Set disable attribute based on a condition for Html.TextBoxFor

Actually, the internal behavior is translating the anonymous object to a dictionary. So what I do in these scenarios is go for a dictionary:

@{
  var htmlAttributes = new Dictionary<string, object>
  {
    { "class" , "form-control"},
    { "placeholder", "Why?" }        
  };
  if (Model.IsDisabled)
  {
    htmlAttributes.Add("disabled", "disabled");
  }
}
@Html.EditorFor(m => m.Description, new { htmlAttributes = htmlAttributes })

Or, as Stephen commented here:

@Html.EditorFor(m => m.Description,
    Model.IsDisabled ? (object)new { disabled = "disabled" } : (object)new { })

One simple approach I have used is conditional rendering:

@(Model.ExpireDate == null ? 
  @Html.TextBoxFor(m => m.ExpireDate, new { @disabled = "disabled" }) : 
  @Html.TextBoxFor(m => m.ExpireDate)
)

I like Darin method. But quick way to solve this,

Html.TextBox("Expiry", null, new { style = "width: 70px;", maxlength = "10", id = "expire-date", disabled = "disabled" }).ToString().Replace("disabled=\"disabled\"", (1 == 2 ? "" : "disabled=\"disabled\""))

The valid way is:

disabled="disabled"

Browsers also might accept disabled="" but I would recommend you the first approach.

Now this being said I would recommend you writing a custom HTML helper in order to encapsulate this disabling functionality into a reusable piece of code:

using System;
using System.Linq.Expressions;
using System.Web;
using System.Web.Mvc;
using System.Web.Mvc.Html;
using System.Web.Routing;

public static class HtmlExtensions
{
    public static IHtmlString MyTextBoxFor<TModel, TProperty>(
        this HtmlHelper<TModel> htmlHelper, 
        Expression<Func<TModel, TProperty>> expression, 
        object htmlAttributes, 
        bool disabled
    )
    {
        var attributes = new RouteValueDictionary(htmlAttributes);
        if (disabled)
        {
            attributes["disabled"] = "disabled";
        }
        return htmlHelper.TextBoxFor(expression, attributes);
    }
}

which you could use like this:

@Html.MyTextBoxFor(
    model => model.ExpireDate, 
    new { 
        style = "width: 70px;", 
        maxlength = "10", 
        id = "expire-date" 
    }, 
    Model.ExpireDate == null
)

---

and you could bring even more intelligence into this helper:

public static class HtmlExtensions
{
    public static IHtmlString MyTextBoxFor<TModel, TProperty>(
        this HtmlHelper<TModel> htmlHelper,
        Expression<Func<TModel, TProperty>> expression,
        object htmlAttributes
    )
    {
        var attributes = new RouteValueDictionary(htmlAttributes);
        var metaData = ModelMetadata.FromLambdaExpression(expression, htmlHelper.ViewData);
        if (metaData.Model == null)
        {
            attributes["disabled"] = "disabled";
        }
        return htmlHelper.TextBoxFor(expression, attributes);
    }
}

so that now you no longer need to specify the disabled condition:

@Html.MyTextBoxFor(
    model => model.ExpireDate, 
    new { 
        style = "width: 70px;", 
        maxlength = "10", 
        id = "expire-date" 
    }
)