What to do when this error occurs when you need to add a new webpart to your gallery of webparts.

The error:

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)

The cause of this error is caused probably for two reasons:

1. You are using either the SmartPart webpart of Jan Tielens.
2. Or ASP.NET Ajax is not installed on the SharePoint server.

All you need to do to correct this is download ASP.NET 2.0 AJAX Extensions 1.0, install it… and do a IISRESET.

Good luck.