Consume REST Web Service VB.NET and Compact Framework 3.5

KennyMcAtee used Ask the Experts™

on 2012-12-31

How do you consume a REST Web service using VB.net. Found a few C# samples but none of them seem to work properly.

Comment

Watch Question

Do more with

EXPERT OFFICE^® is a registered trademark of EXPERTS EXCHANGE^®

View Solution Only

ǩa̹̼͍̓̂ͪͤͭ̓u͈̳̟͕̬ͩ͂̌͌̾̀ͪf̭̤͉̅̋͛͂̓͛̈m̩̘̱̃e͙̳͊̑̂ͦ̌ͯ̚d͋̋ͧ̑ͯ͛̉Glanced up at my screen and thought I had coded the Matrix... Turns out, I just fell asleep on the keyboard.

Trusted Expert

Most Valuable Expert 2011

Top Expert 2015

Commented: 2012-12-31

A RESTful service functions in a similar fashion as a webpage does. Unfortunately, from what I can see, the WebClient class is not contained with CF, so you would have to resort to the HttpWebRequest and HttpWebResponse classes. Depending on the complexity of the service, it could be a simple as:

Imports System.Net
Imports System.IO

Module Module1

    Sub Main()
        Dim request As HttpWebRequest = HttpWebRequest.Create("http://host/path/to/RESTful/service")
        Dim response As HttpWebResponse

        request.Method = WebRequestMethods.Http.Get

        response = request.GetResponse()

        Using reader As New StreamReader(response.GetResponseStream())
            Console.WriteLine(reader.ReadToEnd())
        End Using

    End Sub

End Module

Open in new window

KennyMcAtee

Author

Commented: 2012-12-31

Was hoping to see a step or something in the code that I had missed.. The following code I have working on my laptop without any problems. When I try to port it over to mobile device I am only able to send 2 records. I no longer get a response and triggers a timeout.
To me it seems like some resource is not being released quick enough. Any suggestions?

Private Function SendREST(ByVal strCmd As String) As String

'(string localFile, string uploadUrl)
Dim strAction As String = "POST"
Dim strUrl As String = "WebAddress"

Dim req As HttpWebRequest

''HttpWebRequest req = (HttpWebRequest)WebRequest.Create(uploadUrl);
''req.Method = "PUT";
req = WebRequest.Create(strUrl)
req.Method = "POST"
req.AllowWriteStreamBuffering = True

Dim htmlResponse, jsonResponse As String
Dim client As New REST_Client

lngStart = Date.Now.TimeOfDay.TotalMilliseconds

client.MakeRequest(strUrl, strAction, strCmd, htmlResponse, jsonResponse)

SendREST = jsonResponse
lngEnd = Date.Now.TimeOfDay.TotalMilliseconds
client = Nothing

End Function

Public Class REST_Client
Public Sub MakeRequest(ByVal url As String, ByVal httpVerb As String, ByVal requestBody As String, ByRef htmlResponse As String, ByRef jsonResponse As String) ''As String

Dim reponseAsString As String = ""

Dim intHld As Integer
Dim request = DirectCast(WebRequest.Create(url), HttpWebRequest)
Try

request.Method = httpVerb

request.Timeout = intTO
request.ContentType = "application/json"
request.SendChunked = True
' request.ContentLength = bytearray.length

SetBody(request, requestBody)

Dim response = DirectCast(request.GetResponse(), HttpWebResponse)

ConvertResponseToString(response, htmlResponse, jsonResponse)
Catch ex As Exception
If Err.Number <> 5 Then
MessageBox.Show(Err.Number.ToString & vbCrLf & Err.Description)
Else
errNo = Err.Number
bTimeout = True
End If
''Handle error as desired

Finally
request = Nothing
requestBody = Nothing
End Try

End Sub

Private Sub SetBody(ByVal request As HttpWebRequest, ByVal requestBody As String)
Try
If requestBody.Length > 0 Then

Dim requeststream As Stream = request.GetRequestStream()
Dim writer As New StreamWriter(requeststream)
writer.Write(requestBody)

requeststream.Close()
requeststream.Dispose()
System.GC.Collect()

'Using requestStream As Stream = request.GetRequestStream()
'Using writer As New StreamWriter(requeststream)
' writer.Write(requestBody)
'End Using
'requeststream.Close()
'requeststream.Dispose()
'End Using

End If

Catch ex As Exception
If Err.Number <> 5 Then
MessageBox.Show(Err.Number.ToString & vbCrLf & Err.Description)
Else
errNo = Err.Number
bTimeout = True
End If
End Try
End Sub

