The name '__o' does not exist in the current context

After reading the links given in the comments above, it turns out to be how intellisense handles if blocks.

Mikhail Arkhipov posted an 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.

Noting above, Failure's answer doesn't actually do much harm, other than hiding all intellisense error, which would be known anyway at build time.

Reference: http://youku.io/questions/324366/asp-net-mvc-error-name-o-is-not-declared https://msdn.microsoft.com/en-us/library/t8zbaa6f.aspx

Declare variable __o in code, like this:

public object __o;

Be sure to put it in Master page (if you are using one).

I found out that if I choose Build Only instead of Build + IntelliSense the errors (that are related to IntelliSense) will go away.

Update 1: The Reason

The reason that this is happening is that for codes 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.

Update 2: Getting rid of this error without losing other IntelliSense errors

Click on the filter button on the top left corner of the error list panel and uncheck the CS0103 which the error code for the: The name '__o' does not exist in the current context and these errors will not be shown anymore and you can still have other IntelliSense errors and warnings: