What does DisplayNameFor do?

Question

I'm learning .NET Core with Razor pages, using one of the official tutorials here, and I'm having trouble with this code: @Html.DisplayNameFor(model => model.Movie[0].Title)

The tutorial says:

The DisplayNameExtensions.DisplayNameFor HTML Helper inspects the Title property referenced in the lambda expression to determine the display name. The lambda expression is inspected rather than evaluated. That means there is no access violation when model, model.Movie, or model.Movie[0] is null or empty. When the lambda expression is evaluated, for example, with @Html.DisplayFor(modelItem => item.Title), the model's property values are evaluated.

Which I can't make heads or tails of. What does inspect mean here? Does it mean the lambda function runs in an try/catch, to prevent the access violation errors that the docs speak of? What does it mean exactly?

And in what important way is the second example (with DisplayFor) different? It uses DisplayFor instead of DisplayName, and another change is that it uses ModelItem instead of model. I don't know where it would get ModelItem from, model is made available by @model (...) at the op of the razor page but how ModelItem gets here is not clear to me.

The docs for DisplayNameForare here, but the tutorial links to the non-core docs here, both of which are too terse for me to make much sense of.

Answer 1

DisplayNameFor will look for the Name property of the Display property attribute and print it to your razor page (or the property name itself, if it can't find it). So, if your model is

class Foo
{
    [Display(Name="My name")]
    public string Prop1 { get; set; }

    public string Prop2 { get; set; }
}

It will display My name for Prop1 and Prop2 for Prop2.

The part about it being inspected rather than evaluated, means that it will look at your model definition, it will not try to get a value, so if the model value is null, it will not throw.

On the other hand, DisplayFor will print the value of the selected property, applying any format described by DisplayFormatAttribute, so the model is evaluated and cannot be null.

Typically you will use DisplayNameFor to build the table headers and DisplayFor to build the table data.