ASP.NET MVC Core, How to add a attribute to a input based on the DataType annotation?

Question

How can I add a attribute to a <input /> tag based on the DataType using razor.

For instance if my data does have annotation

[DataType(DataType.Currency)] 
public float Amount {get; set;}

Then I would like to have <input name="Amount" class="currency" > when using the tag helper <input asp-for="Amount " />

You would need to write you own customTagHelper which uses reflection to read the value of the DataType enum to add the class — user3559349
– user3559349, Commented Feb 4, 2018 at 21:54
i created a nuget to help with such things. you can check this: netcoresimple.azurewebsites.net/docs/Generator You can use a generator and create a custom razor template. The template is strongly typed and you can easily access the datatype — Neville Nazerane
– Neville Nazerane, Commented Feb 4, 2018 at 23:10

Munir Husseini · Accepted Answer · 2018-03-05 13:54:33Z

As Stephen Muecke suggested, you'll need to write a TagHelper for this. The TagHelper architecture will provide you with all you need.

using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc.ModelBinding.Metadata;
using Microsoft.AspNetCore.Mvc.ViewFeatures;
using Microsoft.AspNetCore.Razor.TagHelpers;

namespace AnnotationBasedAttributes
{
    [HtmlTargetElement("input")]
    public class MyAwesomeTagHelper : TagHelper
    {
        public ModelExpression AspFor { get; set; }

        public override Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
        {
            if (!(this.AspFor.Metadata is DefaultModelMetadata modelMetadata))
            {
                return Task.CompletedTask;
            }

            var dataTypeAttribute = modelMetadata.Attributes.Attributes.OfType<DataTypeAttribute>().FirstOrDefault();
            if (dataTypeAttribute == null)
            {
                return Task.CompletedTask;
            }

            var type = dataTypeAttribute.DataType.ToString().ToLower();
            output.Attributes.Add(new TagHelperAttribute("class", type));

            return Task.CompletedTask;
        }
    }
}

You don't need to do anything special in your view. Just use the input tag helper as usual:

@model IndexModel

<label asp-for="Amount"></label>
<input asp-for="Amount" />

Just don't forget to tell your app to search for TagHelpers in your assembly. Do this in _ViewImports.cshtml:

@addTagHelper *, MyWebApplication

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