VB.NET/C# and JavaScript communication

Contents

• Introduction
• Background
• Starting point
• Hello World
• Calling JavaScript from VB.NET
• Calling VB.NET from JavaScript
• Passing arguments to functions
• Passing arguments to JavaScript functions
• Passing arrays to JavaScript functions (unsuccessful attempt)
• Passing arrays to JavaScript functions (successful attempt)
• An important fact to remember
• Passing arguments to VB.NET functions
• Returning values from functions
• Returning value from JavaScript functions
• Returning value from VB.NET functions
• Exception handling
• Handle VB.NET exceptions in JavaScript
• Handle JavaScript exceptions in VB.NET
• Event handling
• Handling VB.NET events from JavaScript
• Handling JavaScript events from VB.NET

Introduction

This article is about passing data between VB.NET/C# WinForms and JavaScript.

Before reading further, let me warn you that this article is not about ASP.NET. Concepts covered in the article are applied to Desktop applications.

Background

I was working on a project which required data transfer between my VB.NET WinForms application and the JavaScript (inside an HTML page). Along the way, I hit certain problems, and trying to resolve them cost plenty of time on the web (on Google mostly), so I thought of this article as a platform for developers looking to sort out similar issues. There isn't much detail on this topic on the web apart from a couple of Hello World examples from Microsoft on MSDN.

Starting point

OK, without any further talk, I will dig in to the subject.

Hello World

To start off, we'll start with a very simple example; all this will do is call a JavaScript function from VB.NET to display an alert with message 'Hello world'. Similarly, from the HTML page using JavaScript, we'll call a VB.NET function which will again display a messagebox with the message 'Hello world'. These are the steps you need to do to make it happen:

Calling JavaScript from VB.NET

• In Visual Studio, create a new WinForms application, name it anything you like.
• Add an import statement like Imports System.Security.Permissions in your form1 (main form).
• Add a couple of attributes to form1, like:

<PermissionSet(SecurityAction.Demand, Name:="FullTrust")> _
<System.Runtime.InteropServices.ComVisibleAttribute(True)> _
Public Class Form1
End Class

All they do is tell the .NET Framework that we want fulltrust and make the class visible to COM so this class is visible to JavaScript.

• Add a WebBrowser control to the form and set its url property to c:\temp\mypage.html (just an example path, you should set it to the HTML page you'll be using).
• Add a Button control on the form, and on its click handler, write this code:

Private Sub Button1_Click(ByVal sender As System.Object, _
            ByVal e As System.EventArgs) Handles Button1.Click
    Me.WebBrowser1.Document.InvokeScript("showJavascriptHelloWorld")
End Sub

• In your HTML page, add this code:

<script type="text/javascript">
    function showJavascriptHelloWorld() {
        alert("Hello world");
    }
</script>

Calling VB.NET from JavaScript

• Continuing with the last project, add the following lines of code in your form1:

Public Sub showVbHelloWorld()
    MsgBox("Hello world")
End Sub

• In your form1 load event, add this line of code:

Me.WebBrowser1.ObjectForScripting = Me

What this line does is, it exposes your form1 class to the JavaScript on the HTML page. You could expose any class as long as you make it visible to COM and set its permission to fulltrust.

• In your HTML page, add a Button, and on its click event, call this function:

<script type="text/javascript">
    function showVbHelloWorld() {
        window.external.showVbHelloWorld();
    }
</script>

• That is it. We've accomplished two way communication. But, most developers must have seen similar examples on MSDN or in different forums. This is just the starting point, our next step is to pass arguments to the functions.

Passing arguments to functions

Passing arguments to JavaScript functions

To pass arguments to a JavaScript function, you need to simply declare functions in your JavaScript code like you normally do. In your VB.NET code, you can pass a single argument like:

Me.WebBrowser1.Document.InvokeScript("javascriptFunction", _
   New String() {"Message from vb.net to javascript"})

If you want to pass more than one variable, you can simply add it like:

Me.WebBrowser1.Document.InvokeScript("javascriptFunction", _
   New String() {"Message from vb.net to javascript", _
  "Argument 1","Argument 2" , "Argument n"})

So far, I am only passing string type arguments, but you are not restricted to simple strings. You can easily pass string, integers, booleans without any special additions. E.g.:

Me.WebBrowser1.Document.InvokeScript("javascriptFunction", _
   New Object() {"Message from vb.net to javascript",200,true})

Arguments are not restricted to simple types, you can pass any type you want. The only restriction is you'll have to make it COM visible. In this example, I am going to pass a Person object to JavaScript. To make it work, I'll follow these steps:

• Create a class called Person, like:

