I am currently learning ASP.NET MVC and am presented with the following code snippet:
@Html.TextBoxFor(m => m.FirstName, new { @placeholder = "First name"})
I understand that the lambda expression refers to the model as m, but what is the second argument? It seems to be an anonymous type, but my main question is why is there an @ character in front of placeholder?
The second argument is htmlAttributes which you can use to add additional attributes to the HTML that is generated by the helper (@Html.TextBoxFor is a HTML helper).
The generated code for your example will be along the lines of:
@Html.TextBoxFor(m => m.FirstName, new { @placeholder = "First name"})
<input type="text" name="FirstName" placeholder="First name" />
If you want, you can add more attributes and they will also be added to the generated tag:
@Html.TextBoxFor(m => m.FirstName, new { placeholder = "First name", @class="my_text_box_class" })
<input type="text" name="FirstName" placeholder="First name" class="my_text_box_class" />
It's also possible to override the value of the textbox using Value (upper case V!):
@Html.TextBoxFor(m => m.FirstName, new { placeholder = "First name", Value="John Johnson"})
<input type="text" name="FirstName" placeholder="First name" value="John Johnson" />
The reason for the @ symbol is for when you want an attribute named exactly as a C# reserved word. class, private, namespace etc. It is a way to make the C# complier stop interpreting it as it's C# meaning. The @ symbol is only required for these reserved words.
The @placeholder syntax tells the razor engine that you want to assign a value to an html attribute on the element that will be rendered by the helper. As such, if you needed to set a class to the textbox as well, your anonymous type would change to:
new { @placeholder = "First name", @class = "BoldText" }
As for why you use @, the @ allows you to use reserved keywords as variable names. If you tried to use
class = "BoldText"
you would receive a runtime error in your view since class is a reserved keyword in C#. The prefacing @ ensures that this doesn't happen and is considered a "best practice". The @ would only be necessary before class, not placeholder.
The second parameter expects an anonymous object for the htmlAttributes you would like to specify in your input.
You use the @ symbol to tell the razor engine that you're specifying an htmlAttribute and not a reserved keyword.
placeholderHTML attribute to the input. The @ sign is needed if you use a reserved word (keyword). Otherwise you would get a compilation error. Although, "placeholder" is no keyword, so it is not needed in this case.