Public Sub DrawChart(ByVal i_omsChart As Chart, ByVal i_iChartWidth As Integer, ByVal i_iChartHeight As Integer)

As a 1st parameter it accepts actual chart control from the page, 2nd and 3rd are chart width and height. The method then gets the data for the chart, binds chart to that data, applies chart appearance (colors, series, axises) etc. So drawing a chart is a simple as instantiating the class and calling the method:

Dim oCharting As New CCharting
CCharting.DrawChart(xmsChart,500,300)

where xmsChart is a chart control from HTML markup of the page. The result is displayed on the page:

But now I needed to give access to that chart to external applications, that do not have access neither to chart data nor to Microsoft charting control, may run under different OS's, be Web apps or not. The solution I came up with is to serve chart image via Web Service.

Since CCharting class already had all the chart drawing functionality, I decided to reuse it. A WebService was created with GetChartImage method:

<WebMethod()> _
Public Function GetChartImage(ByVal i_iChartWidth As Integer, ByVal i_iChartHeight As Integer) As Byte()
   Dim oCharting As New CCharting
   Dim omsChart As New Chart
   Dim oStream As New IO.MemoryStream()  

   oCharting.DrawChart(omsChart, i_iChartWidth, i_iChartHeight)
   omsChart.SaveImage(oStream, ChartImageFormat.Png)

   Return oStream.ToArray

End Function

It's very simple. The method accepts chart width and height as parameters and returns chart image in form of a byte array. Notice Line 4 - instead of hard-coded chart control from HTML markup a chart object is created programmaticaly. DrawChart method accept it and draws chart on it (Line 7). Then image is saved in PNG format into memory stream (Line 8). And finally the stream is converted to byte array to be returned as a result (Line 10).

An example of using this Web Service in an external application is a WinForm app. A very basic VB.NET winform app was created with a single PictureBox control (named xpicChart) on the form. A reference to WebService above was added (namespace named WebApi). After that loading image into PictureBox takes just 3 lines of code:

Dim oWebApi As New WebApi.CWebServiceSoapClient
Dim aImage As Byte() = oWebApi.GetChartImage(xpicChart.Width, xpicChart.Height)
xpicChart.Image = Image.FromStream(New IO.MemoryStream(aImage))

1st line creates a WebService client object, 2nd calls the method and retrieves image as a byte array and 3rd loads image into the picture box by converting the byte array into memory stream. The result:

Private Sub xfrm_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
   xCombo.DataSource = GetSomeData()
   xCombo.SelectedIndex = -1
End Sub

Then, when user actually selects an item - SelectedIndexChanged event handler is called to perform action on the selected item:

Private Sub xCombo_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles xCombo.SelectedIndexChanged
   PerformSomeAction(xCombo.SelectedValue)
End Sub

The problem with this approach is when data is bound in Form_Load - SelectedIndexChanged event handler is also called, and when SelectedIndex is set to -1, SelectedIndexChanged event is called again. Since we expect the SelectedIndexChanged to run only when user selects an item - this behavior may cause unwanted consequences.

The solution is to remove Handles VB.NET keyword from event handler declaration:

Private Sub xCombo_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs)
   PerformSomeAction(xCombo.SelectedValue)
End Sub

and programmaticaly assign even handler to the combobox event:

Private Sub xfrm_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
   xCombo.DataSource = GetSomeData()
   xCombo.SelectedIndex = -1
   AddHandler xCombo.SelectedIndexChanged, AddressOf xCombo_SelectedIndexChanged
End Sub

In this case when data is bound to the combobox and SelectedIndex is assigned a value - no event handler is associated with SelectedIndexChanged event. And only after those actions are performed - event handler is added, so the next time user selects an item action is performed.