Question

I’m working with the System.Web.Helpers.WebGrid in an ASP.NET MVC 3 Razor project, and I’m having trouble understanding why the format parameter for a WebGridColumn is a Func<dynamic, object>.

If I create a column like this…

grid.Column(
    format: x => string.Format("{0:d}", x.StartDate)
    );

…I don’t get strong typing on the StartDate property. If I try to get around it like this…

grid.Column(
    format: (MyObjectType x) => string.Format("{0:d}", x.StartDate)
    );

…I’m told at runtime that my lambda can’t be cast to a Func<dynamic, object>. Is there some way I can use a non-dynamic lambda here? Even if it’s just <object, object>?

(I’m in .NET 4.0, and Func<in T, out TResult> is supposed to be contravariant on T, but I’m confused about how covariance and contravariance work with dynamic.)

Answer 1

As far as the type system is concerned, dynamic is the same as object.

They cannot use a strongly-typed delegate because they don’t have a strongly-typed value to pass.

Inside WebGrid, they get an Object from a PropertyDescriptor, and pass that to your delegate.

Covariance won’t help here; had Func<YourType, string> been convertible to Func<object, string>, it would be possible to call it with any other type and get an invalid cast.