Question

I have an HtmlHelper like this: (simplified for clarity)

public static MvcHtmlString MyHelper<TModel>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, object>> expressionId)
{
    // do something here

    tagBuilder.Append(htmlHelper.HiddenFor(expressionId));

    // do something here
}

What happens is that the code works fine when expressionId returns string. But when it returns int, Html.HiddenFor trigger this error:

Templates can be used only with field access, property access, single-dimension array index, or single-parameter custom indexer expressions.

I don’t know exactly what the problem is, but I suspect it has something to do with this:
When I inspect expressionId in debug mode, I can see the expression is: m => Convert(m.Id) instead of the expected m => m.Id.

What bugs me is that, MyHelper is receiving the exact same argument type as HiddenFor. So why can’t I repass that argument?

What should I do?

PS

I’m not passing any nullable expression into MyHelper (I’ve seen some other questions in which this was the answer)

Answer 1

The problem is with the cast expression of the non-nullable type.

Try using a strongly typed helper instead of object:

public static MvcHtmlString MyHelper<TModel, TProperty>(
    this HtmlHelper<TModel> htmlHelper, 
    Expression<Func<TModel, TProperty>> expressionId
)
{
    // do something here

    tagBuilder.Append(htmlHelper.HiddenFor(expressionId));

    // do something here
}