Frequently asked questions

Alex Maitland edited this page Jan 9, 2017 · 94 revisions

Large parts of this document will be replaced with the new General Usage Guide - A must read!

Quick FAQ TOC

This FAQ has ~ 10 items. It tries to showcase some cool CefSharp features and some of the most common gotcha's.

For more hints see the growing list of issues labeled as faq-able!

1. How do you call a Javascript method from .NET?

Simple code may look something like this:

var script = string.Format("document.body.style.background = '{0}'", colors[color_index++]);
if (color_index >= colors.Length)
{
    color_index = 0;
}

browser.GetMainFrame().ExecuteJavaScriptAsync(script);
When can I start executing Javascript?

Unfortunately OnFrameLoadStart is called before the DOM is loaded, so you'll need to use one of the follow: FrameLoadEnd/LoadingStateChanged/IRenderProcessMessageHandler.OnContextCreated. Here are a few examples

browser.RenderProcessMessageHandler = new RenderProcessMessageHandler();

public class RenderProcessMessageHandler : IRenderProcessMessageHandler
{
  // Wait for the underlying `Javascript Context` to be created, this is only called for the main frame.
  // If the page has no javascript, no context will be created.
  void IRenderProcessMessageHandler.OnContextCreated(IWebBrowser browserControl, IBrowser browser, IFrame frame)
  {
    const string script = "document.addEventListener('DOMContentLoaded', function(){ alert('DomLoaded'); });";

    frame.ExecuteJavaScriptAsync(script);
  }
}

//Wait for the page to finish loading (all resources will have been loaded, rendering is likely still happening)
browser.LoadingStateChanged += (sender, args) =>
{
  //Wait for the Page to finish loading
  if (args.IsLoading == false)
  {
    browser.ExecuteJavaScriptAsync("alert('All Resources Have Loaded');");
  }
}

//Wait for the MainFrame to finish loading
browser.FrameLoadEnd += (sender, args) =>
{
  //Wait for the MainFrame to finish loading
  if(args.Frame.IsMain)
  {
    args.Frame.ExecuteJavaScriptAsync("alert('MainFrame finished loading');");
  }
};

Note: Scripts are executed at the frame level, ever page has at least one frame (MainFrame). The IWebBrowser.ExecuteScriptAsync extension method is left for backwards compatibility, you can use it as a shortcut to execute js on the main frame.

2. How do you call a Javascript method that return a result?

If you need to evaluate code which returns a value, use the Task<JavascriptResponse> EvaluateScriptAsync(string script, TimeSpan? timeout) method. Javascript code is executed asynchronously and as such uses the .Net Task class to return a response, which contains error message, result and a success (bool) flag.

// Get Document Height
var task = frame.EvaluateScriptAsync("(function() { var body = document.body, html = document.documentElement; return  Math.max( body.scrollHeight, body.offsetHeight, html.clientHeight, html.scrollHeight, html.offsetHeight ); })();", null);

task.ContinueWith(t =>
{
    if (!t.IsFaulted)
    {
        var response = t.Result;
        EvaluateJavaScriptResult = response.Success ? (response.Result ?? "null") : response.Message;
    }
}, TaskScheduler.FromCurrentSynchronizationContext());

For a more detailed example check out this Gist

Notes

  • Scripts are executed at the frame level, ever page has at least one frame (MainFrame)
  • Only trivial values can be returned (like int, bool, string etc) - not a complex (user-defined) type which you have defined yourself. This is because there is no (easy) way to expose a random Javascript object to the .NET world, at least not today. However, one possible technique is to turn the Javascript object you wish to return to your .NET code into a JSON string with the Javascript JSON.toStringify() method and return that string to your .NET code. Then you can decode that string into a .NET object with something like JSON.net. See this MSDN link for more information. (https://msdn.microsoft.com/en-us/library/ie/cc836459(v=vs.94).aspx)

3. How do you expose a .NET class to Javascript?

See General Usage Guide - How do I expose a .Net class

4. Why do I get an error about "Could not load file or assembly 'CefSharp.Core.dll' or one of its dependencies. The specified module could not be found." when trying to run my CefSharp-based application? It compiles successfully, but does not run? It runs on my developer machine, though throws an exception when I copy it to another computer?

