File not found error when you select New Webpart after installing SmartPart 1.3

Recently, I was asked to install SmartPart 1.3 on WSS 3.0. After installation, we encountered the following error when we attempt to create a new web part in the web part gallery:

File Not Found. at System.Signature._GetSignature(SignatureStruct& signature, Void* pCorSig, Int32 cCorSig, IntPtr fieldHandle, IntPtr methodHandle, IntPtr declaringTypeHandle)
at System.Signature.GetSignature(SignatureStruct& signature, Void* pCorSig, Int32 cCorSig, RuntimeFieldHandle fieldHandle, RuntimeMethodHandle methodHandle, RuntimeTypeHandle declaringTypeHandle)
at System.Signature..ctor(RuntimeMethodHandle methodHandle, RuntimeTypeHandle declaringTypeHandle)
at System.Reflection.RuntimeMethodInfo.get_Signature()
at System.Reflection.RuntimeMethodInfo.GetParameters()
at Microsoft.SharePoint.WebPartPages.SPWebPartSerializer.GetPersonalizableProperties()
at Microsoft.SharePoint.WebPartPages.SPWebPartManager.GetEffectiveWebPartType(Type webPartType, SerializationTarget serializationTarget, Boolean ignoreSupportsAttributeMarkup)
at Microsoft.SharePoint.WebPartPages.SPWebPartManager.GetEffectiveWebPartType(Type aspWebPartType, SerializationTarget serializationTarget)
at Microsoft.SharePoint.ApplicationPages.NewDwp.Page_PreRender(Object sender, EventArgs e)
at System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e)
at System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e)
at System.Web.UI.Control.OnPreRender(EventArgs e)
at Microsoft.SharePoint.WebControls.UnsecuredLayoutsPageBase.OnPreRender(EventArgs e)
at System.Web.UI.Control.PreRenderRecursiveInternal()
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Various sites indicated that Smartpart 1.3 requires AJAX extension 1.0. Because AJAX extension 1.0 is included in .Net Framework 3.5 SP1, I install that instead. However, this did not solve the problem. I found the solution in the comments on this post:

http://www.codeplex.com/smartpart/WorkItem/View.aspx?WorkItemId=2708

Apparently, SmartPart is binding to the older version of the System.Web.Extensions.  If I had installed an earlier version of the framework and the separate AJAX extension 1.0, it may have worked. The article does suggest a workaround.

1. Go the the web.config of your WSS.
2. Locate the section <runtime>, and within that <assemblyBinding>. There should be a list of <dependentAssembly> entries.

3. Add the following entry:

<dependentAssembly>
<assemblyIdentity name="System.Web.Extensions" publicKeyToken="31bf3856ad364e35" culture="neutral" />
<bindingRedirect oldVersion="1.0.61025.0" newVersion="3.5.0.0" />
</dependentAssembly>

What the entry does is redirect binding to the old version of System.Web.Extensions to the new version. The 3.5.0.0 version should have been installed to the GAC when you install Framework 3.5 SP1.

After this change, you will be able to create a new web part in the web part gallery again.