LINQ Programming: Introduction

	Introduction to the LINQ

The Language Integrated Query

Introduction

The Language Integrated Query, abbreviated LINQ, is a technology used to create a sub-list based on an existing list. The new list could be the same as the original, it could be a list of selected values from the original, or it can be a different arrangement of the values of the first list.

Practical Learning: Introducing LINQ

Start Microsoft Visual Studio and create a Windows Forms Application named AltairRealtors1
To create a new class, in the Class View, right-click AltairRealtors1 -> Add -> Class...
Change the name to Property and press Enter

Complete the class as follows:

using System
using System.Collections.Generic
using System.Linq
using System.Text

namespace AltairRealtors1
{
    public enum PropertyType
    {
        Condominium,
        Townhouse,
        SingleFamily,
        Unknown
    }

    public enum PropertyCondition
    {
        Unknown,
        Excellent,
        Good,
        NeedsRepair,
        BadShape
    }

    (Serializable)
    public class Property
    {
        private int nbr
        private PropertyType tp
        private string ct
        private string stt
        private PropertyCondition cond
        private short beds
        private float baths
        private int levels
        private int yr
        private decimal val

        public Property()
        {
            nbr = 0
            tp = PropertyType.Unknown
            ct = "Unknown"
            stt = "AA"
            cond = PropertyCondition.Unknown
            beds = 0
            baths = 0.00F
            levels = 0
            yr = 1900
            val = 0M
        }

        public Property(int propNbr, PropertyType type, string city,
                        string state, PropertyCondition condition,
                        short bedrooms, float bathrooms, int stories,
                        int year, decimal value)
        {
            nbr = propNbr
            tp = type
            ct = city
            stt = state
            cond = condition
            beds = bedrooms
            baths = bathrooms
            levels = stories
            yr = year
            val = value
        }

        public int PropertyNumber
        {
            get { return nbr }
            set { nbr = value }
        }

        public PropertyType Type
        {
            get { return tp }
            set { tp = value }
        }

        public string City
        {
            get { return ct }
            set { ct = value }
        }

        public string State
        {
            get { return stt }
            set { stt = value }
        }

        public PropertyCondition Condition
        {
            get { return cond }
            set { cond = value }
        }

        public short Bedrooms
        {
            get
            {
                if (beds <= 1)
                    return 1
                else
                    return beds
            }
            set { beds = value }
        }

        public float Bathrooms
        {
            get { return (baths <= 0) ? 0.00f : baths }
            set { baths = value }
        }

        public int Stories
        {
            get { return levels }
            set { levels = value }
        }

        public int YearBuilt
        {
            get { return yr }
            set { yr = value }
        }

        public decimal MarketValue
        {
            get { return (val <= 0) ? 0.00M : val }
            set { val = value }
        }
    }
}

In the Solution Explorer, right-click Form1.cs and click Rename
Type AltairRealtors.cs and press Enter twice to display the form

Design the form as follows:

Control

Text

Name

Other Properties

ListView

lvwProperties

Anchor: Top, Bottom, Left, Right

Columns

(Name)	Text	TextAlign	Width
colIndex	#		40
colPropertyNumber	Prop #		55
colDateListed	Date Listed	Center
colPropertyType	Prop Type		85
colCity	City
colState	State
colCondition	Condition
colBedrooms	Beds	Right	65
colBathrooms	Baths	Right	65
colStories	Stories	Right	75
colYearBuilt	Year	Right	70
colMarketValue	Value	Right

Button

btnClose

Anchor: Bottom, Right

Double-click an unoccupied area of the form and implement the Load event as follows:

using System
using System.Collections.Generic
using System.ComponentModel
using System.Data
using System.Drawing
using System.Linq
using System.Text
using System.Windows.Forms
using System.IO
using System.Runtime.Serialization.Formatters.Binary

namespace AltairRealtors1
{
    public partial class AltairRealtors : Form
    {
        public AltairRealtors()
        {
            InitializeComponent()
        }

