Add a post on required HTML attributes in ASP.NET Core

src/posts/RequiredHtmlAttributeAspNet.md

@@ -0,0 +1,51 @@
+---
+title: Add Required HTML Attribute In ASP.NET Core
+description: A tag helper that adds the required attribute to inputs for properties marked as required
+tags: [ Programming, C#, ASP.NET Core ]
+---
+
+By default using the input `asp-for` tag helper on `input`s will not add the `required` attribute for properties marked with `[Required]`. It instead relies on jQuery Validation attributes. This can be a problem if you don't use jQuery and for accessibility.
+
+This tag helper will run alongside the default `asp-for` helper and will add the `required` attribute if the matching property is marked `[Required]`. This will also add the `required` attribute for non-nullable types. It will not add the attribute to checkboxes, as that indicates the checkbox must be checked.
+
+```csharp
+using Microsoft.AspNetCore.Mvc.ViewFeatures;
+using Microsoft.AspNetCore.Razor.TagHelpers;
+using System.ComponentModel.DataAnnotations;
+
+/// <summary>
+/// The default input and select tag helpers don't add the <c>required</c> attribute to required
+/// elements, so this does it instead
+/// </summary>
+[HtmlTargetElement("input", Attributes = "asp-for")]
+[HtmlTargetElement("select", Attributes = "asp-for")]
+[HtmlTargetElement("textarea", Attributes = "asp-for")]
+public class RequiredInputTagHelper : TagHelper
+{
+	public override int Order { get => int.MaxValue; }
+
+	[HtmlAttributeName("asp-for")]
+	public ModelExpression For { get; set; }
+
+	public override void Process(TagHelperContext context, TagHelperOutput output)
+	{
+		base.Process(context, output);
+
+		// Don't do anything if there's already a required attribute
+		if (context.AllAttributes["required"] is not null)
+			return;
+
+		// Don't make check boxes required, that indicates that it must be checked
+		if (this.For.Model is bool)
+			return;
+
+		// The property has [Required] or it is a value type (e.g. int) and is not nullable (e.g. int?)
+		if (this.For.ModelExplorer.Metadata.ValidatorMetadata.Any(a => a is RequiredAttribute)
+			|| (this.For.ModelExplorer.ModelType.IsValueType
+				&& (!this.For.ModelExplorer.ModelType.IsGenericType
+					|| (this.For.ModelExplorer.ModelType.IsGenericType
+						&& this.For.ModelExplorer.ModelType.GetGenericTypeDefinition() != typeof(Nullable<>)))))
+			output.Attributes.Add(new TagHelperAttribute("required"));
+	}
+}
+```