Form visible at runtime, but not at design time

My project has no errors, I'm able to run it via F5, but I can't look at my "main" form, as the [Design] tab tells me, "To prevent possible data loss before loading the designer, the following errors must be resolved: The class name "?" is not a valid identifier for this language." But the Error List that contains that Description says it's at Line 0 and Column 0. Build | Clean Solution did not help. The Call Stack from the [Design] page is: ``` at Microsoft.VisualStudio.Design.Serialization.CodeDom.VSCodeDomParser.CodeTypeDeclarationFromCodeClass(CodeClass vsClass) at Microsoft.VisualStudio.Design.Serialization.CodeDom.VSCodeDomParser.OnNamespacePopulateTypes(Object sender, EventArgs e) at System.CodeDom.CodeNamespace.get_Types() at Microsoft.VisualStudio.Design.Serialization.CodeDom.VSCodeDomParser.Parse(TextReader codeStream) at Microsoft.VisualStudio.Design.Serialization.CodeDom.MergedCodeDomParser.System.CodeDom.Compiler.ICodeParser.Parse(TextReader stream) at System.CodeDom.Compiler.CodeDomProvider.Parse(TextReader codeStream) at Microsoft.VisualStudio.Shell.Design.Serialization.CodeDom.CodeDomDocDataAdapter.get_CompileUnit() at Microsoft.VisualStudio.Design.Serialization.CodeDom.VSCodeDomDesignerLoader.PerformLoad(IDesignerSerializationManager serializationManager) at System.ComponentModel.Design.Serialization.BasicDesignerLoader.BeginLoad(IDesignerLoaderHost host) ``` UPDATE A shutdown of VS2010 and restart solved it. Here: http://msdn.microsoft.com/query/dev10.query?appId=Dev10IDEF1&l=EN-US&k=k(WHYDTELPAGE)%3bk(TargetFrameworkMoniker-%22.NETFRAMEWORK%2cVERSION%3dV4.0%22)&rd=true I found a few suggestions, including this one (but I didn't need to try it, as making VS go to its room for a minute or two before coming back out worked): "delete the [solutionname].suo file which contains designer layout info from your previous session, then start the project and rebuild.."