ASP.NET MVC 2: Where is ModelState.AddUnhandledError Defined?

I've seen ModelState.AddUnhandledError in a few开发者_如何学Python code samples, but it doesn't seem to be part of the ModelStateDictionary class. Presumably it's an extension method?

Is this a standard part of MVC? Anyone know where this is defined?

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No, this is not part of the standard MVC. Presumably it's a custom defined extension method.