CodeProject: JavaScript Tips

CodeProject: JavaScript Tips

Call a C# Method from JavaScript Hosted in a WebBrowser

This sample demonstrates how to call C# from JavaScript. It also shows that parameters can be passed to C# methods.

First, create a Windows Forms application. Then, add a WebBrowser control to your form. Then modify the code for the form so it looks like this:

namespace WindowsFormsApplication6
{
    // This first namespace is required for the ComVisible attribute used on the ScriptManager class.
    using System.Runtime.InteropServices;
    using System.Windows.Forms;

    // This is your form.
    public partial class Form1 : Form
    {
        // This nested class must be ComVisible for the JavaScript to be able to call it.
        [ComVisible(true)]
        public class ScriptManager
        {
            // Variable to store the form of type Form1.
            private Form1 mForm;

            // Constructor.
            public ScriptManager(Form1 form)
            {
                // Save the form so it can be referenced later.
                mForm = form;
            }

            // This method can be called from JavaScript.
            public void MethodToCallFromScript()
            {
                // Call a method on the form.
                mForm.DoSomething();
            }

            // This method can also be called from JavaScript.
            public void AnotherMethod(string message)
            {
                MessageBox.Show(message);
            }
        }

        // This method will be called by the other method (MethodToCallFromScript) that gets called by JavaScript.
        public void DoSomething()
        {
            // Indicate success.
            MessageBox.Show("It worked!");
        }

        // Constructor.
        public Form1()
        {
            // Boilerplate code.
            InitializeComponent();

            // Set the WebBrowser to use an instance of the ScriptManager to handle method calls to C#.
            webBrowser1.ObjectForScripting = new ScriptManager(this);

            // Create the webpage.
            webBrowser1.DocumentText = @"<html>
                <head>
	                <title>Test</title>
                </head>
                <body>
	            <input type=""button"" value=""Go!"" onclick=""window.external.MethodToCallFromScript();"" />
                    <br />
                    <input type=""button"" value=""Go Again!"" onclick=""window.external.AnotherMethod('Hello');"" />
                </body>
                </html>";
        }
    }
}

Note that your application may be part of a namespace other than WindowsFormsApplication6, but the rest of the code should work if you follow the above instructions explicitly. I created this tip/trick because somebody asked me a question and they didn’t understand this sample that I sent them to. This tip/trick makes the sample more understandable by fixing the two bugs I spotted, adding the using statements that weren’t mentioned, and by heavily commenting the code. Hopefully the rest of you will find this of use as well.

String.Format in JavaScript

Introduction

C# and VB.NET have a nifty Format() function that allows you to pass in a template string (a string that contains placeholders) and some parameters, and it replaces the placeholders from the template string with the parameter values. For example:

String.Format("Hello {0}.", "world"); // Hello world.

While JavaScript doesn’t come with a function to do this, we can create one that acts very similar.

Example

Here is a complete sample that shows how to create a Format() function in JavaScript:

<html>
<head>
	<title>JavaScript String.Format</title>
	<script type="text/javascript">

		// This is the function.
		String.prototype.format = function (args) {
			var str = this;
			return str.replace(String.prototype.format.regex, function(item) {
				var intVal = parseInt(item.substring(1, item.length - 1));
				var replace;
				if (intVal >= 0) {
					replace = args[intVal];
				} else if (intVal === -1) {
					replace = "{";
				} else if (intVal === -2) {
					replace = "}";
				} else {
					replace = "";
				}
				return replace;
			});
		};
		String.prototype.format.regex = new RegExp("{-?[0-9]+}", "g");

		// Sample usage.
		var str = "She {1} {0}{2} by the {0}{3}. {-1}^_^{-2}";
		str = str.format(["sea", "sells", "shells", "shore"]);
		alert(str);

	</script>
</head>
</html>

Save the above to an HTML file and run it in your browser. You will see a dialog that says:

She sells seashells by the seashore. {^_^}

How it Works

A regular expression is used to find numbers wrapped with curly braces. The number in the curly brace corresponds to the argument index to lookup. The value at that argument index will be inserted in place of the curly brace placeholder. For convenience, I’ve also created some special placeholders. If “{-1}” is passed in, it will be replaced with “{” and if “{-2}” is passed in, it will be replaced with “}”.

JavaScript Regular Expression

While I’ve seen a few implementations of this functionality in JavaScript, none of them seemed to use this approach. Some looked at each character in the string using a loop, and others used a regular expression to search for each placeholder (“{0}”, “{1}”, and so on) in succession. However, this implementation takes advantage of an overload of the replace() function that takes a regular expression and a function as parameters. The regular expression is used to find placeholders to replace and the function is passed each of those placeholders, returning the string to replace it with. This is useful because the string only needs to be scanned once, so performance is improved and we avoid the issue of recursive replaces (e.g., when “{0}” is replaced with “{0}”).

See Also