Dissecting Razor, part 7: Helpers

We’ll continue our trek into Razor’s class-level features with helpers.

Helpers are one of Razor’s unique features.  They encapsulate blocks of HTML and server-side logic into reusable page-level methods. 

You can define a helper by writing @helper MethodName(parameters) { ... }.  Inside the code block, you can put any markup or server-side code.  The contents of a helper are parsed as a code block (like the contents of a loop or if block), so any non-HTML-like markup must be surrounded by <text> tags or prefixed by the @: escape.

Here is a simple example:

<!DOCTYPE html>
<html>
    <body>
        @helper NumberRow(int num) {
            var square = num * num;
            <tr>
                <td>@num</td>
                <td>@square</td>
                <td>@(num % 2 == 0 ? "Even" : "Odd")</td>
            </tr>
        }
        <table>
            <thead>
                <tr>
                    <th>Number</th>
                    <th>Square</th>
                    <th>Eveness</th>
                </tr>
            </thead>
            <tbody>
                @for (int i = 0; i < 10; i++) {
                    @NumberRow(i)
                }
            </tbody>
        </table>
    </body>
</html>

Note that code statements (such as the square declaration) can go directly inside the helper body without being wrapped in code blocks – the direct contents of the helper is a code block, not markup.  Like any other code block, HTML-like markup is automatically treated as markup instead of code.

Like @functions blocks, helper methods can go anywhere in the source file; the physical location of the block is ignored.

Here is the generated source for the above example: (with blank lines and #line directives stripped for clarity)

public class _Page_Razor_Helpers_cshtml : System.Web.WebPages.WebPage {

    public System.Web.WebPages.HelperResult NumberRow(int num) {
        return new System.Web.WebPages.HelperResult(__razor_helper_writer => {

            var square = num * num;

            WriteLiteralTo(@__razor_helper_writer, "            <tr>\r\n                <td>");
            WriteTo(@__razor_helper_writer, num);

            WriteLiteralTo(@__razor_helper_writer, "</td>\r\n                <td>");
            WriteTo(@__razor_helper_writer, square);

            WriteLiteralTo(@__razor_helper_writer, "</td>\r\n                <td>");
            WriteTo(@__razor_helper_writer, num % 2 == 0 ? "Even" : "Odd");

            WriteLiteralTo(@__razor_helper_writer, "</td>\r\n            </tr>\r\n");

        });
    }
    public _Page_Razor_Helpers_cshtml() {
    }
    protected ASP.global_asax ApplicationInstance {
        get {
            return ((ASP.global_asax)(Context.ApplicationInstance));
        }
    }
    public override void Execute() {
        WriteLiteral("<!DOCTYPE html>\r\n<html>\r\n    <body>\r\n        ");
        WriteLiteral("\r\n        <table>\r\n            <thead>\r\n                <tr>\r\n                   " +
        " <th>Number</th>\r\n                    <th>Square</th>\r\n                    <th>E" +
        "veness</th>\r\n                </tr>\r\n            </thead>\r\n            <tbody>\r\n");

        for (int i = 0; i < 10; i++) {
            Write(NumberRow(i));
        }

        WriteLiteral("            </tbody>\r\n        </table>\r\n    </body>\r\n</html>\r\n");
#error

    }
}

Helpers are compiled as class-level methods that take a parameter set and return a System.Web.WebPages.HelperResult.  (This class name is configured by the RazorHostFactory)

Notice the the contents of the helper method are inside a lambda expression that takes a parameter named __razor_helper_writer.  This construction allows the helper to write directly to the HTTP response stream instead of assembling a giant string and then writing the string all at once.

The HelperResult constructor takes an Action<TextWriter> which contains the contents of the helper block.  The class implements IHtmlString and calls the action from the constructor to generate HTML.  However, under normal circumstances, this IHtmlString implementation is never called.

Calls to helper methods (@NumberRow(i)) are passed to the Write(HelperResult) overload.  This overload calls HelperResult.WriteTo(writer), which passes the page’s TextWriter directly to the helper’s lambda expression.  Thus, the lambda expression can write directly to the page’s output stream, without passing the output as a parameter to the helper method.

