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copied!<p>There has been a breaking change/bug in ASP.NET MVC 3 in the <code>System.Web.Mvc.ModelMetadata. FromLambdaExpression</code> method which explains the exception you are getting:</p> <p>ASP.NET MVC 2.0:</p> <pre><code>... case ExpressionType.MemberAccess: { MemberExpression body = (MemberExpression) expression.Body; propertyName = (body.Member is PropertyInfo) ? body.Member.Name : null; containerType = body.Member.DeclaringType; flag = true; break; } ... </code></pre> <p>ASP.NET MVC 3.0</p> <pre><code>... case ExpressionType.MemberAccess: { MemberExpression body = (MemberExpression) expression.Body; propertyName = (body.Member is PropertyInfo) ? body.Member.Name : null; containerType = body.Expression.Type; flag = true; break; } ... </code></pre> <p>Notice how the <code>containerType</code> variable is assigned a different value. So in your case in ASP.NET MVC 2.0 it was assigned the value of <code>IOwned</code> which is the correct declaring type of the <code>AuthorId</code> property whereas in ASP.NET MVC 3.0 it is assigned to <code>IActivity</code> and later when the framework tries to find the property it crashes.</p> <p>That's the cause. As far as the resolution is concerned I would wait for some official statement from Microsoft. I can't find any relevant information about this in the Release Notes document. Is it a bug or some <em>feature</em> which needs to be workarounded here?</p> <p>For now you could either use the non-strongly typed <code>Html.Hidden("AuthorId")</code> helper or specify <code>IOwned</code> as type for your control (I know both suck).</p>

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