ASP.NET MVC: Custom Html Helpers in Razor

Question

I am having difficulty with Html Helpers when used with Razor. Said helpers worked fine in MVC 2 with the web form view engine. But not in razor. The error I get at runtime is:

Compiler Error Message: CS1502: The best overloaded method match for 'System.Web.WebPages.WebPageExecutingBase.Write(System.Web.WebPages.HelperResult)' has some invalid arguments  Source Error:   Line 1:  @using Wingspan.Web.Mvc; Line 2:  @Html.IncrementalMenu(MenuBlock.Site) 

Expanding the Show Detailed Compiler Output reveals:

d:\...\Views\Shared\MenuTop.cshtml(2,1): error CS1502: The best overloaded method match for 'System.Web.WebPages.WebPageExecutingBase.Write(System.Web.WebPages.HelperResult)' has some invalid arguments d:\...\Views\Shared\MenuTop.cshtml(2,7): error CS1503: Argument 1: cannot convert from 'void' to 'System.Web.WebPages.HelperResult' 

That indicates to me that razor doesn't like my helper, IncrementalMenu, returning void (which works fine in MVC 2 web form engine views).

I get no errors at Compile time, although the line of code (@Html.IncrementalMenu(...)) is red underlined with the following message:

Cannot implicitly convert type 'void' to 'object' 

IncrementalMenu is in the Wingspan.Web.Mvc namespace. It's signature is as follows:

public static void IncrementalMenu(this HtmlHelper html, MenuBlock menuBlock) {     // Uses an HtmlTextWriter to render a menu from the sitemap } 

I'm blowed if I know what is wrong...

PS:

The MenuBlock parameter is just an enum that identifies how the menu should render. Don't fixate on this as that is fine.

Answer 1

You can call your helper like this:

@{ Html.IncrementalMenu(MenuBlock.Site); } 

WebForms syntax

<% Html.IncrementalMenu(MenuBlock.Site); %> 

You just call your method, and the return value (if there is any) is ignored.

Code like this expects a return value, and writes the return value to the html stream:

@Html.YourHelper() 

Webforms syntax:

<%: Html.YourHelper() %> 

The same, if result value != IHtmlString:

<%= Server.HtmlEncode(Html.YourHelper()) %>