Looking inside the helper, we see that all content is passed to WriteTo and WriteLiteralTo methods, as opposed to the Write and WriteLiteral methods used by the rest of the page.

Helper methods cannot call the normal Write* methods since they aren’t necessarily writing to the current output (even though they usually do).  Therefore, they call these Write*To methods, which accept the TextWriter as a parameter.  These static methods are inherited from the WebPageExecutingBase class; their names are also configured by the RazorHostFactory.  The @ in the parameter is a little-used C# syntactictal feature that allows keywords to be used as identifiers; it has nothing to do with Razor’s use of the @ character.

Since helpers are compiled as normal methods, they can do almost anything that a normal method can.  However, because their contents are compiled inside a lambda expression, they have some limitations.  For example, helpers cannot use ref or out parameters, since they cannot be used inside lambda expressions.  Helpers can take params arrays or optional parameters.

Also, Razor’s C# code parser doesn’t support generic helper methods, although there is no reason that they couldn’t be supported in a later release.

The VB.Net code parser also doesn’t explicitly support generic helpers.  However, because VB.Net generics use parentheses, a VB.Net helper can declare a generic type parameter instead of a normal parameter list.

For example:

<!DOCTYPE html>
<html>

    <body>
        @Helper PrintType(Of T)
            @GetType(T)
        End Helper
        @PrintType(Of Dictionary(Of Integer, List(Of String)))()
    </body>

</html>

This trick is not very useful.

Next Time: Static Helpers

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The title "Dissecting Razor, Part 7: Helpers" should be consistent throughout the series, with a brief tagline or teaser highlighting specific aspects of Razor's Helpers. The introduction should set the stage for a thorough exploration of Razor's Helpers, ensuring each aspect is explained to cater to readers with varying levels of familiarity. Visual aids, such as code snippets or diagrams, should be mentioned to enhance clarity and understanding. The closing integration should integrate the insights from Part 7 into the broader Dissecting Razor series, leaving readers with a clear understanding of how Helpers contribute to Razor's functionality.


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This review provides a detailed and comprehensive examination of Razor Helpers, a crucial aspect of Razor's 'Dissecting Razor' series. It breaks down complex concepts, making it accessible to both experienced and new developers. The review emphasizes the importance of Helpers in improving code readability and maintainability. It offers practical examples and real-world examples, showcasing how to effectively use Razor Helpers in web development projects. The review balances technical depth with user-friendly explanations, making it a valuable resource for developers.

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Razor helpers are a powerful tool for encapsulating reusable logic and markup, but they also have limitations. They can be used in large or complex pages, and developers should be cautious when using them. To maintain code consistency, developers should follow a naming convention for helpers, such as using PascalCase for helper method names and camelCase for parameters. To improve Razor helpers, it is recommended to ensure consistent indentation, casing for keywords and identifiers, and remove unnecessary spaces or extra newlines. Error handling mechanisms should be included, especially within lambda expressions where exceptions might occur. Clarifications should be provided about the @ symbol in the parameter of WriteTo and WriteLiteralTo methods. Suggestions for improvement include adding a more diverse set of examples, including a section on best practices and potential pitfalls when working with helpers, correcting typos, and using consistent terminology. Testing helper methods is encouraged to ensure they behave as expected, especially when dealing with more complex logic. Overall, Razor helpers offer valuable insight into code reusability and maintainability. accidente de camión

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Razor Helpers are reusable code snippets that generate HTML markup or perform common tasks within Razor views or pages. They can be categorized into HTML, URL, partial, anti-forgery, and authentication helpers. Helpers promote code reusability, maintainability, and consistency, and can be invoked using Razor syntax. They can interact with models and data, support localization and accessibility, and undergo thorough testing.

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Dissecting Razor, part 7: Helpers provides great insights into how helpers can streamline and enhance your coding process. These tools are essential for writing cleaner and more efficient code. It's similar to choosing the right accessories to complement a leather jacket mens outfit – the right helpers can really make your development work stand out.

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