edg3/experiment_software_csharp_12_net_8 · Datasets at Hugging Face

title stringlengths 3 46	content stringlengths 0 1.6k
17:591	...
17:592	asp-route-{action method parametern}=`valuen`>
17:593	put anchor text here
17:594	</a>
17:595
17:596	Below is an example of its usage:
17:597	<a asp-controller=`Home` asp-action=`Index`>
17:598	Back Home
17:599	</a>
17:600
17:601	The HTML created is:
17:602	<a href=`Home/Index`>
17:603	Back Home
17:604	</a>
17:605
17:606	It might appear that there is almost no advantage in using the tag helper. However, this isn’t true! The advantage is that whenever the routing rules change, the tag helper automatically updates the href that it generates to conform to the new routing rules.
17:607	A similar syntax is added to the form tags:
17:608	<form asp-controller=`{controller name}`
17:609	asp-action=`{action method name}`
17:610	asp-route-{action method parameter1}=`value1`
17:611	...
17:612	asp-route-{action method parametern}=`valuen`
17:613	...
17:614	>
17:615	...
17:616
17:617	The script tag is enhanced with attributes that allow us to fall back to a different source if the download fails. Typical usage is to download scripts from some cloud service to optimize the browser cache and to fall back to a local copy of the script if there is a failure. The following code uses the fallback technique to download the bootstrap JavaScript file:
17:618	<script src=`https://stackpath.bootstrapcdn.com/
17:619	bootstrap/4.3.1/js/bootstrap.bundle.min.js`
17:620	asp-fallback-src=`~/lib/bootstrap/dist/js/bootstrap.bundle.min.js`
17:621	asp-fallback-test=`window.jQuery && window.jQuery.fn && window.jQuery.fn.modal`
17:622	crossorigin=`anonymous`
17:623	integrity=`sha384-xrRywqdh3PHs8keKZN+8zzc5TX0GRTLCcmivcbNJWm2rs5C8PRhcEn3czEjhAO9o`>
17:624	</script>
17:625
17:626	asp-fallback-test contains a JavaScript test that verifies whether the download succeeded. In the preceding example, the test verifies whether a JavaScript object has been created.
17:627	All HTML tags that admit an src attribute, that is, the img and the script tags, can be added as an asp-append-version attribute set to true. The asp-append-version attribute set to true doesn’t change the syntax of the img and script tags; it just adds a hash to the src query string to prevent cashing each time the image or script file changes. Here is an example:
17:628	<img src=`~/images/myImage.png` asp-append-version=`true`>
17:629
17:630	Which is rendered as:
17:631	<img src=`/images/myImage.png?v=kM_dqr9NVtnMdsM2MUgdskVVFD`>
17:632
17:633	The hash passed in the v query parameter is computed from the content of the image file, so it changes whenever the image changes, thereby preventing the browser from rendering an old cached copy of the image.
17:634	The ~/ symbol is not a feature specific to the img tag helper and, instead, a Razor native feature you can use in all links contained in any tag of a Razor file. It stands for the application root. It is not equivalent to the HTML / symbol that stands for the root of the domain, because ASP.NET Core applications can also be placed in subfolders of the domain. So ~/ translates as / only when the application is placed in the domain root; otherwise, it translates as /{application subfolder name}/.
17:635	The environment tag can be used to select different HTML for different environments (development, staging, and production). Its typical usage is selecting the debug versions of JavaScript files during development, as shown in this example:
17:636	<environment include=`Development`>
17:637	@development version of JavaScript files@
17:638	</environment>
17:639	<environment exclude=`Development`>
17:640	@development version of JavaScript files @
17:641	</environment>
17:642
17:643	There is also a cache tag, which caches its content in memory to optimize rendering speed:
17:644	<cache>
17:645	@* heavy to compute content to cache *@
17:646	</cache>
17:647
17:648	By default, content is cached for 20 minutes, but the tag has attributes that must be defined when the cache expires, such as expires-on=`{datetime}`, expires-after=`{timespan}`, and expires-sliding=`{timespan}`. Here, the difference between expires-sliding and expires-after is that, in the second attribute, the expiration time count is reset each time the content is requested. The vary-by attribute causes the creation of a different cache entry for each different value passed to vary-by. There are also attributes such as vary-by-header, which creates a different entry for each different value assumed by the request header specified in the vary-by-cookie attribute, and so on.
17:649	All input tags – that is, textarea, input, and select – have an asp-for attribute that accepts a properties path rooted in the view’s ViewModel as their value. For instance, if the view has a Person ViewModel, we may have something like this:
17:650	<input type=`text` asp-for`Address.Town`/>
17:651
17:652	The first thing the preceding code does is assign the value of the Town nested property to the value attribute of the input tag. In general, if the value is not a string, it is converted into a string using the current request culture.
17:653	However, it also sets the name of the input field to Address.Town and the id of the input field to Address_Town. This is because dots are not allowed in tag ids.
17:654	A prefix can be added to these standard names by specifying it in ViewData.TemplateInfo.HtmlFieldPrefix. For instance, if the previous property is set to MyPerson, the name becomes MyPerson.Address.Town.
17:655	If the form is submitted to an action method that has the same Person class as one of its parameters, the name of Address.Town that’s given to the input field will cause the Town property of this parameter to be filled with the input field. In general, the string contained in the input field is converted into the type of property it has been matched with, using the current request culture. Summing this up, the names of input fields are created in such a way that a complete Person model can be recovered in the action method when the HTML page is posted.
17:656	The same asp-for attribute can be used in a label tag to cause the label to refer to the input field with the same asp-for value.
17:657	The following code is an example of an input/label pair:
17:658	<label asp-for=`Address.Town`></label
17:659	<input type=`text` asp-for=`Address.Town`/>
17:660
17:661	When no text is inserted into the label, the text shown in the label is taken from a Display attribute that decorates the property (Town, in this example), if any; otherwise, the name of the property is used.
17:662	If span or div contains an asp-validation-for=`Address.Town` error attribute, then validation messages concerning the Address.Town input will be inserted automatically inside that tag. The validation framework will be described in the Understanding the connection between ASP.NET Core MVC and design principles section.
17:663	It is also possible to automatically create a validation error summary by adding the attribute that follows a div or a span:
17:664	asp-validation-summary=`ValidationSummary.{All, ModelOnly}`
17:665
17:666	If the attribute is set to ValidationSummary.ModelOnly, only messages that are not associated with specific input fields will be shown in the summary, while if the value is ValidationSummary.All, all error messages will be shown.
17:667	The asp-items attribute can be applied to any select tag to automatically generate all the select options. It must be passed an IEnumerable<SelectListItem>, where each SelectListItem contains both the text and value of an option. SelectListItem also contains an optional Group property that you can use to organize the options shown in select into groups.
17:668	Here is an example of how to use asp-items:
17:669	...
17:670	@{
17:671	var choices = new List<SelectListItem>
17:672	{
17:673	new SelectListItem {Value=`value1`, Text=`text1`, Group=`group1`},
17:674	new SelectListItem {Value=`value2`, Text=`text2`, Group=`group1`}
17:675	...
17:676	new SelectListItem {..., Group=`group2`}
17:677	...
17:678	}
17:679	}
17:680	<select asp-for=`MyProperty` asp-items=`choices`>
17:681	<option value=``>Select a value</option>
17:682	</select>
17:683	...
17:684
17:685	When added, option tags are placed before the ones generated by asp-items.
17:686	The next topic shows how to reuse view code.
17:687	Reusing view code
17:688	ASP.NET Core MVC includes several techniques for reusing view code, with the most important being the layout page.
17:689	In each web application, several pages share the same structure, for instance, the same main menu or the same left or right bar. In ASP.NET Core, this common structure is factored out in views called layout pages/views.
17:690