This is a common error, typically one of the following

  1. VC++ 2012/2013 Redistributable Package is required in order to run CefSharp on non developer machines. See FAQ #6 below for more information. You can include the required dlls as part of your application.
  2. Not all dependencies are present in the executing folder. CefSharp includes unmanaged dll's and resources, these are copied to the executing folder via two .props file which are included in your project when you install the Nuget packages. See list of required files below, make sure the required files are present.
  3. You packaged your application for distribution via an installer and it doesn't run on the target machine. Installers don't include the unmanaged resources by default, you'll need to add them manually. For ClickOnce, see #1314 for some pointers and solutions other users have come up with.

A list of required files can be found here: Output files description (Redistribution)

NOTE: This also applies if you get a FileNotFoundException when initializing the WPF control in XAML.

NOTE 2: If compiling from source (not recommended, use the Nuget packages) and you notice that you can no longer build in debug mode, but release builds work just fine you may need to repair your version of Visual Studio. This happens in rare cases where you will get the same exact message as a missing unmanaged .dll file as shown above.

5. Why does the Visual Studio WPF designer not work when I add a ChromiumWebBrowser to my app?

This is unfortunate, and a consequence of the fact that CefSharp is written in C++ code which in turn references CEF DLL:s. That scenario isn't so well-supported by the Visual Studio designer. This forum thread explains it a bit further and also gives a possible workaround. I think (unfortunately) that the easiest workaround is to just accept the fact that it's not easy to support and just live with it, but there are also other approaches if you really feel you need it.

Update It may be possible to use DelayLoadDLLs for libcef.dll and make it possible to add designer support, see https://github.com/cefsharp/CefSharp/issues/1702

6. How do I include the Visual Studio C++ 2012/2013 redistributables on the target app?

CefSharp requires the VC++ Runtime. You have a few options for installing/including this with your app:

  • You can install VC++ on every machine on which you wish to run your CefSharp based application. Once installed updates can then be managed via Windows Update.
  • You can either set the Visual Studio C++ redistributables as pre-requisites of the installer (i.e. ClickOnce or WiX Toolset)
  • By copying over to your project the contents of this folder (Only present if you have the matching version of Visual Studio installed):
# For VC++ 2012 (x86)
C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\redist\x86\Microsoft.VC110.CRT
# For VC++ 2012 (x64)
C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\redist\x64\Microsoft.VC110.CRT

# For VC++ 2013 (x86)
C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\redist\x86\Microsoft.VC120.CRT
# For VC++ 2013 (x64)
C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\redist\x64\Microsoft.VC120.CRT

With the 3rd approach you won't need to install the prerequisite Visual C++ 2012/2013 Runtime Files to your client. For the time being the plan is to stay on VC2013 for official CefSharp binary releases. We might switch to VC2015 when feasible. If you build your own from source what you deploy of course has to match your build environment. For the official Nuget releases see the Releases Branches table for details.

For more information see the Redistributing Visual C++ Files on MSDN. To download visit Visual Studio C++ 2012/2013 redistributables

A new option is JetBrains have a Microsoft.VC120.CRT.JetBrains package on Nuget.org, appears to include the relevant files, you would still need to hook up some sort of post build task to copy them to your bin folder.

Note When building from source make sure you compile in Release mode when deploying to machines that don't have Visual Studio installed. Visual C++ uses a different set of run-time libraries for Debug and Release builds. The Debug run-time libraries are only installed with Visual Studio. If you use the official Nuget packages they're already built in Release mode and you can subsequently build your app in debug mode as only the Visual C++ projects need to be compiled in Release mode.

