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 Web Service Consumption
The following example shows you how to read a tag in VB.NET by consuming the OPC Web Client Web Service.
 Reading a Single Tag:
Simply copy and paste the following code into your application:
'
'the following constants are used here to help make the code more readable
'from an educational perspective.
'Feel free to modify the contents of the values below.
Const strPCName = ""
Const strOPCServer = "OPCLabs.KitServer.2"
Const strTag = "Simulation.Random"
Const strAccessPath = ""
Const iDataType = 0
Try
'define our OPC Web Client
Dim objOWC As New EasyDAWS
'define the object to receive the value, quality and timestamp
Dim objTagVTQ As IVTQNet
'now to make the call to read the tag
'NOTE: that we are using "readItem" and NOT "readItemValue". This is because
'I DO WANT the quality & timestamp values
objTagVTQ = objOWC.ReadItem( _
strPCName, _
strOPCServer, _
strTag)
'now to show the value to the screen, but lets do the right thing here and
'check the QUALITY of the TAG first!!!!
'
'0 = bad
'192 = good
If Not objTagVTQ Is Nothing Then
If objTagVTQ.Quality >= 192 Then
MessageBox.Show("Value: " & objTagVTQ.Value.ToString)
Else
MessageBox.Show("Value: " & objTagVTQ.Value.ToString _
& " is NOT GOOD quality, quality : " _
& objTagVTQ.Quality.ToString)
End If
End If
'now to destroy the objects
objTagVTQ = Nothing
objOWC = Nothing
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
Reading Multiple Tags within a single transaction:
Simply copy and paste the following code into your application:
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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