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Back to Services Source Examples
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Using the OPCData.NET Components within a VB.NET NT Service Application
The following example shows you one possible way in which you could use the OPCData.NET Components within an NT Service application.
This example is not for all developers as NT Service development is not easy. Therefore this and other NT Service based examples should only be used by experienced NT Service programmers.
How this example is constructed
This example actually uses a Windows Form to display some options allowing the end-user to specify the OPC Server, Tag and PC etc. to read. The values are then read and displayed to the form.
A Form was used for simplicity in showing this example. Please note that a Windows Form is NOT required to use the OPC Web Client within an NT Service.
This example contains the following form:
The code in this example is applied to the Click event handler for the button in this window. The textfields in this window are identified as "txt" and then the name, i.e. "txtPC", "txtOPC", "txtTag" and "txtAccess".
IMPORTANT NOTE:
When using an NT Service with Windows Forms you must ensure that the Service
properties (within the CONTROL PANEL > Administrative Tools > Services) are set to allow the service to interact with the desktop. If this setting is not applied then you will not see the Form!
 Reading a Tag
Try
'
'define our OPC Web Client object
Dim objOWC As New
EasyOPCDALib.EasyDA3
'
'define the tag object and get a reading from the
'OPC Server
Dim objVTQ As EASYOPCDALib.VTQ = objOWC.ReadItem( _
txtPC.Text, _
txtOPC.Text, _
txtTag.Text, _
txtAccess.Text, _
0)
'
'check if any errors occurred
If Err.Number = 0 Then
labelValue.Text = objVTQ.Value
labelQuality.Text = objVTQ.Quality
Else
labelValue.Text = "Error " & Err.Number
End If
'
'clean-up
'NOTE: in your application you probably would not destroy
'the objects as we are here. You would most likely
'want to keep them alive as long as possible.
objVTQ = Nothing
objOWC = Nothing
Catch ex As Exception
labelValue.Text = "Error: " & ex.Message
End Try
Reading Multiple Tags within a single transaction
Try
'
'create ourselves an opc web client object
Dim EasyDA As New OPCLabs.EasyOPCDANet.EasyDAWS
'
'now create an array to receive our values into
Dim Values As Array
'
'now request the read of multiple items
'I have actually broken this single instruction over multiple lines
'to try and make it more readable.
'You will notice that there are essentially 3 parameters:
' (1) the computer name for the opc server, which is blank in this case
' (2) the opc server
' (3) the array of items to read
Values = EasyDA.ReadMultipleItemValues _
( _
"", _
"OPCLabs.KitServer.2", _
New Object() {"Simulation.Random", _
"Trends.Ramp (1 min)", _
"Trends.Sine (1 min)", _
"Simulation.Register_I4"} _
)
'
'now to take a look at what we received
If Not Values Is Nothing Then
'
'iterate thru the array
For I As Integer = LBound(Values) To UBound(Values)
'
'lets check to make sure that the value is not an error code type
If VarType(Values(I)) <> VariantType.Error Then
'
'no error here, so lets display the value to a textbox
TextBox1.Text += "Element " & I & ": " & Values(I) & vbCrLf
Else
'
'lets call the "raiseError" method and raise the error returned in
'the value
EasyDA.RaiseError(Values(I))
'
'display an error to the textbox
TextBox1.Text += vbCrLf & "** " _
& Err.Source _
& ": " & Err.Description _
& " (0x" & Hex(Err.Number) & ")" _
& " **"
End If
Next
End If
Catch webex As Web.Services.Protocols.SoapException
'display exception to screen
If webex.InnerException Is Nothing Then
MessageBox.Show(webex.Message)
Else
MessageBox.Show(webex.InnerException.Message)
End If
Catch ex As Exception
'
'display an exception to screen
If ex.InnerException Is Nothing Then
MessageBox.Show(ex.Message)
Else
MessageBox.Show(ex.InnerException.Message)
End If
End Try
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