7. How do I override Javascript window.alert actions and similar?

The ChromiumWebBrowser controls provide a property called JsDialogHandler. This property can be set to a class of yours, which can be used to override the behavior of these dialogs.

8. Where are the CefSharp3 binaries?

CefSharp3 is released as Nuget packages. Please see https://github.com/cefsharp/CefSharp/blob/master/README.md#nuget-packages for the latest stable and pre-release versions.

For a very simple example projects using the NuGet packages, see the CefSharp.MinimalExample repository. Clone it/download the source if you want a really small and simple example of how CefSharp3 can be used.

Please Note: Platform Target
You must select either x86 or x64 when using these packages. If you select AnyCPU the NuGet magic won't work currently.
See steps to configure your solution here.

9. Windows XP/2003 Support

We no longer support Windows XP/2003. The official Nuget packages are not tested on Windows XP/2003. You may have some luck using newer versions that target VC++ 2013 (There is a known issue with the VC++ 2012 packages). (Remembering that the CefSharp.Example.* projects are compiled using .Net 4.5, so you'll have to re-target them and update code accordingly should you wish to use the provided examples).

Please don't create new issues related to Windows XP. The following https://github.com/cefsharp/CefSharp/wiki/Windows-XP-No-Longer-Supported Wiki page may be useful and can be edited by anyone with a GitHub account, so feel free to contribute any information you think may prove useful.

10. What files do I need to include when I redistribute an application that uses CefSharp?

See: Output files description (Redistribution)

11. Build process can't copy CEF files

Sometimes the build process can't copy CEF files and will retry many times before it finally fails. This happens when CefSharp.BrowserSubprocess.exe is still running even though the application was closed.
The solution is to manually kill the CefSharp.BrowserSubprocess.exe using i.e. Task Manager.

12. Why does IndexedDB return a QuotaExceededError?

When working with IndexedDB you must set the CefSettings CachePath to a directory where the current user has write access. In most cases you can use code like this to create a cache directory and pass the setting to Cef during Initialize():

// On Win7 this will create a directory in AppData.
var cache = System.IO.Path.Combine(Environment.GetFolderPath (Environment.SpecialFolder.ApplicationData), System.IO.Path.Combine("MyApplication","cache"));
if (!System.IO.Directory.Exists (cache))
    System.IO.Directory.CreateDirectory (cache);

// Set the CachePath during initialization.
var settings = new CefSettings(){CachePath = cache};
Cef.Initialize (settings);

13. How do you handle a Javascript event in C#?

public class BoundObject
{
    public void OnFrameLoadEnd (object sender, FrameLoadEndEventArgs e)
    {
      if(e.Frame.IsMain)
      {
        browser.ExecuteScriptAsync(@"
          document.body.onmouseup = function()
          {
            bound.onSelected(window.getSelection().toString());
          }
        ");
      }
    }

    public void OnSelected(string selected)
    {
        MessageBox.Show("The user selected some text [" + selected + "]");
    }
}

// ...

// After your ChromiumWebBrowser has been instantiated (for WPF directly after `InitializeComponent();` in the control constructor).
var obj = new BoundObject();
browser.RegisterJsObject("bound", obj);
browser.FrameLoadEnd += obj.OnFrameLoadEnd ;

Please note:

  • RegisterJsObject should be called directly after you create an instance of ChromiumWebBrowser
  • "bound" is the name of the object that will be created as a top-level object accessible to any JS scripts running on the webpage. You can use whatever you like, but ensure it does not conflict with an existing JS object, and that the names match in C# and JS.
  • This sample injects JS code into the webpage on-the-fly using the FrameLoadEnd event. If you have control of the webpage, you can simply add the JS code into the webpage.
  • We call the OnSelected C# method with a single string argument. You can use any number of arguments, or none, but they must all be primitive types.
  • Ensure you cast complex JS objects to strings or convert them to JSON when passing them.
  • Since CefSharp will be searching for the method OnSelected by its name, ensure your obfuscator does NOT rename the method to something else, or it will not be executed.