17:591

...

17:592

asp-route-{action method parametern}=`valuen`>

17:593

put anchor text here

17:594

</a>

17:595

17:596

Below is an example of its usage:

17:597

17:598

Back Home

17:599

</a>

17:600

17:601

The HTML created is:

17:602

17:603

Back Home

17:604

</a>

17:605

17:606

It might appear that there is almost no advantage in using the tag helper. However, this isn’t true! The advantage is that whenever the routing rules change, the tag helper automatically updates the href that it generates to conform to the new routing rules.

17:607

A similar syntax is added to the form tags:

17:608

<form asp-controller=`{controller name}`

17:609

asp-action=`{action method name}`

17:610

asp-route-{action method parameter1}=`value1`

17:611

...

17:612

asp-route-{action method parametern}=`valuen`

17:613

...

17:614

>

17:615

...

17:616

17:617

The script tag is enhanced with attributes that allow us to fall back to a different source if the download fails. Typical usage is to download scripts from some cloud service to optimize the browser cache and to fall back to a local copy of the script if there is a failure. The following code uses the fallback technique to download the bootstrap JavaScript file:

17:618

17:619

bootstrap/4.3.1/js/bootstrap.bundle.min.js`

17:620

asp-fallback-src=`~/lib/bootstrap/dist/js/bootstrap.bundle.min.js`

17:621

asp-fallback-test=`window.jQuery && window.jQuery.fn && window.jQuery.fn.modal`

17:622

crossorigin=`anonymous`

17:623

integrity=`sha384-xrRywqdh3PHs8keKZN+8zzc5TX0GRTLCcmivcbNJWm2rs5C8PRhcEn3czEjhAO9o`>

17:624

</script>

17:625

17:626

asp-fallback-test contains a JavaScript test that verifies whether the download succeeded. In the preceding example, the test verifies whether a JavaScript object has been created.

17:627

All HTML tags that admit an src attribute, that is, the img and the script tags, can be added as an asp-append-version attribute set to true. The asp-append-version attribute set to true doesn’t change the syntax of the img and script tags; it just adds a hash to the src query string to prevent cashing each time the image or script file changes. Here is an example:

17:628

17:629

17:630

Which is rendered as:

17:631

17:632

17:633

The hash passed in the v query parameter is computed from the content of the image file, so it changes whenever the image changes, thereby preventing the browser from rendering an old cached copy of the image.