Private Sub ConvertResponseToString(ByVal response As HttpWebResponse, ByRef htmlResponse As String, ByRef jsonResponse As String) 'As String

htmlResponse = "Status code: " & CInt(response.StatusCode) & " " & Convert.ToString(response.StatusCode) & " " & vbCr & vbLf
Try

For Each key As String In response.Headers.Keys
htmlResponse += String.Format("{0}: {1} " & vbCr & vbLf, key, response.Headers(key))
Next

jsonResponse = New StreamReader(response.GetResponseStream()).ReadToEnd()
Catch ex As Exception
MessageBox.Show(Err.Number.ToString & vbCrLf & Err.Description)

End Try

End Sub
End Class

Vadim Rapp

Verified Expert

Commented: 2013-06-16

When I faced this problem, I did a little research, and ended with writing my own class for that. The request is composed simply by concatenation of request elements plus all the tags. Here's the piece that parses the response, see if it helps.

    Private Function ExtractElement(ByVal Source As String, ByVal Node As String) As Object
        Dim L As String, c1 As String = "", c2 As String = ""
        L = Split(Source, """" & Node & """:")(1)
        If L.StartsWith("""") Or L.StartsWith("{") Or L.StartsWith("[") Then
            c1 = Left(L, 1)
            If c1 = """" Then c2 = """"
            If c1 = "{" Then c2 = "}"
            If c1 = "[" Then c2 = "]"
            ExtractElement = Extr(L, c1, c2)
        Else
            ExtractElement = L.Split(",")(0)
        End If
        Exit Function
    End Function

    Private Function Extr(ByVal Str As String, ByVal Sep As String, ByVal ClosingSep As String) As String
        Dim Pos1 As Integer, Pos2 As Integer, cnt As Integer = 0
        For i = 1 To Len(Str)
            If Mid(Str, i, Len(ClosingSep)) = ClosingSep And cnt > 0 Then
                cnt = cnt - 1
                If cnt = 0 Then Pos2 = i
            ElseIf Mid(Str, i, Len(Sep)) = Sep Then
                If cnt = 0 Then Pos1 = i
                cnt = cnt + 1
            End If
            If cnt = 0 Then
                If Pos2 = 0 Then
                    Return ""
                Else
                    Return Mid(Str, Pos1 + Len(Sep), Pos2 - Len(Sep) - 1)
                End If
            End If
        Next i
        Return ""
    End Function

    Private Function ExtractArray(ByVal Source As String, ByVal Node As String) As Object
        Dim L As String, StartTag As String = "[", ClosingTag As String = "]", Cnt As Long = 0, i As Long, j As Long, j1 As Long
        If Not Source.StartsWith(",") Then Source = "," & Source
        L = Split(Source, ",""" & Node & """:")(1)
        ' find closing tag of starttag
        For i = 1 To Len(L)
            If Mid(L, i, 1) = StartTag Then Cnt += 1
            If Mid(L, i, 1) = ClosingTag Then Cnt -= 1
            If Cnt = 0 Then
                L = Mid(L, 2, i - 2)
                Dim r()
                ReDim r(0)
                ' find array members: comma-separated {...},{...},....
                For j = 1 To Len(L)
                    If Mid(L, j, 1) = "{" Then
                        Cnt = Cnt + 1
                        If Cnt = 1 Then j1 = j
                    End If
                    If Mid(L, j, 1) = "}" Then
                        Cnt = Cnt - 1
                        If Cnt = 0 Then
                            If r(UBound(r, 1)) <> "" Then ReDim Preserve r(UBound(r, 1) + 1)
                            r(UBound(r, 1)) = Mid(L, j1 + 1, j - j1 - 1)
                        End If
                    End If
                Next j
                Return r
                Exit Function
            End If
        Next i
        Return Nothing
    End Function

Open in new window

KennyMcAtee

Commented: 2014-04-01

Fixed solution. Was not closing all resources so portable was running out of resources.

KennyMcAtee

Author

Commented: 2014-04-06

Answers provided did not fix my problem. I know it was running out of resources, just assumed it was a problem with the garbage collector. Tried adding the close in a few places that seemed logical and low and behold we have a solution

fernando Rodriguez

Commented: 2016-10-10

I won try

Do more with

Submit tech questions to Ask the Experts™ at any time to receive solutions, advice, and new ideas from leading industry professionals.

Start 7-Day Free Trial

Consume REST Web Service VB.NET and Compact Framework 3.5

Premium Content

Premium Content

Explore our Suite of Services