ASP.NET 3.5 For Dummies®

21.6. Some Common Error Messages and Where to Look

Runtime errors that happen often enough find their way onto Frequently Asked Question (FAQ) lists. In this section, I show you some exceptions that you're likely to encounter and provide you some guidance on resolving them.

21.6.1. System.Security.SecurityException

Your pages are running fine on your computer but fail when you post them to the Web site on the Internet. The exception complains: The application attempted to perform an operation not allowed by the security policy.

Web hosting companies usually lock the trust level of sites to medium rather than full. The following setting (at the machine-level web.config file) overrides whatever you try to set in your Web site:

<trust level="Medium" originUrl="" />

If a page or called component tries to use the file system (usually outside your Web site), Registry, event log, OLEDB data access, or restricted assemblies, the constraint throws a SecurityException that crashes the page.

Ask the Web host to review the trust level and perhaps allow an override or exception. To locate information on appropriate settings, search Microsoft's Web site for How to use medium trust in ASP.NET. If you're using a third-party component, check with the vendor whether their component requires full trust and if so, what you can do to make it work under medium trust.

21.6.2. System.NullReferenceException

People ask about this one all over the Internet: Object reference not set to an instance of an ...