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Implementing a Basic Web Service in C#

To demonstrate asynchronous Web Service invocation, I've implemented a sample application that provides information about airports. This is the kind of information a pilot might use, and is similar to the information you might find in an airport facilities directory (AFD). The factual information for the sample program was taken from AirNav.com (see Figure 1). AirNav.com provides a disclaimer that encourages pilots to obtain a current AFD for their specific region. The basic idea behind the sample Web Service is that a pilot could type in the region or state and the locator would find an airport in that area. The information returned by the Web Service might include runway information, details about field FBOs (fixed base operators), and related goods and services.

Figure 1Figure 1 AirNav.com is a useful resource for pilots.

Listing 1 provides a couple of enumerated types and classes that we might implement to support the Web Service. I prefer to implement this kind of code in a class library, distinct from the Web Service project, to facilitate reuse in another context. (For example, I might want to create a Windows version of an application based on this code.)

Listing 1—Code To Implement an Airport Locator Application

using System;
using System.Collections;
using System.Drawing;
using System.IO;

namespace AirportLocatorLibrary
{
 public enum Region
 {
  NorthEast, SouthEast, MidWest,
  SouthWest, NorthWest,
  Mexico, Canada
 }

 public enum State
 {
  Alabama, Alaska, Arkansas, Connecticut, Ohio, Michigan,
  Indiana, Illinois, Kansas, California, NewYork, NewJersey,
  Florida, Washington, Oregon, Texas, Nevada /* etc */
 }

 public class AirportLocator : ReadOnlyCollectionBase
 {
  public AirportLocator(): base() {}
  public AirportLocator(Region region) : base()
  {
   InitializeByRegion(region);
  }

  public AirportLocator( State state ) : base()
  {
   InitializeByState(state);
  }

  public Airport this[int index]
  {
   get{ return (Airport)InnerList[index]; }
  }

  public int Add(Airport value)
  {
   return InnerList.Add(value);
  }

  private void InitializeByRegion(Region region)
  {
   if( region != Region.MidWest ) return;
   InitializeByState(State.Michigan);
  }

  private void InitializeByState(State state)
  {
   if( state != State.Michigan ) return;

   Airport airport = new Airport();
   airport.Airspace = Airspace.ClassE;
   airport.Name = "Mason Jewitt";
   airport.FboName = "Aerogenesis";
   airport.AwosFrequency = 119.425;
   airport.UnicomFrequency = 122.7;
   airport.CallSign = "KTEW";
   airport.FieldElevation = 919;
   airport.Position = new Position(
    "42-33.94610N / 084-25.39312W", _
    "42-33-56.766N / 084-25-23.587W");
   airport.VorFrequency = 110.8;
   airport.State = State.Michigan;

   airport.Description =
    "Small airport. (Note: Spartan Wings flying club)";

   Runway runway = new Runway();
   runway.Length = 4000;
   runway.Width = 75;
   runway.Number = 27;

   airport.Runways = new Runway[]{runway};
   airport.NearestCity = "Mason";

   Add(airport);
  }
 }

 public enum Airspace
 {
  ClassA, ClassB, ClassC, ClassD, ClassE, ClassG
 }

 public class Position
 {
  string longitude;
  string latitude;

  public Position(){}
  public Position(string longitude, string latitude)
  {
   this.longitude = longitude;
   this.latitude = latitude;
  }

  public string Longitude
  {
   get{ return longitude; }
   set{ longitude = value; }
  }

  public string Latitude
  {
   get{ return latitude; }
   set{ latitude = value; }
  }
 }

 public class Airport
 {
  private string name;
  private string callSign;
  private Position position;
  private Airspace airspace;
  private State state;
  private string nearestCity;
  private Runway[] runways;
  private string description;
  private double towerFrequency;
  private double unicomFrequency;
  private double atisFrequency;
  private double awosFrequency;
  private double vorFrequency;
  private string fboName;
  private double fieldElevation;

  public Airport(){}

  public string Name
  {
   get{ return name; }
   set{ name = value; }
  }

  public string CallSign
  {
   get{ return callSign; }
   set{ callSign = value; }
  }

  public Position Position
  {
   get{ return position; }
   set{ position = value; }
  }

  public Airspace Airspace
  {
   get{ return airspace; }
   set{ airspace = value; }
  }

  public State State
  {
   get{ return state; }
   set{ state = value; }
  }

  public string NearestCity
  {
   get{ return nearestCity; }
   set{ nearestCity = value; }
  }

  public Runway[] Runways
  {
   get{ return runways; }
   set{ runways = value; }
  }

  public string Description
  {
   get{ return description; }
   set{ description = value; }
  }

  public double TowerFrequency
  {
   get{ return towerFrequency; }
   set{ towerFrequency = value; }
  }

