By Tim Gaunt in ASP.Net — 22 Aug 2007

String.Format- Exception of type System.Web.HttpUnhandledException was thrown.

TypeSystem.FormatExceptionMessageException of type 'System.Web.HttpUnhandledException' was thrown. StackTraceat System.Text.StringBuilder.FormatError() at System.Text.StringBuilder.AppendFormat(IFormatProvider provider, String format, Object[] args) at System.String.Format(IFormatProvider provider, String format, Object[] args) Error Line0

Just got that message (or at most "Exception of type 'System.Web.HttpUnhandledException' was thrown")? Puzzled? I was the first time I got it, I've been meaning to post about it for quite some time now so seeing as I got it again today I took the hint.

The error is horrifingly obvious when you know about it, in short, you've no doubt got some code that looks like this:

String.Format("<html><head><styletype=\"text/css\">body{color: #fff;}</style><body>...");

Can you spot it now? Notice your style declaration is using the curly brackets? Basically String.Format is interpreting that as a placeholder i.e. {0} and is throwing a wobbly.

The solution is simple too, just replace all opening/closing brackets with two i.e:

String.Format("<html><head><styletype=\"text/css\">body{{color: #fff;}}</style><body>...");

I hope that helps someone out there :)

P.S. Watch out for methods that use String.Format as they may catch you out in the same way -i.e. Subject of System.Net.Mail.MailMessage

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