MVC Razor: How to reference helper parameter in Razor markup?

Question

I am wanting to pass a template parameter to my helper method for rendering errors. I have seen several examples where this is done but appear to require that the variables in the template are in scope when the helper is invoked.

e.g. Expression of HelperResult to format item from a list

I am hoping to do something like:

    public static MvcHtmlString ErrorBlock<TModel>(this HtmlHelper helper, TModel model, string @class = null, object context = null, string view = null, object attributes = null, Func<ErrorModel,HelperResult> errorTemplate = null)
        where TModel : ErrorModel

...

@ShopMR.ErrorBlock(Model, errorTemplate: r => @<div>@r.Message</div>)

But I get the following compiler errors:

Cannot convert lambda expression to intended delegate type because some of the return types in the block are not implicitly convertible to the delegate return type
Cannot convert lambda expression to type 'HelperResult' because it is not a delegate type

I have tried creating a delegate but it results in the same error. Is this possible? Should my func return some other type that can be compiled/evaluated as Razor text?

Sam.C · Accepted Answer · 2016-08-29 18:33:03Z

1

To get rid of the compilation errors change the rasor to this :

@ShopMR.ErrorBlock(Model, errorTemplate:r => new HelperResult( x => { x.WriteLine($"<div>r.Message</div>" ); } )

The errorTemplate was expecting a lambda expression that returns HelperResult Template.

answered Aug 29, 2016 at 18:33

Sam.C

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1 Comment

Kristian Oye Over a year ago

That fixed the issue although I was hoping there was a cooler solution that would still allow me to leverage compile-time checking. Thank you so much!

Kristian Oye · Accepted Answer · 2016-08-29 19:18:27Z

After blundering around I came up with something I liked a little better (although I am sure there is a cleaner solution)

    public static MvcHtmlString ErrorBlock<TModel>(this HtmlHelper helper, TModel model, 
        string @class = null, object context = null, string view = null, object attributes = null, 
        Func<ErrorModel, Func<ErrorModel,IHtmlString>> errorTemplate = null)
        where TModel : BaseModel
    {
    ...
        if (errorTemplate != null)
        {
            var formattedErrors = errors.Select(e => errorTemplate?.Invoke(e)?.Invoke(e)?.ToHtmlString() ?? string.Empty);
            tb.InnerHtml = string.Join("", formattedErrors);
        }
    }
    ....

This allowed me to use my intended syntax in calling the extension:

@ShopMR.ErrorBlock(Model, errorTemplate: r => @<div>@r.Message</div>)

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