Getting property attributes in TagHelpers

Question

Some of model properties has "Required" data annotation, that I need to read in a TagHelper class.

public partial class Sale
{
    [Required]
    public string CustomerId { get; set; }
    ...

In the sales view I create a custom select for customer:

<customer asp-for="CustomerId " value="@Model.CustomerId"></customer>

And in the CustomerTagHelper class there is the process method:

public override void Process(TagHelperContext context, TagHelperOutput output)
{

How can I discover at this point, if the current bind property has the "required" attribute? I´m using asp-net core.

Answer 1

You can access custom attribute through ModelExpression.

public class CustomTagHelper : TagHelper
{
    [HtmlAttributeName("asp-for")]
    public ModelExpression For { get; set; }

    public override void Process(TagHelperContext context, TagHelperOutput output)
    {            
        CustomAttribute attribute = For.Metadata
                                       .ContainerType
                                       .GetProperty(For.Name)
                                       .GetCustomAttribute(typeof(CustomAttribute)) 
                                       as CustomAttribute;
        if (attribute != null)
        {
            output.Attributes.Add("some-attr", attribute.Text);
        }
    }
}

And then just use it in your template <custom asp-for="SomeProp"></custom>.

Answer 2

The tag helper doesn't know about anything other than what you provide as input for its attributes. So you want to create a tag helper that you can use as follows:

@model WebApplication4.Models.Sale
...
<customer asp-for="CustomerId" />

Then you would declare a property of type ModelSource associated with the asp-for attribute. That would give you access to not just the value of the property but also metadata like the following (and more!):

• property value: source.Model
• property name: source.Name
• container model type: source.Metadata.ContainerType
• IsRequired flag: source.Metadata.IsRequired

You will also get intellisense in VS to select one of properties in your model for the asp-for model and it will throw an error if the value isnt the name of a model property.

---

As an example, take a look at this tag helper:

public class CustomerTagHelper: TagHelper
{
    [HtmlAttributeName("asp-for")]
    public ModelExpression Source { get; set; }

    public override void Process(TagHelperContext context, TagHelperOutput output)
    {
        output.TagName = "p";
        output.TagMode = TagMode.StartTagAndEndTag;

        var contents = $@"
            Model name: {Source.Metadata.ContainerType.FullName}<br/>
            Property name: {Source.Name}<br/>
            Current Value: {Source.Model}<br/> 
            Is Required: {Source.Metadata.IsRequired}";

        output.Content.SetHtmlContent(new HtmlString(contents));
    }
}

Then if you had these 2 models:

public class Sale
{
    [Required]
    public string CustomerId { get; set; }
}
public class Promotion
{        
    public string CustomerId { get; set; }
}

Which are used in these 2 actions and views:

public IActionResult Sale()
{
    return View();
}

@model WebApplication4.Models.Sale
...
<customer asp-for="CustomerId" />


public IActionResult Promotion()
{
    return View(new Models.Promotion { CustomerId = "abc-123" });
}

@model WebApplication4.Models.Promotion
...
<customer asp-for="CustomerId" />

Will produce these outputs:

Tag helper for: WebApplication4.Models.Sale
Property name: CustomerId
Current Value: 
Is Required: True

Model name: WebApplication4.Models.Promotion
Property name: CustomerId
Current Value: abc-123
Is Required: False