In my unity project, I extended System.Net.WebClient class to add timeout property from this question.
class WebDownload : WebClient
{
    public int Timeout { get; set; }
    public WebDownload() : this(60000) { }
    public WebDownload(int timeout)
    {
        this.Timeout = timeout;
    }
    protected override WebRequest GetWebRequest(Uri address)
    {
        var request = base.GetWebRequest(address);
        if (request != null)
        {
            request.Timeout = this.Timeout;
        }
        return request;
    }
}
After I save my file, Visual Studio 2013 gives this error :

Even if Visual studio gives that error, program compiled without error and I can use my WebDownload object without problem. I tried to do same thing for an empty project and this time it did not give that error.
Is this a bug in Visual Studio 2013 Unity Tool?
Here is the error stack (in case image is not readable) :
at System.ComponentModel.TypeDescriptor.AddAttributes(Object instance, Attribute[] attributes) at Microsoft.VisualStudio.Design.VSDesignSurface.CreateDesigner(IComponent component, Boolean rootDesigner) at System.ComponentModel.Design.DesignerHost.AddToContainerPostProcess(IComponent component, String name, IContainer containerToAddTo) at System.ComponentModel.Design.DesignerHost.PerformAdd(IComponent component, String name) at System.ComponentModel.Design.DesignerHost.System.ComponentModel.Design.IDesignerHost.CreateComponent(Type componentType, String name) at System.ComponentModel.Design.Serialization.DesignerSerializationManager.CreateInstance(Type type, ICollection arguments, String name, Boolean addToContainer) at System.ComponentModel.Design.Serialization.DesignerSerializationManager.System.ComponentModel.Design.Serialization.IDesignerSerializationManager.CreateInstance(Type type, ICollection arguments, String name, Boolean addToContainer) at System.ComponentModel.Design.Serialization.TypeCodeDomSerializer.Deserialize(IDesignerSerializationManager manager, CodeTypeDeclaration declaration) at System.ComponentModel.Design.Serialization.CodeDomDesignerLoader.PerformLoad(IDesignerSerializationManager manager) at Microsoft.VisualStudio.Design.Serialization.CodeDom.VSCodeDomDesignerLoader.PerformLoad(IDesignerSerializationManager serializationManager) at System.ComponentModel.Design.Serialization.BasicDesignerLoader.BeginLoad(IDesignerLoaderHost host)
 
    