  public double UnicomFrequency
  {
   get{ return unicomFrequency; }
   set{ unicomFrequency = value; }
  }

  public double AtisFrequency
  {
   get{ return atisFrequency; }
   set{ atisFrequency = value; }
  }

  public double AwosFrequency
  {
   get{ return awosFrequency; }
   set{ awosFrequency = value; }
  }

  public double VorFrequency
  {
   get{ return vorFrequency; }
   set{ vorFrequency = value; }
  }

  public string FboName
  {
   get{ return fboName; }
   set{ fboName = value; }
  }

  public double FieldElevation
  {
   get{ return fieldElevation; }
   set{ fieldElevation = value; }
  }
 }

 public enum Direction
 {
  East, North, West, South
 }

 public class Runway
 {
  private int number;
  private double length;
  private double width;
  private double elevation;
  private string unit;
  private Direction prevailingWind;

  public int Number
  {
   get{ return number; }
   set{ number = value; }
  }

  public int Heading
  {
   get{ return number * 10; }
  }

  public double Length
  {
   get{ return length; }
   set{ length = value; }
  }

  public double Width
  {
   get{ return width; }
   set{ width = value; }
  }

  public string Unit
  {
   get{ return unit; }
   set{ unit = value; }
  }

  public double Elevation
  {
   get{ return elevation; }
   set{ elevation = value; }
  }

  public Direction PrevailingWind
  {
   get{ return prevailingWind; }
   set{ prevailingWind = value; }
  }
 }
}

The code in Listing 1 implements a ReadOnlyCollectionBase, a strongly typed collection, but we could have just as easily used an ADO.NET DataSet. The code has types that describe things like runways, radio frequencies, and airport locations.

The code in Listing 1 is not a Web Service; rather, Listing 1 provides a partial, general solution to managing information about airports. Comparatively, the Web Service itself is the easy part.

The Web Service as I defined it accepts information indicating the general area in which we want to search for airports, and returns a strongly typed collection of Airport objects. (Because I only have one test airport, in the InitializeByState method, the AirportLocator will always return Mason Jewitt's airport information. A real application would probably read this information from a persistent data store—perhaps a database or XML file.) Listing 2 shows the Web Service code, most of which is generated when you create an ASP.NET Web Service project.

Listing 2—The Web Service Code

using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Diagnostics;
using System.Web;
using System.Web.Services;

namespace AirportLocator
{
 /// <summary>
 /// Summary description for Service1.
 /// </summary>
 public class Service1 : System.Web.Services.WebService
 {
  public Service1()
  {
   //CODEGEN: Call is required by the ASP.NET Web Services Designer
   InitializeComponent();
  }

  #region Component Designer generated code
  //Required by the Web Services Designer
  private IContainer components = null;

  /// <summary>
  /// Required method for Designer support - do not modify
  /// the contents of this method with the code editor.
  /// </summary>
  private void InitializeComponent()
  {
  }

  /// <summary>
  /// Clean up any resources being used.
  /// </summary>
  protected override void Dispose( bool disposing )
  {
   if(disposing && components != null)
   {
    components.Dispose();
   }
   base.Dispose(disposing);
  }
  #endregion

  [WebMethod()]
  public AirportLocatorLibrary.AirportLocator GetAirports(string state)
  {
   return new AirportLocatorLibrary.AirportLocator(
    AirportLocatorLibrary.State.Michigan);
  }

 }
}

This code may look daunting until I tell you that we only have to add the six lines in bold beginning with the WebMethodAttribute and ending with the method curly brace. (By convention we drop the Attribute suffix when applying attributes, and web methods are public.) The rest of the code is part of the ASP.NET Web Service project template.

The rest of our job is just programming, taken care of by the Microsoft Web Services technology. To recap, this code invokes the AirportLocatorLibrary.AirportLocator factory method, which constructs our list of Airports. Here are the steps for creating the Web Service:

  1. Create a new class library project in a new solution.

  2. Add the code from Listing 1 to a .cs module.

  3. Add an ASP.NET Web Service project to the same solution as the one containing the class library.

  4. In the ASP.NET Web Service project, add a project reference to the class library containing the Aiport code.

  5. Add the bold case code from Listing 2 to the Web Service Service1.asmx.cs code behind the file.

Make the Web Service project the startup project, and press F5 to run the Web Service.

When you run the Web Service directly, you get a test page similar to the one shown in Figure 2. Click the GetAiports link (this is the web method defined in Listing 2) to test the web method.

TIP

To ensure that you have a distinct name for your Web Service, change the WebServiceAttribute namespace argument before deploying your Web Service. For example, place this immediately before the class header in Listing 2:

[WebService(Namespace = http://www.softconcepts.com")]

Figure 2Figure 2 When you browse to an .asmx page, a test page is created that provides a simple interface allowing you to test web methods.

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