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when deciding on which ActionResult to return from a Controller Action I decided to use the ternary operators as opposed to the lengthier if-else. Here is my issue...

this code works

return
    ModelState.IsValid ?
    (ActionResult) RedirectToAction("Edit", new { id = id }) :
    View(new EditViewModel(updatedCategory));

but this doesn't 

return 
     ModelState.IsValid ?
     RedirectToAction("Edit", new { id = id }) :
     View(new EditViewModel(updatedCategory));

I would not have to do the explicit casting if using an if-else. Plus both RedirectToAction() and View() return an ActionResult derivative.

I like the terseness of this code but that casting doesn't seem right. Can anyone enlighten me?

Though I'm sure this is obvious, the EditViewModel is a view model for my Edit action and updatedCategory is an EF4 object. But I don't think this is relevant to the issue.

ok... I just realized what I was doing is unnecessary because regardless I am going back to the Edit action with the updatedCategory, so I don't need to make sure the Model is valid. I am still curious to know the answer to the question if anyone can help.

Derrick W
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2 Answers2

9

I believe it's because the arguments when using the ?: operator have to be convertable between themselves, e.g. in condition ? x : y you need to be able to convert x to y or y to x. Then the type of the result is the least specific of the two. So if x was an object and y a string then you can cast a string to an object and the result would be of type object.

In your example x is a RedirectToRouteResult and y is a ViewResult. You cannot convert a RedirectToRouteResult to a ViewResult or vice versa. You can convert them both to an ActionResult however, which is why if you cast to an ActionResult it works - the type of x is then an ActionResult,y can be converted to an ActionResult and the overall result is of type ActionResult.

Hope I've explained myself correctly there... Afraid I don't know the exact semantics of the ?: operator as I rarely use it myself...

MrKWatkins
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    C# Reference, http://msdn.microsoft.com/en-us/library/ty67wk28.aspx: "...Either the type of first_expression and second_expression must be the same, or an _implicit_ conversion must exist from one type to the other." +1 – Andras Vass Jan 28 '11 at 14:51
  • makes perfect sense. I do appreciate the speedy response. @Andras thanks for the reference. I should have looked more into the C# side of it. I got hung up on the ASP.NET MVC side when clearly this is a language issue rather than a framework issue. – Derrick W Jan 28 '11 at 22:11
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The data types have to be exactly the same on the assignment variable and both return types here is the most simple example I can think of:

int originalValue = 10;
int? value = (originalValue != 10) ? null : originalValue;

//Which is very easily fixed with type casting as you have done

int? value = (originalValue != 10) ? null : (int?)originalValue;
Jessie Lulham
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