Template error when refining a lambda expression for use in MVC InputFor

Question

In my application I have several (>10) places where I have a model like this:

public interface IOptionList
{
    string Name;
    bool Checked;
}

public class Option : IOptionList { }

public class MyModel
{
    public Option[] Options = 
    {
        new Option { Name = "Option 1", Checked = false }
    };

    // etc... for many more implementations of IOptionList
}

These are used to generate CheckboxLists in views like so:

@for (int i = 0; i < Model.Options.Length; i++)
{
    <div>
        @Html.CheckBoxFor(x => x.Options[i].Checked)
        @Html.LabelFor(x => x.Options[i].Checked, Model.Options[i].Name)
    </div>
}

Because this is used so many times I'd like to simplify my models by writing an HtmlHelper extension to generate the lists like this:

@Html.CheckBoxFormGroupFor(x => x.Options, Model.Options)

and my current attempt looks like this:

public static CheckBoxFormGroupFor<TModel, TItem>(this HtmlHelper<TModel>html,
    Expression<Func<TModel, TItem[]>> expression, TItem[] values)
{
    var sb = new StringBuilder();

    for (int i = 0; i < values.Length; i++)
    {
        var indexExpression = Expression.ArrayIndex(expression.Body, Expression.Constant(i));
        var checkedAccessExpression = Expression.Property(indexExpression, typeof(ICheckboxList), "Checked");
        var lambda = Expression.Lambda<Func<TModel, bool>>(checkedAccessExpression, Expression.Parameter(typeof(TModel)));
        var cbx = html.CheckBoxFor(x => lambda.Compile()(x));
        var lbl = html.LabelFor(x => lambda.Compile()(x), values[i].Name);

        var div = new TagBuilder("div");
        div.InnerHtml = $"{cbx}{lbl}";
        sb.Append(div);
    }
    return new HtmlString(sb.ToString());
}

which as I understand extends the initial lambda expression x => x.Options to access the correct property of the object at the correct array index, however this gives me a template error with the message

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

As far as I can see I'm only doing a single-D array index followed by property access so I'm not sure why I'm seeing this. I'd previously tried var cbx = html.CheckBoxFor(lambda); but this doesn't work as the parameter x is only defined in the scope of the view.

Assuming what I want is even possible can anyone help with how to achieve it? I'm new to manipulating Expressions in this way.

Answer 1

You are close. Using directly the lambda variable

var cbx = html.CheckBoxFor(lambda);
var lbl = html.LabelFor(lambda, values[i].Name);

is indeed the right way. Just make sure the lambda expression your are composing uses the same parameter as the expression argument (since it's bound to the body used in the new expression), i.e. here

var lambda = Expression.Lambda<Func<TModel, bool>>(
    checkedAccessExpression,
    Expression.Parameter(typeof(TModel)));

replace Expression.Parameter(typeof(TModel) with expression.Parameters[0]:

var lambda = Expression.Lambda<Func<TModel, bool>>(
    checkedAccessExpression,
    expression.Parameters[0]));