Conditional attribute rendering: null value attributes are sometimes unexpectedly rendered

[tormnator]

This issue is about conditional attribute rendering where sometimes null value attributes are unexpectedly rendered. I stumbled upon this today during work in a asp.net core mvc 7 application (and VS 2022), and it took me a while to nail down the behavior. I haven't found any documentation or issues stating whether this is a bug, as designed, or otherwise.

When rendering something like <div @(something) style="@null">, then you would expect the style attribute not to be output in the final html. But in my testing that is not the case. But, if you place the @(something) behind the style attribute, then the style attribute is omitted as expected. Below is a more thorough test:

@{
  var aString = new HtmlString("string");
  var aStringClassEq = "class=\"string\"";
  var anHtmlString = new HtmlString("HtmlString");
  var anHtmlStringClassEq = new HtmlString("class=\"HtmlString\"");

  // 1. Regular strings:
  <div name="works-1" class="@aString" style="@null">Styles</div>
  <div name="works-1b" class="@Html.Raw(aString)" style="@null">Styles</div>
  <div name="fails-1a" @aStringClassEq style="@null">Styles</div>
  <div name="fails-1b" @Html.Raw(aStringClassEq) style="@null">Styles</div>

  // 2. HtmlString:
  <div name="works-2a" class="@anHtmlString" style="@null">Styles</div>
  <div name="works-2b" class="@Html.Raw(anHtmlString)" style="@null">Styles</div>
  <div name="fails-2a" @anHtmlStringClassEq style="@null">Styles</div>
  <div name="fails-2b" @Html.Raw(anHtmlStringClassEq) style="@null">Styles</div>

  <div name="works-3" htmlString style="@null">Styles</div>
  <div name="fails-3b" @aString style="@null">Styles</div>
  <div name="fails-3a" @anHtmlString style="@null">Styles</div>

  @* Placing an attribute with null value *first* always works: *@
  <div name="works-4a" style="@null" @aStringClassEq>Styles</div>
  <div name="fails-4b" style="@null" @Html.Raw(aStringClassEq)>Styles</div>
  <div name="fails-4c" style="@null" @anHtmlStringClassEq>Styles</div>
  <div name="fails-4d" style="@null" @Html.Raw(anHtmlStringClassEq)>Styles</div>
  <div name="works-4e" style="@null" @aString>Styles</div>
  <div name="works-4f" style="@null" @anHtmlString>Styles</div>
}

I expected all the div elements with name="fails-*" to not output the empty styles attribute, but here's the actual output:

  <div name="works-1" class="string">Styles</div>
  <div name="works-1b" class="string">Styles</div>
  <div name="fails-1a" class=&quot;string&quot; style="">Styles</div>
  <div name="fails-1b" class="string" style="">Styles</div>
  <div name="works-2a" class="HtmlString">Styles</div>
  <div name="works-2b" class="HtmlString">Styles</div>
  <div name="fails-2a" class="HtmlString" style="">Styles</div>
  <div name="fails-2b" class="HtmlString" style="">Styles</div>
  <div name="works-3" htmlString>Styles</div>
  <div name="fails-3b" string style="">Styles</div>
  <div name="fails-3a" HtmlString style="">Styles</div>
  <div name="works-4a" class=&quot;string&quot;>Styles</div>
  <div name="works-4b" class="string">Styles</div>
  <div name="works-4c" class="HtmlString">Styles</div>
  <div name="works-4d" class="HtmlString">Styles</div>
  <div name="works-4e" string>Styles</div>
  <div name="works-4f" HtmlString>Styles</div>

[chsienki]

@jjonescz Please take a look, thanks