Add CSS Class to html element in a TagHelper

Question

In an ASP.NET Core View I have the following:

<div class="message" message=""></div>

And I the a Tag Helper with the following TagHelper:

[HtmlTargetElement("div", Attributes = MessageName)]
public class MessageTagHelper : TagHelper {

  private const String MessageName = "message";

  [HtmlAttributeName(MessageName)]
  public String Message { get; set; }

  [HtmlAttributeNotBound, ViewContext]
  public ViewContext ViewContext { get; set; }


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

    String message;
    Boolean defined = ViewContext.HttpContext.Request.Cookies.TryGetValue("_message", out message);

    if (defined) {
      ViewContext.HttpContext.Response.Cookies.Delete("_message");
      output.Attributes.Add("class", "active");
      output.Content.Append(message);
    }
  }
}

On the following code line I need to add "active" as CSS Class.

output.Attributes.Add("class", "active");

However, this does nothing. Message remains only with class "message".

What am I missing?

Alvord

Answer 1

As of ASP.NET Core 2.1 you can use the TagHelperOutputExtensions class provided by Microsoft to add your "active" CSS class in one line of code.

using Microsoft.AspNetCore.Mvc.TagHelpers;
using System.Text.Encodings.Web;
...
public override void Process(TagHelperContext context, TagHelperOutput output)
{
    ...
    output.AddClass("active", HtmlEncoder.Default);
    ...
}

Lorraine

Answer 2

@Shyju's answer is mostly correct, but there's a lot of extraneous code that's unnecessary. The following is enough to handle it in ASP.NET Core 2.0:

var classes = output.Attributes.FirstOrDefault(a => a.Name == "class")?.Value;
output.Attributes.SetAttribute("class", $"active {classes}");

Pillar

good! but this doesn't check for duplicate values if the class already contains new values to add. So, I post a more complete version here – Lewls

Answer 3

I think you should remove the existing TagHelperAttribute for css class from the Attributes collection and add a new one which has all the classes (Existing and your new "active" class)

The below code should work.

var existingClass = output.Attributes.FirstOrDefault(f => f.Name == "class");
var cssClass = string.Empty;
if (existingClass != null)
{
    cssClass = existingClass.Value.ToString();       
}
cssClass = cssClass + " active";
var ta = new TagHelperAttribute("class", cssClass);
output.Attributes.Remove(existingClass);
output.Attributes.Add(ta);

Clearness

FYI output.Attributes.Remove(existingClass); should be located inside the if statement, otherwise it's trying to remove a null value. – Boson

existingClass.Value.ToString() won't work because existingClass.Value is of type Microsoft.AspNetCore.Mvc.TagHelpers.TagHelperOutputExtensions.ClassAttributeHtmlContent class. – Rooseveltroost

To be precise it will work if there is only one class name. But not for 2 or more, for example <div class="message text" ></div> won't return classes in ToString(). – Rooseveltroost

My example above with <div../> is not correct. Actually method .ToString() works fine if there were no tag-helpers which added some extra classes via output. Attributes.Add("class", ...) before our tag-helper is invoked. If they did then method .ToString() won't return expected names of css classes. It will return "ClassAttributeHtml‌​Content" C# class name instead in ASP.NET Core 2. – Rooseveltroost

yea. There was no built in support for multiple classes. There was a new extension method added (AddClasses) to handle this. I will update the answer with that when i play around with it later. – Clearness

Boy oh boy is this UGLY that we have to do it this way. I will be writing an extension method to greatly alleviate this pain, but I wonder why the team can't see the Need for Simplicity® here. – Buell

Answer 4

The other answers here does not check current css class to avoid duplicate values. Here, I wrote an extension to add a css class with proper checks to ensure clean html at output:

public static class GeneralExtionsions
{
    //the white space chars valid as separators between every two css class names
    static readonly char[] spaceChars = new char[] { ' ', '\t', '\r', '\n', '\f' };

    /// <summary> Adds or updates the specified css class to list of classes of this TagHelperOutput.</summary>
    public static void AddCssClass(this TagHelperOutput output, string newClass)
    {
        //get current class value:
        string curClass = output.Attributes["class"]?.Value?.ToString(); //output.Attributes.FirstOrDefault(a => a.Name == "class")?.Value?.ToString();

        //check if newClass is null or equal to current class, nothing to do
        if (string.IsNullOrWhiteSpace(newClass) || string.Equals(curClass, newClass, StringComparison.OrdinalIgnoreCase))
        {
            return;
        }

        //append newClass to end of curClass if curClass is not null and does not already contain newClass:
        if (!string.IsNullOrWhiteSpace(curClass) 
            && curClass.Split(spaceChars, StringSplitOptions.RemoveEmptyEntries).Contains(newClass, StringComparer.OrdinalIgnoreCase)
            )
        {
            newClass = $"{curClass} {newClass}";
        }

        //set new css class value:
        output.Attributes.SetAttribute("class", newClass);

    }

}

Lewls