<PermissionSet(SecurityAction.Demand, Name:="FullTrust")> _
<System.Runtime.InteropServices.ComVisibleAttribute(True)> _
Public Class Person
    Public Sub New(ByVal firstName As String, _
           ByVal lastName As String, ByVal age As Integer)
        Me._firstName = firstName
        Me._lastName = lastName
        Me._age = age
    End Sub

    Private _firstName As String
    Private _lastName As String
    Private _age As Integer

    Public Function getFirstName() As String
        Return Me._firstName
    End Function

    Public Function getLastName() As String
        Return Me._lastName
    End Function

    Public Function getAge() As Integer
        Return Me._age
    End Function

End Class

• In your form1, call it like this:

Me.WebBrowser1.Document.InvokeScript("sendPerson", _
   New Person() {New Person("John", "Smith", 37)})

• In your JavaScript, you can use the Person object like:

<script type="text/javascript">
    function sendPerson(person) {
        alert("First Name:" + person.getFirstName() + 
              "Last Name:" + person.getLastName() + 
              "Age:" + person.getAge());
    }
</script>

• Most developers should be aware of this, but I must point out that when you pass your object to JavaScript, it is fully exposed (all public methods/properties), and JavaScript can modify your object. As an example, if I add this function in my Person class:

Public Sub setFirstName(ByVal firstName as String)
    Me._firstName=firstName
End Sub

Now, in your JavaScript, modify the function sendPerson like:

function sendPerson(person) {
    person.setFirstName('Roger');
}

After this call, your person will have first name as 'Roger' instead of 'John'. To avoid this situation, there are two options. Make setFristName private/protected, but that means you won't be able to access this method from VB.NET. The better solution is to add an attribute to setFirstName, like:

<System.Runtime.InteropServices.ComVisibleAttribute(False)> _
Public Sub setFirstName(ByVal firstName as String)
    Me._firstName=firstName
End Sub

Now, your function is visible in your VB.NET code, but JavaScript won't be able to call it, and as a result modify it in case you don't want to expose a particular method to JavaScript for any reason.

Passing arrays to JavaScript functions (unsuccessful attempt)

OK, passing arrays to JavaScript. Some developers might think why is it a separate section. The reason is it is not straightforward to pass arrays as most people would think. If you try this code:

Dim firstNames() As String = {"John", "Richard", "Micheal"}
Dim lastNames() As String = {"Smith", "Stone", "Day"}
Me.WebBrowser1.Document.InvokeScript("passNameArrays", _
                        New Object() {firstNames, lastNames})

In your JavaScript, if you try doing this:

<script type="text/javascript">
    function passNameArrays(firstNames, lastNames) {
        alert(firstNames[0]);
    }
</script>

you'll get a JavaScript error (if error messages are enabled).

So, why didn't that work, you must be asking. The reason is pretty simple. You can only pass simple types, and arrays are not simple types. You'll have to put a workaround to pass arrays.

Passing arrays to JavaScript functions (successful attempt)

OK, it is understood that we can't pass arrays to JavaScript (at least, I am not aware of a way for passing them; if anyone knows a method, they are welcome to correct me). The simplest workaround is to create a type which wraps your array and passes that type instead of a simple array. E.g., you could wrap your array of strings like:

<PermissionSet(SecurityAction.Demand, Name:="FullTrust")> _
<System.Runtime.InteropServices.ComVisibleAttribute(True)> _
Public Class myArr
    Implements IList(Of String)

    Private _list As New List(Of String)

    Public Sub New()

    End Sub
    Public Sub New(ByVal arr() As String)
        For Each Str As String In arr
            Me._list.Add(Str)
        Next
    End Sub

    Public Sub Add(ByVal item As String) _
           Implements System.Collections.Generic.ICollection(Of String).Add
        Me._list.Add(item)
    End Sub

    Public Sub Clear() _
           Implements System.Collections.Generic.ICollection(Of String).Clear
        Me._list.Clear()
    End Sub

    Public Function Contains(ByVal item As String) As Boolean _
           Implements System.Collections.Generic.ICollection(Of String).Contains
        Return _list.Contains(item)
    End Function

    Public Sub CopyTo(ByVal array() As String, ByVal arrayIndex As Integer) _
           Implements System.Collections.Generic.ICollection(Of String).CopyTo

    End Sub

    Public ReadOnly Property Count() As Integer _
           Implements System.Collections.Generic.ICollection(Of String).Count
        Get
            Return Me._list.Count
        End Get
    End Property

       'Rest of the class method needs to be implemented here
End Class

And now, if we take on the example from the previous section where we unsuccessfully tried to pass an array of first and last names, let's give it another try:

Dim firstNames As New myArr(New String() {"John", "Richard", "Micheal"})
Dim lastNames As New myArr(New String() {"Smith", "Stone", "Day"})
Me.WebBrowser1.Document.InvokeScript("passNameArrays", _
               New Object() {firstNames, lastNames})

In your JavaScript, you could use it like:

<script type="text/javascript">

    function passNameArrays(firstNames, lastNames) {
        alert(firstNames.item(0));
    }
</script>

And now, you should see an alert with the text "John". All the rest of the functions like count, indexOf etc. are available from JavaScript.

