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Custom helper for surrounding a block in markup with MVC

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This article illustrates how to create a custom helper for ASP.NET MVC to use in views that surrounds a block of markup.

You may have seen or used the following code in your MVC applications:

@using (Html.BeginForm())
{
    // ...
}

It surrounds the markup inside the block with a <form> tag and includes attributes based on what is passed through the parameter list. The way it works is HtmlHelper.BeginForm() returns a MvcForm object which on creation uses HtmlHelper to write markup to the page. When the using block comes to an end the Dispose method on MvcForm is used to output the closing tag.

I found myself needing to do this exact thing, so I downloaded the MVC source and had a look at how it all worked. The specific use case I needed it for was to output a fancy box around a block of markup, the box had a header and contained several tags before the content. Unfortunately I couldn’t place it in the layout as there could be multiple of them on a single page.

Here is an example of how to create a custom helper, this once surrounds the block in a simple div, take particular note of the Begin() and End() methods.

public class MvcDiv : IDisposable
{
    private bool _disposed;
    private readonly FormContext _originalFormContext;
    private readonly ViewContext _viewContext;
    private readonly TextWriter _writer;

    public MvcDiv(ViewContext viewContext)
    {
        if (viewContext == null)
        {
            throw new ArgumentNullException("viewContext");
        }

        _viewContext = viewContext;
        _writer = viewContext.Writer;
        _originalFormContext = viewContext.FormContext;
        viewContext.FormContext = new FormContext();

        Begin();
    }

    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    public void Begin()
    {
        _writer.Write("<div>");
    }

    private void End()
    {
        _writer.Write("</div>");
    }

    protected virtual void Dispose(bool disposing)
    {
        if (!_disposed)
        {
            _disposed = true;
            End();

            if (_viewContext != null)
            {
                _viewContext.OutputClientValidation();
                _viewContext.FormContext = _originalFormContext;
            }
        }
    }

    public void EndForm()
    {
        Dispose(true);
    }
}

Currently you can call this like so:

@using (new MvcDiv(Html.ViewContext))
{
    // ...
}

Alternative you could implement a Html.BeginDiv() extension method which would look like this:

public static class HtmlHelperExtensions
{
    public static MvcDiv BeginDiv(this HtmlHelper htmlHelper)
    {
        return new MvcDiv(htmlHelper.ViewContext);
    }
}

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