Errore MVC ASP.NET: "Nome '__o' non dichiarato."

Question

Ho un sacco di questi errori sul mio file View aspx.

Ma, si costruisce e funziona bene.

Cosa sono? Come faccio a sbarazzarmene?

Answer 1

Mikhail Arkhipov pubblicato in explanation and workaround in the ASP.NET forums:

We have finally obtained reliable repro and identified the underlying issue. A trivial repro looks like this:

<% if (true) { %> 
    <%=1%> 
<% } %> 
<%=2%> 

In order to provide intellisense in <%= %> blocks at design time, ASP.NET generates assignment to a temporary __o variable and language (VB or C#) then provide the intellisense for the variable. That is done when page compiler sees the first <%= ... %> block. But here, the block is inside the if, so after the if closes, the variable goes out of scope. We end up generating something like this:

if (true) { 
    object @__o; 
    @__o = 1; 
} 
@__o = 2; 

The workaround is to add a dummy expression early in the page. E.g. <%="" %>. This will not render anything, and it will make sure that __o is declared top level in the Render method, before any potential ‘if’ (or other scoping) statement.