SharePoint 2010 Error Solution : A Web Part or Web Form Control on this Page cannot be displayed or imported. The type could not be found or it is not registered as safe.

Introduction

An occours when a user tries to add a webpart on the page. The description of the error is "A Web Part or Web Form Control on this Page cannot be displayed or imported. The type could not be found or it is not registered as safe.".

Test Cases

Here, I am creating a small solution to show why I got this issue.

1. Open Visual Studio 2010. Create a project with name "Sample".
2. Add a WebPart to the project with name "SomeWebPart"(Note : Mind it WebPart not VisualWebPart).
3. Add a class library to the solution with name "Helper".
4. Add a "Helper.snk" to the project by Right Clicking "Helper" > Properties > Signing > Choose a strong name...
5. Once that is done the structure will look like image given below
   
6. Helper.Enums Code

   using System;
   using System.Collections.Generic;
   using System.Linq;
   using System.Text;
   
   namespace Helper
   {
       public class Enums
       {
           public enum SomeType
           {
               A = 0,
               B = 1,
               C = 2
           }
       }
   }
   

7. SomeWebPart.cs Code

   using System;
   using System.ComponentModel;
   using System.Web;
   using System.Web.UI;
   using System.Web.UI.WebControls;
   using System.Web.UI.WebControls.WebParts;
   using Microsoft.SharePoint;
   using Microsoft.SharePoint.WebControls;
   
   namespace Sample.SomeWebPart
   {
       [ToolboxItemAttribute(false)]
       public class SomeWebPart : WebPart
       {
           /// <summary>
           /// The enum which resides in enum
           /// </summary>
           public Helper.Enums.SomeType SomeTypeEnum { get; set; }
   
           /// <summary>
           /// Create child control
           /// </summary>
           protected override void CreateChildControls()
           {
   
           }       
           
       }
   }

8. Deploy to the site. Try adding the WebPart and it will give the above mentioned error.

Problem

The problem is "Sample.dll" created for the WebPart gets added to the wsp and gets added to GAC automatically but the "Helper.dll" is not considered as a safe one and it stays out of GAC and so while adding the WebPart, it blows and the error shows on the alert window.

Solution

1. Sample > Package > Open "Package.package"
2. At the bottom you will find an option title "Advanced".
   
3. As shown in the image select "Add Assembly from Project Output...".
   
4. In the dialog select "Helper" will auto populate the Helper details.
5. Safe Controls section add a new item.
6. Fill the Namespace and Assembly with name "Helper".
   
7. Click "OK" and exit the dialog. You can see the settings of Helper added on the Advanced screen.
   
8. Deploy the WebPart and try adding it again. It should work.

Hope this helped!