Why doesn't this string.Format() return string, but dynamic?

Question

@{
    ViewBag.Username = "Charlie Brown";
    string title1 = string.Format("Welcome {0}", ViewBag.Username);
    var title2 = string.Format("Welcome {0}", ViewBag.Username);
}

In the MVC view I use the values like this:

@Html.ActionLink(title1, "Index")
@Html.ActionLink(title2, "Index")

Here, the title1 works fine. But the title2 ActionLink failed with a compiler error:

CS1973: 'System.Web.Mvc.HtmlHelper' has no applicable method named 'StandardHeader' but appears to have an extension method by that name. Extension methods cannot be dynamically dispatched. Consider casting the dynamic arguments or calling the extension method without the extension method syntax.

string.Format() has quite a few overloads, but the return type is always string. Why does the variable declaration using var fail here?

I doubt that you got that error message from the code you've given. I suspect it's a different bit of code. — Jon Skeet
– Jon Skeet, Commented Feb 12, 2016 at 15:49

camden_kid · Accepted Answer · 2016-02-12 20:23:24Z

16

Okay, so we already know from comments and other answers that the problem is in dynamic. Since dynamic is bound on runtime, only at that time is the overload resolution and type validating is done.

So: if at least one of the parameters is dynamic, the overload resolution is done at runtime.

That is why this obvious mistake is allowed:

dynamic x = "";
int i = string.Format("{0}", x);

It doesn't bother if there isn't a string.Format overload that returns an int. It evaluates that later on.

edited Feb 12, 2016 at 20:23

camden_kid

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answered Feb 12, 2016 at 15:55

Patrick Hofman

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9 Comments

Servy Over a year ago

All expressions are evaluated at runtime, with the occasional exception of some operations on compile time literals. This is not specific to dynamic. What's deferred here is overload resolution and the the type of the expression is determined at runtime. That's different.

Patrick Hofman Over a year ago

Okay, didn't know that. Thanks @Servy

Blaise Over a year ago

Cool. That explains why. Thanks.

Patrick Hofman Over a year ago

@Servy There is no overload resolution with int i = x; but it still compiles. Any thoughts on that? (From your comment I understood it shouldn't work, or did I read wrong?)

Servy Over a year ago

@PatrickHofman dynamic is implicitly convertible to any type.

|

Servy · Accepted Answer · 2016-02-12 15:49:02Z

11

The error message is telling you exactly what's wrong here:

Extension methods cannot be dynamically dispatched. Consider casting the dynamic arguments or calling the extension method without the extension method syntax.

title2 is of type dynamic. You need to cast it to string, since you know that's what it is.

answered Feb 12, 2016 at 15:49

Servy

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10 Comments

CodeCaster Over a year ago

But string.Format() doesn't return dynamic.

Lee Over a year ago

@David - It will be dynamic if ViewBag.Username is dynamic. dynamic applies to the entire expression.

Servy Over a year ago

@CodeCaster Every method can return dynamic if any of its arguments is dynamic.

Jon Skeet Over a year ago

It would be worth explaining why the type of title2 is dynamic here. The explanation Lee gave is the crucial bit of this question, IMO.

Jon Skeet Over a year ago

@Servy: Nit pick: if the argument is dynamic. Making the parameter type dynamic has no effect.

|

Anwar Ul-haq · Accepted Answer · 2016-02-12 16:00:53Z

7

It's the view bag which is dynamic.

If you use actual username (instead of ViewBag.UserName)it would work. Or cast (string)ViewBag.Username into string.

answered Feb 12, 2016 at 16:00

Anwar Ul-haq

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