Add additional content into a known content placeholder from a user control.

Answer 1

I figured it out. The following JavascriptInjector class will work, although I don't know what the implications are of calling the render method in the PreRender. Also think I may need to do something to figure out which type of HtmlTextWriter that the base application is using.

"JavaScriptInjector.cs"

using System;
using System.IO;
using System.Linq;
using System.Web.UI;
using System.Web.UI.WebControls;

public class JavascriptInjector : PlaceHolder
{
    private bool performRegularlly;

    public string InjectInto
    {
        get { return this.ViewState["InjectInto"] as string; }
        set { this.ViewState["InjectInto"] = value; }
    }

    protected override void OnInit(EventArgs e)
    {
        base.OnInit(e);

        this.PreRender += this.__PreRender;
    }

    protected override void Render(HtmlTextWriter writer)
    {
        if (this.performRegularlly)
        {
            base.Render(writer);
        }
    }

    private void __PreRender(object sender, EventArgs e)
    {
        if (string.IsNullOrEmpty(this.InjectInto))
        {
            goto performRegularlly;
        }

        var injectInto = this.FindControlRecursively(this.Page);

        if (injectInto == null)
        {
            goto performRegularlly;
        }

        performRegularlly = false;

        using (var stringWriter = new StringWriter())
        using (var writer = new HtmlTextWriter(stringWriter))
        {
            base.Render(writer);

            writer.Flush();

            injectInto.Controls.Add(new LiteralControl(stringWriter.GetStringBuilder().ToString()));
        }

        this.Controls.Clear();

        return;

        performRegularlly: this.performRegularlly = true;
    }

    private Control FindControlRecursively(Control current)
    {
        var found = current.FindControl(this.InjectInto);

        if (found != null)
        {
            return found;
        }

        foreach (var child in current.Controls.Cast<Control>())
        {
            return this.FindControlRecursively(child);
        }

        return null;
    }
}