Is there a way to use the Sitecore MVC Field helpers and render a General Link with stuff appended to the query string?

Question

I have an item with a General Link field named "MyLinkField". In my view I have the following:

@Sitecore.Html().Field("MyLinkField")

That works great to output the link. Now I have added a multiselect field to the source item called "QueryStringItems". This allows the author to select one or more query string content items. Each query string content item has a field called "Key" and field called "Value". This allows us to have a central listing of query string key/value pairs. We want the author to be able to easily select which key/value pairs they would like to add to the query string for a given link.

Is this even possible using the out of the box Sitecore helpers? Is there some way I could write a custom helper to extend the out of the box one to also include any selected QueryStringItems?

UPDATE: Building off of Dan's answer, here is the code that I got to work both with Internal and External General Links.

public class CustomLinkRenderer : LinkRenderer
{
    public CustomLinkRenderer(Item item) : base(item)
    { }

    protected override string GetUrl(XmlField field)
    {
        if(field?.InnerField?.Item != null)
        {
            MultilistField queryStringItemsField = (MultilistField)field.InnerField.Item.Fields["Query String Items"];
            Item[] queryStringItems = queryStringItemsField?.GetItems();
            if(queryStringItems != null && queryStringItems.Length > 0)
            {
                SafeDictionary<string> existingQueryString = WebUtil.ParseQueryString(field.GetAttribute("querystring"));
                foreach(Item queryStringItem in queryStringItems)
                {
                    existingQueryString[queryStringItem["Key"]] = queryStringItem["Value"];
                }
                field.SetAttribute("querystring", WebUtil.BuildQueryString(existingQueryString, false));

                var linkType = field.GetAttribute("linktype");
                if(linkType == "external")
                {
                    var url = field.GetAttribute("url");
                    if(!string.IsNullOrEmpty(url))
                    {
                        if (!url.StartsWith("/", StringComparison.InvariantCulture) && url.IndexOf("://", StringComparison.InvariantCulture) < 0)
                            url = "http://" + url;
                        var queryString = WebUtil.BuildQueryString(existingQueryString, false);

                        if (url.Contains("?"))
                        { url += "&" + queryString; }
                        else
                        { url += "?" + queryString; }

                        return url;
                    }
                }
                else
                {
                    return (new LinkUrl()).GetUrl(field, this.Item.Database);
                }
            }
        }
        return base.GetUrl(field);
    }
}

Answer 1

I'm not sure if there's a way using the helpers available in the view, but you could certainly replace a few layers of the renderField pipeline to look for any QueryStringItems data and append it to the links.

You'll need to extend a few classes:

1. Sitecore.Pipelines.RenderField.GetLinkFieldValue

using Sitecore.Pipelines.RenderField;

public class GetLinkFieldValueWithQueryStringItems : GetLinkFieldValue
{
    protected override LinkRenderer CreateRenderer(Item item)
    {
        return new LinkRendererWithQueryStringItems(item);
    }
}

2. Sitecore.Xml.Xsl.LinkRenderer

using Sitecore.Xml.Xsl;

/// <summary>
/// Implements the Link Renderer.
/// </summary>
public class LinkRendererWithQueryStringItems : LinkRenderer
{
    protected virtual string GetUrl(XmlField field)
    {
        if (field?.InnerField?.Item != null)
        {
            MultilistField queryStringItemsField = (MultilistField)field.InnerField.Item.Fields["QueryStringItems"];
            Item[] queryStringItems = queryStringItemsField?.GetItems();
            if (queryStringItems != null && queryStringItems.Length > 0)
            {
                SafeDictionary<string> existingQueryString = WebUtil.ParseQueryString(field.GetAttribute("querystring"));
                foreach (Item queryStringItem in queryStringItems)
                {
                    existingQueryString[queryStringItem["Key"]] = queryStringItem["Value"];
                }
                field.SetAttribute("querystring", WebUtil.BuildQueryString(existingQueryString, false));
                return (new LinkUrl()).GetUrl(field, this.Item.Database);
            }
        }
        return base.GetUrl(field)
    }
}

And the obligatory patch config

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <pipelines>
        <renderField>
            <processor patch:instead="processor[@type='Sitecore.Pipelines.RenderField.GetLinkFieldValue, Sitecore.Kernel']" type="Custom.Web.Pipelines.RenderField.GetLinkFieldValueWithQueryStringItems, Custom.Web"/>
        </renderField>
    </pipelines>
  </sitecore>
</configuration>

Full disclosure: I've not tested this code exactly as-is. If you run into issues that need to be resolved, please comment below so I can fix them here for future users.