Code generated HTML attributes

Razor is great when it comes to conditional HTML output, or outputting HTML in a for-loop, but when it comes to conditional code within the element itself things are a bit more tricky. For example , the following code does not compile because you cannot add C# control blocks inside the < and > of an element.

<img
  @foreach(var nameAndValue in AdditionalAttributes)
  {
    @nameAndValue.Key = @nameAndValue.Value
  } 
  src="https://randomuser.me/api/portraits/lego/1.jpg" />

@code
{
  Dictionary<string, object> AdditionalAttributes;

  protected override void OnInitialized()
  {
    AdditionalAttributes = new Dictionary<string, object>
    {
      ["id"] = "EmmetImage",
      ["alt"] = "A photo of Emmet"
    };
    base.OnInitialized();
  }
}

The next approach we might attempt is to write a method that returns a string and call that inside the < and > characters.

<div @IfYouCanSeeThisTextThenTheCodeWasNotExecutedHere />
<span>@IfYouCanSeeThisTextThenTheCodeWasNotExecutedHere</span>

@code
{
  string IfYouCanSeeThisTextThenTheCodeWasNotExecutedHere = "The code here was executed";
}

But this doesn't work either. The preceding example would output the following HTML.

<div @ifyoucanseethistextthenthecodewasnotexecutedhere=""></div>
<span>The code here was executed</span>

Razor will only execute C# code in the following places:

1. Inside an element's content area, for example <span>@GetSomeHtml()</span>.
2. When determining a value to assign into an element's attribute, for example <img src=@GetTheImageForTheUrl() />.
3. Within the @code section.

The technique we need to employ to generate one or more attributes + values for a HTML element is called "Attribute splatting". Attribute splatting involves assigning a Dictionary<string, object> to an attribute with the special name @attributes.

<div @attributes=MyCodeGeneratedAttributes/>

@code
{
  Dictionary<string, object> MyCodeGeneratedAttributes;

  protected override void OnInitialized()
  {
    MyCodeGeneratedAttributes = new Dictionary<string, object>();
    for(int index = 1; index <= 5; index++)
    {
      MyCodeGeneratedAttributes["attribute_" + index] = index;
    }
  }
}

The preceding code will output a <div> with 5 attributes.

<div attribute_1="1" attribute_2="2" attribute_3="3" attribute_4="4" attribute_5="5"></div>

Special cases

Some HTML attributes, such as readonly and disabled require no values - their mere presence on the element is sufficient for them to be effective. In fact, even apply a value such as false will still activate them. The following <input> element will be both readonly and disabled.

<input readonly="false" disabled="false"/>

In razor views the rule is slightly different. If we output readonly=@IsReadOnly or disabled=@IsDisabled - whenever the value being assigned is false razor will not output the attribute at all; when the value being assigned is true razor will output the element without assigning a value.

<input readonly=@true disabled=@false/> will result in razor generated HTML that does not include the disabled attribute at all.