Call JavaScript from C# code, passing parameters and returning results.

I came across a need to call JavaScript methods on HTML code inside a web page from a C# WinForms application. How could that language gap be bridged?

It turns out that Type.InvokeMember() does the trick when using a [Internet Explorer] WebBrowser control to load a web page.

using mshtml;
...
private AxSHDocVw.AxWebBrowser axWebBrowser1;
...
///
 /// Retrieves a reference to the HTML element containing a DIV. /// JavaScript methods are implemented on this element itself. /// 

/// A reference to the HTML element holding the JavaScript methods
private HTMLDivElementClass GetContainer()
{
 HTMLDivElementClass container = null;
 IHTMLDocument2 doc = axWebBrowser1.Document as IHTMLDocument2;

 if (null != doc)
 {
  foreach (IHTMLElement element in doc.all)
  {
   if (element.id == "My_Container")
   {
    container = element as HTMLDivElementClass;
    break;
   }
  }
 }

 return container;
}

///
 /// Gets the text from the container /// 

/// A string containing the text of the container
private string GetText()
{
 string result = null;
 HTMLDivElementClass div = GetContainer();
 if (null != div)
 {
  Type type = div.GetType();
  result = (string) type.InvokeMember(
    "GetText",
    BindingFlags.InvokeMethod,
    null,
    div,
    null);
 }   

 return result;
}

The HTML page loaded in the WebBrowser control has a DIV element with an id of “My_Container”, attached to which is a method called GetText(), which returns some arbitrary text.

Having acquired a reference to the DIV on which the JavaScript method is declared (using GetContainer()), the JavaScript method is called using the InvokeMember() method of the Type class instance; the return value is cast to a string.

Discussion

There is every reason why this should NOT work, but the implementers of the WebBrowser control decided that JavaScript methods living within the DOM should be accessible as first class object members at runtime (in this case, via IDispatch). Nice!

Result

C# code can call arbitrary JavaScript within an HTML page! The only limitation is that the return types of the JavaScript methods can be only simple types (even DateTime is too complex).