No Parameterless Constructor defined for the Object

So we had created a new Web Part and using SharePoint Designer we put it on to a pagelayout, then deployed as a Feature.

Fantastic

Then when creating a page based on the layout we got this error "No Parameterless Constructor defined for the Object"

So a bit of digging on the internet (and reading the error message) we discover that the problem can be caused by the Web Part not having a Constructor without any parameters.

Something like this.

namespace WebPart1
{
[Guid("4547a00e-f630-4117-922e-f075866be9ec")]
public class WebPart1 : System.Web.UI.WebControls.WebParts.WebPart
{
public WebPart1()
{
}
}
}


However the code did have a parameterless constructor. So I spent the next couple of hours adding parameterless constructors to all of the class files.

Unfortunately this was still to no avail.

Eventually I got to the point when I uninstalled the feature, then completely into Central Admin, retracted the solution, then deleted it.

Then deleted the feature folder in the 12 hive (c:\Program Files\Common Files\Microsoft Shared\web server extensions)

Then re-installed the feature et voila!

So I suspect SharePoint had done some caching along the way, and it was only by doing an uninstall was this error cleared from it's cache.