How to register a Web Part as safe?

Question

At launching "Preview in Browser" (by pressing F12) of a page from SPD 2010 I am getting the error:

• soap:ServerServer was unable to process request. ---> A Web Part or Web Form Control on this Web Part Page cannot be displayed or imported because it is not registered as safe on this site. You may not be able to open this page in an HTML editor that is compatible with Microsoft SharePoint Foundation, such as Microsoft SharePoint Designer. To fix this page, contact the site administrator to have the Web Part or Web Form Control configured as safe. You can also remove the Web Part or Web Form Control from the page by using the Web Parts Maintenance Page. If you have the necessary permissions, you can use this page to disable Web Parts temporarily or remove personal settings. For more information, contact your site administrator.

Well, this is floating error that I sometime have, sometimes not.

What are the probable causes of it?

Answer 1

Make sure that all web parts on that page have a corresponding safe control entry in web.config. Here is an example of one of the SharePoint assemblies on how to register all web parts in a given namespace as safe:

 <configuration>
  <SharePoint>
    <SafeControls>
      <SafeControl Assembly="Microsoft.SharePoint, Version=11.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" Namespace="Microsoft.SharePoint.WebPartPages" TypeName="*" Safe="True" AllowRemoteDesigner="True" SafeAgainstScript="False" />

Custom developed web parts get the safe control entry automatically added, when it is created in Visual Studio 2010. Refactoring the name or namespace of the web part class will break that safe control entry.

Answer 2

Check this out on msdn : Step 4: Add your Web Part to the Safe Controls List

Answer 3

I know this is an old post, but this havent solved my issue.

The solution in my case was: Type ?contents=1 after the aspx page in your browser. This opens up the webpart maintenance page. If you see ErrorWebPart, check the box before it, and click delete. After this, you'll be able to open the page from SharePoint Designer.