Friday, February 5, 2016

Tag Helpers in ASP.NET Core 1.0

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This article is for those who would like to use Tag Helpers over old Razor/HTML helpers. Tag Helpers, another new feature of ASP.NET Core. Let’s quickly have a look at it.

What are Tag Helpers?
As per ASP.NET documentation:
                    "Tag Helpers enable server-side code to participate in creating and rendering HTML elements in Razor files".

Tag Helpers are somewhat similar to HTML Helpers/Razor Helpers introduced in previous version of ASP.NET MVC but those helpers were not that easy to understand for web designers because of inline C# method calls. As web designers are more inclined towards HTML, a simpler and comfortable approach was required. Fortunately, ASP.NET Core 1.0 provided that. Now one need not to use HTML Helpers to build cshtml forms. Means when you had to write this:




Now you can write this:




As you can see above format is very easy to understand as it is purely a HTML syntax.

How Tag Helpers work?

When you create a new ASP.NET web application in Visual Studio, it adds Microsoft.AspNet.Tooling.Razor to the project.json file. This is the package that adds Tag Helper tooling and enables intelliSense in Visual Studio. In order to make our Razor views aware of Tag Helpers, a special file named _ViewImports.cshtml is used. This file is stored in Views folder. By default this file will be there. By any chance if file is not there, then create it and add following line in it:




Pre-defined Tag Helpers

ASP.NET Core 1.0 includes a list of pre-defined Tag Helpers:

Anchor - generates hyperlink
Input - generates input elements
Select – generates dropdownlist
Cache – manages partial page caching
Form – generates form element
Script – processes script tag
Link – processes link element
Label – outputs label element
Option – targets individual options in a list                   
TextArea – processes textarea tag
Environment – controls rendering of content
ValidationMessage – generates validation error
ValidationSummary – provides validation summary message
I’ll try to brief about all above Tag Helpers in coming posts. Now what if existing tags doesn't fulfill our need? In that case, we have to go ahead and invent something else. Should we proceed to create Tag Helper based on our requirement? 
Creating Custom Tag Helpers
As part of this article, we will create a new Tag Helper for appreciating someone. Our tag will be <appreciate>. For example:
<appreciate person-name-for-appreciation="shweta"></appreciate>
 
The server will use appreciate Tag Helper to convert that markup into following:
<label>Great Work, Shweta</label>
In order to create this new Tag Helper, we have to create a new class named AppreciateTagHelper and inherit TagHelper in that. Make sure to add the reference of Microsoft.AspNet.Razor.TagHelpers in this new class. Sample code is as:










Note - it is not mandatory to suffix TagHelper in class name. It is just a good practice.
To make this Tag Helper available to all Razor views, we have to add addTagHelper directive to _ViewImports.cshtml file as:
@addTagHelper "*,CustomTagHelper"
 
Above statement signifies that all the custom Tag Helpers (denoted by wild card character) will be available from assembly named CustomTagHelper. If interested, instead of * you can go with fully qualified names also.
Next step is to update our views. Let’s go ahead and append below line in view:
<appreciate person-name-for-appreciation="shweta"></appreciate>
Good thing is, you will get intelliSense for your custom Tag Helper also J
Run your application and you will find that appreciate Tag Helper is replaced with your label markup with output as:
 






Hope you enjoyed learning this new feature.

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