched this video:
http://msdn2.microsoft.com/en-ca/vbasic/bb737878.aspx

This tutorial assumes that you have a primary key in all your tables, and a dbml file created.

Add an entry:

      Try
                Dim db As New dbDataContext
                Dim client As New client
                client.firstname = fnametxt.Text.Trim.ToUpper
                client.lastname = lnametxt.Text.Trim.ToUpper
                client.email = emailtxt.Text.Trim.ToUpper
                client.address = addresstxt.Text.Trim.ToUpper
                client.country = countrytxt.Text.Trim.ToUpper
                           
                db.clients.InsertOnSubmit(client)
                db.SubmitChanges()
                              
            Catch ex As Exception
               'log your error here. Handle carefully.                
            End Try

Now we edit the same client:

Try
                Dim client = (From p In db.clients _
                                 Where p.id = id _
                                 Select p).Single
                client.firstname = fnametxt.Text.Trim.ToUpper
                client.lastname = lnametxt.Text.Trim.ToUpper
                client.email = emailtxt.Text.Trim.ToUpper
                client.address = addresstxt.Text.Trim.ToUpper
                client.country = countrytxt.Text.Trim.ToUpper
             
                db.SubmitChanges()
        
            Catch ex As Exception
                 'log the error
            End Try

Note: How I have brackets around my query then a .single. The .single function returns ONE RECORD. If there are more then 1 records then it will THROW an EXCEPTION. You can alternatively use .First to get the FIRST record of many if you think there are many records.

again db.submitChanges is required to DO anything

Code Block 3: Adding a picture using LINQ to SQL field "image". This code gets an image from the picture box. To load a picturebox, please do a search on MSDN for picturebox class.

 
Try
            Dim db As New dbDataContext
            Dim entry As New inventory
                         
                Using picture As Image = PictureBox1.Image
                    Using stream As New IO.MemoryStream
                        picture.Save(stream, Imaging.ImageFormat.Jpeg)
                        entry.picture = stream.GetBuffer
                    End Using
                End Using

                db.inventories.InsertOnSubmit(entry)
                db.SubmitChanges()

                MessageBox.Show("Item has been added, closing")
                
            End If

        Catch ex As Exception
            MessageBox.Show(ex.Message)
        End Try

There will be instances when you have autoincrement field called "id" or w/e. Obviuosly you dont assign id a value when submitting a new entry. But there will be times when you add an item, and you need to use the newly entered id to do another task.

 Dim db As New dbDataContext
                    Dim newO As New order
                   'order has a autoincrement field "id"
                    newO.clientid = clientid
                    newO.clientname = clientlbl.Text
                                
                    db.orders.InsertOnSubmit(newO)
                    db.SubmitChanges()

                    ordernumber = newO.id

If you notice newO.id will be assigned the id of the item you just entered. This will throw an exception if you called it before submitChanges was called.

[mendhak]

If you have a structure or class that you want returned from your LINQ query, you can do that too as LINQ allows you to shape your results.

Let's say you have

 struct ImageLink
        {
            public string DestinationUrl;
            public string ThumbnailUrl;
        }

(ImageLink represents a small thumbnail that links off to another page, like on Flickr)

You can then do

var ResultItems = from i in SearchResults.Results
                        select new ImageLink { DestinationUrl = i.DisplayUrl, ThumbnailUrl = i.Image.ThumbnailURL };

You then get an IEnumerable List of a structure or class that your code is already aware of so that you can deal with it appropriately. Use ResultItems.ToList() to convert it to a List<ImageLinks> if that makes you comfortable.