        private void AltairRealtors_Load(object sender, EventArgs e)
        {
            Property() lstProperties = new Property()
            {
                new Property(524880, PropertyType.SingleFamily, "Silver Spring", "MD",
                        PropertyCondition.Good, 4, 2.50f, 3, 1995, 495880.00M),
                new Property(688364, PropertyType.SingleFamily, "Alexandria", "VA",
                        PropertyCondition.Excellent, 4, 3.5f, 2, 2000, 620724.00M),
                new Property(611464, PropertyType.SingleFamily, "Laurel", "MD",
                        PropertyCondition.Good, 0, 0F, 2, 0, 422625.00M),
                new Property(749562, PropertyType.Townhouse, "Gettysburg", "WV",
                        PropertyCondition.Good, 3, 2.5F, 3, 2002, 425400.00M),
                new Property(420115, PropertyType.Unknown, "Washington", "DC",
                        PropertyCondition.Unknown, 2, 0F, 0, 1982, 312555.00M),
                new Property(200417, PropertyType.Condominium, "Germantown", "MD",
                        PropertyCondition.Excellent, 2, 1f, 0, 0, 215495.00M),
                new Property(927474, PropertyType.Townhouse, "Arlington", "VA",
                        PropertyCondition.BadShape, 4, 2.5f, 3, 1992, 415665.00M),
                new Property(682630, PropertyType.SingleFamily, "Martinsburg", "WV",
                        PropertyCondition.Good, 4, 3.5f, 3, 2005, 325000.00M),
                new Property(288540, PropertyType.Condominium, "Silver Spring", "MD",
                        PropertyCondition.Good, 1, 1f, 0, 2000, 242775.00M),
                new Property(247472, PropertyType.SingleFamily, "Silver Spring", "MD",
                        PropertyCondition.Excellent, 3, 3f, 3, 1996, 625450.00M),
                new Property(297446, PropertyType.Townhouse, "Laurel", "MD",
                        PropertyCondition.Unknown, 4, 1.5F, 2, 2002, 412885.00M),
                new Property(924792, PropertyType.SingleFamily, "Washington", "DC",
                        PropertyCondition.Good, 5, 3.5F, 3, 2000, 555885.00M),
                new Property(294796, PropertyType.SingleFamily, "Falls Church", "VA",
                        PropertyCondition.Excellent, 5, 2.5f, 2, 1995, 485995.00M),
                new Property(811155, PropertyType.Condominium, "Alexandria", "VA",
                        PropertyCondition.Good, 1, 1.0F, 0, 2000, 352775.00M),
                new Property(447597, PropertyType.Townhouse, "Hyattsville", "MD",
                        PropertyCondition.Excellent, 3, 2f, 3, 1992, 365880.00M),
                new Property(297415, PropertyType.Townhouse, "ashington", "DC",
                        PropertyCondition.Good, 4, 3.5f, 1, 2004, 735475.00M),
                new Property(475974, PropertyType.SingleFamily, "Gaithersburg", "MD",
                        PropertyCondition.Unknown, 4, 2.5f, 1, 1965, 615775.00M),
                new Property(927409, PropertyType.Condominium, "McLean", "VA",
                        PropertyCondition.Excellent, 1, 1f, 12, 2006, 485900.00M),
                new Property(304750, PropertyType.Condominium, "Washington", "DC",
                        PropertyCondition.Unknown, 2, 2f, 6, 1992, 388665.00M),
                new Property(207850, PropertyType.Townhouse, "Rockville", "MD",
                        PropertyCondition.Good, 3, 2.5F, 2, 1988, 525995.00M)
            }

            FileStream stmProperties = null
            BinaryFormatter bfmProperties = new BinaryFormatter()

            // If this directory doesn't exist, create it
            Directory.CreateDirectory(@"C:\Altair Realtors")
            // This is the file that holds the list of properties
            string Filename = @"C:\Altair Realtors\Properties.atr"

            // Find out if there is already a file that contains a list of properties.
            // If that file exists, open it to get it ready for the new properties.
            if (File.Exists(Filename))
            {
                stmProperties = new FileStream(Filename,
                                              FileMode.Open,
                                              FileAccess.Read,
                                              FileShare.Read)

                try
                {
                    // Retrieve the list of items from file
                    lstProperties =
                        (Property())bfmProperties.Deserialize(stmProperties)
                }
                finally
                {
                    stmProperties.Close()
                }
            }

            // Save the list of properties
            stmProperties = new FileStream(Filename,
                                          FileMode.Create,
                                          FileAccess.Write,
                                          FileShare.Write)

            try
            {
                bfmProperties.Serialize(stmProperties, lstProperties)
            }
            finally
            {
                stmProperties.Close()
            }
        }
    }
}