I have implemented the class as IList(Of String), but you could implement it as IList(Of Object) and pass any type of array (as long as they are COM visible).

An important fact to remember

One important fact which most developers should be aware of is that you are not bound to pass a set number of arguments to the JavaScript function. You could pass any number of arguments to the JavaScript function, e.g., our last JavaScript function could be written and used like this:

<script type="text/javascript">
    function passNameArrays() {
        alert(arguments[0].item(0));
    }
</script>

Passing arguments from JavaScript to VB.NET functions

We have spend enough time passing arguments to JavaScript. Let's move on to the next step, passing arguments to VB.NET functions. You could simply do it like this in JavaScript:

<script type="text/javascript">
    function sendPerson() {
        window.external.sendPerson("John", "Smith", 26);
    }
</script>

and on your VB.NET side, you could define the function to receive these arguments, like:

Public Sub sendPerson(ByVal firstName As String, _
           ByVal lastName As String, ByVal age As Integer)
    MsgBox(String.Format("First name: {0} Last Name: {1} Age: {2}", _
                         firstName, lastName, age))
End Sub

and you are done.

Again, you are not bound to pass simple types as arguments. You could pass JavaScript objects as arguments, if required. Take a look at the last example in a different way. The JavaScript will be:

<script type="text/javascript">
    function sendPerson() {
        var person = new Array();
        person["firstName"] = "John";
        person["lastName"] = "Smith";
        person["age"] = 26;
        window.external.sendPerson(person );
    }
</script>

and you can use the Person object on the VB.NET side, like:

Public Sub sendPerson(ByVal person As Object)
    MsgBox(String.Format("First name: {0} Last Name: {1} Age: {2}", _
           person.firstName, person.lastName, person.age))
End Sub

similar example but different method to achieve the same result.

Returning values from functions

Returning a value from a JavaScript function to VB.NET

What about returning a value from functions? Simple, you could easily get the value from a JavaScript function; e.g., if we call this line in VB.NET:

Dim str As String = _
    Me.WebBrowser1.Document.InvokeScript("getJavascriptString")
MsgBox(str)

and in JavaScript, we could write the function to return a string like:

<script type="text/javascript">
    function getJavascriptString() {
        return "String returned from javascript";
    }
</script>

Again, you are not bound to simple types. You can return your custom JavaScript objects very easily. E.g., let's create a Person class in JavaScript this time, and write a function which will return the Person object back to VB.NET.

<script type="text/javascript">
    function person(name,age) {
        this.name = name;
        this.age = age;
        this.getName = function() {  return this.name; }
        this.getAge = function() { return this.age; }
    }
    
    function getPersonObject() {

        myPerson = new person("Chris McCreadie", 48);
        return myPerson;
    }
</script>

On your VB.NET side, you can do something like this (must warn you the code below is not going to work):

Dim jsPerson As Object = Me.WebBrowser1.Document.InvokeScript("getPersonObject")
MsgBox(String.Format("Name: {0} Age:{1}", jsPerson.getName(), jsPerson.getAge()))

If you try the above code, the first line will run fine, but at runtime (as it is late binding), you'll receive an exception on the second line. If you are asking why it doesn't work, frankly speaking, I don't know, but if you know the answer, you are welcome to explain it.

So, what is the solution? The solution is pretty simple. You take help from System.Reflection, and modify the above code to something like this:

Dim jsPerson As Object = Me.WebBrowser1.Document.InvokeScript("getPersonObject")
Dim jsPersonType As Type = jsPerson.GetType()
Dim name As String = jsPersonType.InvokeMember("getName", _
         Reflection.BindingFlags.InvokeMethod, Nothing, jsPerson, Nothing)
Dim age As Integer = jsPersonType.InvokeMember("getAge", _
        Reflection.BindingFlags.InvokeMethod, Nothing, jsPerson, Nothing)
MsgBox(String.Format("Name: {0} Age: {1}", name, age))

The above code should work perfectly and give you the desired results. I won't go in to the details of explaining the InvokeMember function, there's plenty of help on MSDN and Google for that.

What's the next step now? We can access simple types and complex (custom) types. Can we access the objects (HTML buttons, text fields, div) on the page? Yes, we can. And surprisingly, they are easier to access. Let's pull our sleeves and write a simple function in JavaScript which will return a Button object which happens to be on the page.

