.NET CORE MvcHtmlString(TextboxFor, LabelFor).ToString() Error

问题

Hello i must convert MvcHtmlString to string. I was using before this code.(.net framework)

TagBuilder div = new TagBuilder("div");
div.MergeAttribute("class", "form-group");
var label = helper.LabelFor(expression, new { @class = "control-label col-lg-1" });
div.InnerHtml += label.ToString();

Now I am developing .net core and I get error at

div.InnerHtml += label.ToString();

Error Image

Returns a string that represent the current object. Reference to type 'HtmlString' claims it is defined in 'System.Web', but it could not be found. Module 'System.Web, Version=4.0.0.0, Culture=neutral, PublicyToken=b03f5f7f11d50a3a should be referenced

Please help me. I must use this method or alternative. I am trying build a form.

回答1:

This line below causing error because TagBuilder.InnerHtml is a read only property and you can't use += operator to assign HTML string:

div.InnerHtml += label.ToString();

What you should do is using AppendHtml() against existing LabelFor helper:

div.InnerHtml.AppendHtml(label);

Take note that .NET Core MVC doesn't use System.Web namespace, it uses Microsoft.AspNetCore parent namespace instead. You should try IHtmlContent to build your own custom helper instead of MvcHtmlString, by following these steps:

1) Include all using statements provided below to enable IHtmlContent and IHtmlHelper instance.

using Microsoft.AspNetCore.Html;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.AspNetCore.Mvc.ViewFeatures;

2) Create your custom helper using Microsoft.AspNetCore.Mvc.Rendering.IHtmlHelper as parameter instead of System.Web.Mvc.HtmlHelper, shown in example below:

public static IHtmlContent CustomLabelFor<TModel, TProperty>(this IHtmlHelper<TModel> helper, Expression<Func<TModel, TProperty>> expression, object htmlAttributes)
{
    string result;

    TagBuilder div = new TagBuilder("div");
    div.MergeAttribute("class", "form-group");
    var label = helper.LabelFor(expression, new { @class = "control-label col-lg-1" });
    div.InnerHtml.AppendHtml(label);

    using (var sw = new System.IO.StringWriter())
    {
        div.WriteTo(sw, System.Text.Encodings.Web.HtmlEncoder.Default);
        result = sw.ToString();
    }

    return new HtmlString(result);
}

Note: This example helper already tested in VS 2017, .NET Core 2.1.

Related issue:

Create Custom HTML Helper in ASP.Net Core

回答2:

Have you tried this ?:

div.InnerHtml += label.ToHtmlString(); 

来源：https://stackoverflow.com/questions/53413899/net-core-mvchtmlstringtextboxfor-labelfor-tostring-error