Execute the application to create the list
Close the form and return to your programming environment
Return to the form and double-click the Close button

Change the file as follows:

using System
using System.Collections.Generic
using System.ComponentModel
using System.Data
using System.Drawing
using System.Linq
using System.Text
using System.Windows.Forms
using System.IO
using System.Runtime.Serialization.Formatters.Binary

namespace AltairRealtors1
{
    public partial class AltairRealtors : Form
    {
        Property() lstProperties

        public AltairRealtors()
        {
            InitializeComponent()
        }

        private void AltairRealtors_Load(object sender, EventArgs e)
        {
            FileStream stmProperties = null
            BinaryFormatter bfmProperties = new BinaryFormatter()

            // This is the file that holds the list of properties
            string Filename = @"C:\Altair Realtors\Properties.atr"

            // Find out if there is already a file that contains a list of properties.
            // If that file exists, open it.
            if (File.Exists(Filename))
            {
                stmProperties = new FileStream(Filename,
                                              FileMode.Open,
                                              FileAccess.Read,
                                              FileShare.Read)

                try
                {
                    // Retrieve the list of items from file
                    lstProperties = (Property())bfmProperties.Deserialize(stmProperties)
                }
                finally
                {
                    stmProperties.Close()
                }
            }
        }

        private void btnClose_Click(object sender, EventArgs e)
        {
            Close()
        }
    }
}

Introduction to LINQ

To support the ability to query a list, you can use the Language Integrated Query, abbreviated LINQ. To use LINQ in your application, you must include the System.Core.dll assembly in your program. If you started your application as an empty project:

On the main menu, you can click Project -> Add Reference...
In the Solution Explorer, you can right-click the name of the project and click Add Reference...
In the Class View, you can right-click the name of the project and click Add Reference...

In the .NET tab of the Add Reference dialog box, you can click System.Core

Add Reference

Then click OK. You must then use the System.Linq namespace in your code or you can include the using System.Linq line in your list of namespaces.

If you create an application by selecting the Windows Forms Application option from the New Project dialog box, the studio would add the necessary assemblies to your project and the necessary namespaces to your code file.

Creating a Query

To query a list, you write a statement using words and operators of the LINQ. The most fundamental operation you can perform on LINQ consists of creating, also referred to as selecting, or querying, a list of values, from an existing list such as an array. The basic formula to use is:

Dim SubListName = From ValueHolder In List Select ValueHolder

The Dim keyword, the assignment operator "=", the From keyword, the In keyword, the Select keyword, and the semicolon are required.

The SubListName is a name of a new variable that will hold the list of values produced by this operation.

The ValueHolder is the name of a variable that will be used to identify each resulting member of this operation. This variable will be equivalent to getting each member of the list and that responds to a condition.

The List factor represents the name of the variable that you would have created already. The List can be an array. Here is an example:

Imports System
Imports System.Linq
Imports System.Drawing
Imports System.Windows.Forms

Module Exercise

    Public Class WinControls
        Inherits Form

        Private lbxNumbers As ListBox

        Dim components As System.ComponentModel.Container

        Public Sub New()
            InitializeComponent()
        End Sub

        Public Sub InitializeComponent()
            lbxNumbers = New ListBox
            lbxNumbers.Location = New System.Drawing.Point(12, 12)
            lbxNumbers.Width = 100

            Text = "Numbers"
            MinimizeBox = False
            MaximizeBox = False
            Controls.Add(lbxNumbers)
            Size = New System.Drawing.Size(130, 145)
            StartPosition = FormStartPosition.CenterScreen
        End Sub

        Private Sub ExerciseLoad(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
            Dim Numbers() = {12.44, 525.38, 6.28, 2448.32, 632.04}

            Dim Nbrs = From N In Numbers Select N

            For Each Member In Nbrs
                lbxNumbers.Items.Add(Member)
            Next
        End Sub

        Public Shared Function Main() As Integer

            Application.Run(New WinControls())
            Return 0

        End Function

    End Class

End Module

This would produce:

Numbers

To make the code easier to read, you can spread the select statement to various lines. Here is an example:

Dim Nbrs = From N
           In Numbers
           Select N