<script type="text/javascript">
    function getHtmlButton() {
    //assuming there's a button with id myBtn on the page
        return document.getElementById('myBtn');
    }
</script>

Now, on the VB.NET side, we could do something like this:

Dim htmlButton As Object = Me.WebBrowser1.Document.InvokeScript("getHtmlButton")
htmlButton.value = "Set from vb.net"

The above code will change the button text to "Set from vb.net". We could slightly improve the above code by doing a couple of things:

• Add a reference in your project for "Microsoft HTML Object Library", can be found under the COM section.
• And now, change the first line of the above code to:

Dim htmlButton As mshtml.HTMLInputElement = _
    Me.WebBrowser1.Document.InvokeScript("getHtmlButton")

What these changes will do is, firstly, it will make our htmlButton object a strongly type one instead of the Object type. Secondly, you'll get intellisence from Visual Studio, and it will make your job easier in trying to figure out what method/properties to call/set on the htmlButton.

I would give another example just to show the possibilities.

'assuming JavaScript got a function which will return a div on the page
Dim htmlDiv As mshtml.HTMLDivElement = _
    Me.WebBrowser1.Document.InvokeScript("getHtmlDiv")
htmlDiv.style.background = "Red"

Most of the types in MSHTML are self explanatory, and you can guess what will be converted from the HTML to the MSHTML type. But if you can't tell, here's a small tip: Set your variable to type Object initially, and once you have received an HTML object from JavaScript (assuming you've setup a break point), hover your mouse over it and Visual studio will let you know the exact type, or alternatively, you can look in the Autos or Local window in Visual Studio.

A tip: All (most) developers know this, but if anyone doesn't, here is a little tip if you want more than one value back from JavaScript. If you are passing objects, JavaScript can modify those objects. If we take the example where we created a list(of String) in VB.NET and if we need to get the list of first names and last names from JavaScript, we could do something like this:

Dim firstNames As New myArr()
Dim lastNames As New myArr()
Me.WebBrowser1.Document.InvokeScript("getNames", _
               New Object() {firstNames, lastNames})

and on the JavaScript side, we could do something like:

function getNames(firstNames, lastNames) {
    firstNames.add("John");
    firstNames.add("Chris");
    lastNames.add("Martin");
    lastNames.add("Churn");
}

and as a result, we could easily get multiple values back from JavaScript. I agree this is not the best example, but I hope you get the concept I am trying to explain.

Returning a value from a VB.NET function to JavaScript

I am sure I don't need this section, but just for completeness, I'll give an example here. On the VB.NET side, we could write a simple function to return the current date and time like:

Public Function getCurrentTime() As DateTime
    Return DateTime.Now
End Function

and on the JavaScript side:

function getCurrentTime() {
    alert(window.external.getCurrentTime());
}

Ask me a question :-) I know you must be asking a question right now. Have I done some mistake or submitted an incomplete code? How can I return a DateTime from VB.NET when it is not a simple type and I haven't added any wrapper class around it? The answer is it's a fully working code, and even though DateTime is not a simple type, it's one of the special types which automatically get converted to the JavaScript Date type. Every other concept in returning values from VB.NET is explained above, and I don't think any further explanation is needed. You can return string, integer, boolean, datetime, without any special arrangements, but for other types, you'll have to expose the type/class to COM for it to be visible.

Exception handling

So far, all the examples I have done (for simplicity), I haven't done any exception handling, but that doesn't mean in a production code you'll be doing this. Every call from one platform to the other must be done with an assumption that it wont' work and something at some point will go wrong. As JavaScript is an interpreted language, even if there's some mistake in the script, it might not be visible until runtime when you call JavaScript from your VB.NET code. Similarly, when you call VB.NET from JavaScript, if an exception is thrown from the VB.NET function, it won't crash your program/exe but will be thrown back to the JavaScript and you'll get a JavaScript error dialog from Internet explorer.

Handle VB.NET exceptions in JavaScript

Coming very soon...

Handle JavaScript exceptions in VB.NET

Coming very soon...

Event Handling

Handling VB.NET events from JavaScript

Coming soon...

Handling JavaScript events from VB.NET

Coming soon...

Points of interest

Coming very soon...

History

• 12/04/2009: First iteration, just written the first section called Hello world.
• 13/04/2009: Added section for passing arguments to JavaScript.
• 14/04/2009: Added sections for passing arrays to JavaScript and passing arguments to a VB.NET function.
• 16/04/2009: Added section for returning a value from a JavaScript function to VB.NET.
• 19/04/2009: Finished section for returning a value from JavaScript. Added section for returning a value from a VB.NET function to JavaScript.