17:634

The ~/ symbol is not a feature specific to the img tag helper and, instead, a Razor native feature you can use in all links contained in any tag of a Razor file. It stands for the application root. It is not equivalent to the HTML / symbol that stands for the root of the domain, because ASP.NET Core applications can also be placed in subfolders of the domain. So ~/ translates as / only when the application is placed in the domain root; otherwise, it translates as /{application subfolder name}/.

17:635

The environment tag can be used to select different HTML for different environments (development, staging, and production). Its typical usage is selecting the debug versions of JavaScript files during development, as shown in this example:

17:636

17:637

@*development version of JavaScript files*@

17:638

</environment>

17:639

17:640

@*development version of JavaScript files *@

17:641

</environment>

17:642

17:643

There is also a cache tag, which caches its content in memory to optimize rendering speed:

17:644

<cache>

17:645

@* heavy to compute content to cache *@

17:646

</cache>

17:647

17:648

By default, content is cached for 20 minutes, but the tag has attributes that must be defined when the cache expires, such as expires-on=`{datetime}`, expires-after=`{timespan}`, and expires-sliding=`{timespan}`. Here, the difference between expires-sliding and expires-after is that, in the second attribute, the expiration time count is reset each time the content is requested. The vary-by attribute causes the creation of a different cache entry for each different value passed to vary-by. There are also attributes such as vary-by-header, which creates a different entry for each different value assumed by the request header specified in the vary-by-cookie attribute, and so on.

17:649

All input tags – that is, textarea, input, and select – have an asp-for attribute that accepts a properties path rooted in the view’s ViewModel as their value. For instance, if the view has a Person ViewModel, we may have something like this:

17:650

17:651

17:652

The first thing the preceding code does is assign the value of the Town nested property to the value attribute of the input tag. In general, if the value is not a string, it is converted into a string using the current request culture.

17:653

However, it also sets the name of the input field to Address.Town and the id of the input field to Address_Town. This is because dots are not allowed in tag ids.

17:654

A prefix can be added to these standard names by specifying it in ViewData.TemplateInfo.HtmlFieldPrefix. For instance, if the previous property is set to MyPerson, the name becomes MyPerson.Address.Town.

17:655

If the form is submitted to an action method that has the same Person class as one of its parameters, the name of Address.Town that’s given to the input field will cause the Town property of this parameter to be filled with the input field. In general, the string contained in the input field is converted into the type of property it has been matched with, using the current request culture. Summing this up, the names of input fields are created in such a way that a complete Person model can be recovered in the action method when the HTML page is posted.

17:656

The same asp-for attribute can be used in a label tag to cause the label to refer to the input field with the same asp-for value.

17:657

The following code is an example of an input/label pair:

17:658

<label asp-for=`Address.Town`></label

17:659

17:660

17:661

When no text is inserted into the label, the text shown in the label is taken from a Display attribute that decorates the property (Town, in this example), if any; otherwise, the name of the property is used.

17:662

If span or div contains an asp-validation-for=`Address.Town` error attribute, then validation messages concerning the Address.Town input will be inserted automatically inside that tag. The validation framework will be described in the Understanding the connection between ASP.NET Core MVC and design principles section.

17:663

It is also possible to automatically create a validation error summary by adding the attribute that follows a div or a span:

17:664

asp-validation-summary=`ValidationSummary.{All, ModelOnly}`

17:665

17:666

If the attribute is set to ValidationSummary.ModelOnly, only messages that are not associated with specific input fields will be shown in the summary, while if the value is ValidationSummary.All, all error messages will be shown.

17:667

The asp-items attribute can be applied to any select tag to automatically generate all the select options. It must be passed an IEnumerable<SelectListItem>, where each SelectListItem contains both the text and value of an option. SelectListItem also contains an optional Group property that you can use to organize the options shown in select into groups.

17:668

Here is an example of how to use asp-items:

17:669

...

17:670

@{

17:671

var choices = new List<SelectListItem>

17:672

{

17:673

new SelectListItem {Value=`value1`, Text=`text1`, Group=`group1`},

17:674

new SelectListItem {Value=`value2`, Text=`text2`, Group=`group1`}

17:675

...

17:676

new SelectListItem {..., Group=`group2`}

17:677

...

17:678

}

17:679

}

17:680

17:681

<option value=``>Select a value</option>

17:682

</select>

17:683

...

17:684

17:685

When added, option tags are placed before the ones generated by asp-items.

17:686

The next topic shows how to reuse view code.

17:687

Reusing view code

17:688

ASP.NET Core MVC includes several techniques for reusing view code, with the most important being the layout page.

17:689

In each web application, several pages share the same structure, for instance, the same main menu or the same left or right bar. In ASP.NET Core, this common structure is factored out in views called layout pages/views.

17:690