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

Question

I had the same problem with something like

@foreach (var item in Model)
{
    @Html.DisplayFor(m => !item.IsIdle, "BoolIcon")
}

I solved this just by doing

@foreach (var item in Model)
{
    var active = !item.IsIdle;
    @Html.DisplayFor(m => active , "BoolIcon")
}

When you know the trick, it's simple.

The difference is that, in the first case, I passed a method as a parameter whereas in the second case, it's an expression.

The template it is referring to is the Html helper DisplayFor.

DisplayFor expects to be given an expression that conforms to the rules as specified in the error message.

You are trying to pass in a method chain to be executed and it doesn't like it.

This is a perfect example of where the MVVM (Model-View-ViewModel) pattern comes in handy.

You could wrap up your Trainer model class in another class called TrainerViewModel that could work something like this:

class TrainerViewModel
{
    private Trainer _trainer;

    public string ShortDescription
    {
        get
        {
            return _trainer.Description.ToString().Substring(0, 100);
        }
    }

    public TrainerViewModel(Trainer trainer)
    {
        _trainer = trainer;
    }
}

You would modify your view model class to contain all the properties needed to display that data in the view, hence the name ViewModel.

Then you would modify your controller to return a TrainerViewModel object rather than a Trainer object and change your model type declaration in your view file to TrainerViewModel too.

I ran into a similar problem with the same error message using following code:

@Html.DisplayFor(model => model.EndDate.Value.ToShortDateString())

I found a good answer here

Turns out you can decorate the property in your model with a displayformat then apply a dataformatstring.

Be sure to import the following lib into your model:

using System.ComponentModel.DataAnnotations;

The ...For extension methods on the HtmlHelper (e.g., DisplayFor, TextBoxFor, ElementFor, etc...) take a property and nothing else. If you don't have a property, use the non-For method (e.g., Display, TextBox, Element, etc...).

The ...For extension methods provides a way of simplifying postback by naming the control after the property. This is why it takes an expression and not simply a value. If you are not interested in this postback facilitation then do not use the ...For methods at all.

Note: You should not be doing things like calling ToString inside the view. This should be done inside the view model. I realize that a lot of demo projects put domain objects straight into the view. In my experience, this rarely works because it assumes that you do not want any formatting on the data in the domain entity. Best practice is to create a view model that wraps the entity into something that can be directly consumed by the view. Most of the properties in this view model should be strings that are already formatted or data for which you have element or display templates created.