Make Yahoo! Web Service REST Calls With VB.NET

Welcome, guest Sign In

My Projects
Tools & Services
APIs and Services
- Flickr
- Geo
- Mail
- Search
- Social
- Updates
- YAP
- YQL
Developer Tools
- Hadoop
- YUI
- YSlow
- View all »
Resources
- Blog
- YDN Theater
- Hack U

YDN .Net Developer Center Using REST

Make Yahoo! Web Service REST Calls With VB.NET

The .NET Framework provides classes for performing HTTP requests. This HOWTO describes how to perform both GET and POST requests.

Overview
Simple GET Requests
Simple POST Requests
HTTP Authenticated Requests
Error Handling
Further Reading

Overview

The System.Net namespace contains the HttpWebRequest and HttpWebResponse classes which fetch data from web servers and HTTP based web services. Often you will also want to add a reference to System.Web which will give you access to the HttpUtility class that provides methods to HTML and URL encode and decode text strings.

Yahoo! Web Services return XML data. While some web services can also return the data in other formats, such as JSON and Serialized PHP, it is easiest to utilize XML since the .NET Framework has extensive support for reading and manipulating data in this format.

Simple GET Requests

The following example retrieves a web page and prints out the source.

VB.NET GET Sample 1

Simple POST Requests

Some APIs require you to make POST requests. To accomplish this we change the request method and content type and then write the data into a stream that is sent with the request.

VB.NET POST Sample 1

' We use the HttpUtility class from the System.Web namespace
		Imports System.Web

Dim request As HttpWebRequest
		Dim response As HttpWebResponse = Nothing
		Dim reader As StreamReader
		Dim address As Uri
		Dim appId As String
		Dim context As String
		Dim query As String
		Dim data As StringBuilder
		Dim byteData() As Byte
		Dim postStream As Stream = Nothing

address = New Uri("http://api.search.yahoo.com/ContentAnalysisService/V1/termExtraction")

' Create the web request
		request = DirectCast(WebRequest.Create(address), HttpWebRequest)

' Set type to POST
		request.Method = "POST"
		request.ContentType = "application/x-www-form-urlencoded"

' Create the data we want to send
		appId = "YahooDemo"
		context = "Italian sculptors and painters of the renaissance" _
		   & "favored the Virgin Mary for inspiration"
		query = "madonna"

data = New StringBuilder()
		data.Append("appid=" + HttpUtility.UrlEncode(appId))
		data.Append("&context=" + HttpUtility.UrlEncode(context))
		data.Append("&query=" + HttpUtility.UrlEncode(query))

' Create a byte array of the data we want to send
		byteData = UTF8Encoding.UTF8.GetBytes(data.ToString())

' Set the content length in the request headers
		request.ContentLength = byteData.Length

' Write data
		Try
			postStream = request.GetRequestStream()
			postStream.Write(byteData, 0, byteData.Length)
		Finally
			If Not postStream Is Nothing Then postStream.Close()
		End Try

Try
			' Get response
			response = DirectCast(request.GetResponse(), HttpWebResponse)

' Get the response stream into a reader
			reader = New StreamReader(response.GetResponseStream())

' Console application output
			Console.WriteLine(reader.ReadToEnd())
		Finally
			If Not response Is Nothing Then response.Close()
		End Try

HTTP Authenticated requests

The del.icio.us API requires you to make authenticated requests, passing your del.icio.us username and password using HTTP authentication. This is easily accomplished by adding an instance of NetworkCredentials to the request.

VB.NET HTTP Authentication

Error Handling

Yahoo! offers many REST based web services but they don't all use the same error handling. Some web services return status code 200 (OK) and a detailed error message in the returned XML data while others return a standard HTTP status code to indicate an error. Please read the documentation for the web services you are using to see what type of error response you should expect. Remember that HTTP Authentication is different from the Yahoo! Browser-Based Authentication.

Calling HttpRequest.GetResponse() will raise an exception if the server does not return the status code 200 (OK), the request times out or there is a network error. Redirects are, however, handled automatically.

Here is a more full featured sample method that prints the contents of a web page and has basic error handling for HTTP error codes.

VB.NET GET Sample 2

Public Shared Sub PrintSource(ByVal address As Uri)

Dim request As HttpWebRequest
		Dim response As HttpWebResponse = Nothing
		Dim reader As StreamReader
		Dim sbSource As StringBuilder

If address Is Nothing Then Throw New ArgumentNullException("address")

Try
			' Create and initialize the web request
			request = DirectCast(WebRequest.Create(address), HttpWebRequest)
			request.UserAgent = ".NET Sample"
			request.KeepAlive = False
			request.Timeout = 15 * 1000

' Get response
			response = DirectCast(request.GetResponse(), HttpWebResponse)

If request.HaveResponse = True AndAlso Not (response Is Nothing) Then

' Get the response stream
				reader = New StreamReader(response.GetResponseStream())

' Read it into a StringBuilder
				sbSource = New StringBuilder(reader.ReadToEnd())

' Console application output
				Console.WriteLine(sbSource.ToString())
			End If
		Catch wex As WebException
			' This exception will be raised if the server didn't return 200 - OK
			' Try to retrieve more information about the network error
			If Not wex.Response Is Nothing Then
				Dim errorResponse As HttpWebResponse = Nothing
				Try
					errorResponse = DirectCast(wex.Response, HttpWebResponse)
					Console.WriteLine( _
					  "The server returned '{0}' with the status code {1} ({2:d}).", _
					  errorResponse.StatusDescription, errorResponse.StatusCode, _
					  errorResponse.StatusCode)
				Finally
					If Not errorResponse Is Nothing Then errorResponse.Close()
				End Try
			End If
		Finally
			If Not response Is Nothing Then response.Close()
		End Try

End Sub

Make Yahoo! Web Service REST Calls With VB.NET

Overview

Simple GET Requests

VB.NET GET Sample 1

Simple POST Requests

VB.NET POST Sample 1

HTTP Authenticated requests

VB.NET HTTP Authentication

Error Handling

VB.NET GET Sample 2

Further reading

Yahoo! Forum Discussions