jrummell.ToString()

Have you ever had to create a large web form for users to fill out and then receive an email copy after its submitted? That can be tedious work. The first few times I did it, I used a StringBuilder to build the email HTML one control at a time. Later, I viewed the HTML output of the page and replaced all input controls with spans, and then put that HTML in a StringBuilder. Either of these methods work, but it gets real annoying when I later have to add a field or two to the form and therefore to the email HTML.

I knew there had to be a way to do this programmatically without copying and pasting into a StringBuilder. Well, there is. Here’s a rather common code snippet that does just this:

public static string GetRenderedHtml(this Control control)
{
StringBuilder sbHtml = new StringBuilder();
using (StringWriter stringWriter = new StringWriter(sbHtml))
using (HtmlTextWriter textWriter = new HtmlTextWriter(stringWriter))
{
control.RenderControl(textWriter);
}
return sbHtml.ToString();
}

This is great! Let’s try it out on this simple example:

<div id="divForm" runat="server">
<fieldset class="inputArea">
<legend>Contact</legend>
<asp:Label runat="server" AssociatedControlID="txtName">
Name</asp:Label>
<asp:TextBox runat="server" ID="txtName" />
<asp:Label runat="server" AssociatedControlID="txtEmail">
Email</asp:Label>
<asp:TextBox runat="server" ID="txtEmail" />
<asp:Label runat="server" AssociatedControlID="txtWebsite">
Website</asp:Label>
<asp:TextBox runat="server" ID="txtWebsite" />
<asp:Label runat="server" AssociatedControlID="txtComment">
Comment</asp:Label>
<asp:TextBox runat="server" ID="txtComment" TextMode="MultiLine" Rows="4" cols="30" />
<asp:Button ID="btnSubmit" runat="server" Text="Submit" OnClick="btnSubmit_Click" />
</fieldset>
</div>

protected void btnSubmit_Click(object sender, EventArgs e)
{
txtRenderedHtml.Text = divForm.GetRenderedHtml();
}

Here is what we get:

Control 'txtName' of type 'TextBox' must be placed inside a form tag with runat=server.

So how do you get around that? Well, lets think about this. I’m trying to capture a form and render it as HTML to be included in an email, so I don’t want any TextBoxes. Lets replace the TextBoxes (and any other editable controls) with Labels and try again.

public static void ReplaceEditableControls(this Control control)
{
// don't bother with controls that aren't visible
if (!control.Visible)
{
return;
}
ListControl listControl = control as ListControl;
IButtonControl buttonControl = control as IButtonControl;
IValidator validator = control as IValidator;
IEditableTextControl textControl = control as IEditableTextControl;
UpdatePanel updatePanel = control as UpdatePanel;
if (validator != null || buttonControl != null)
{
control.Visible = false;
}
else if (listControl != null && listControl.SelectedItem != null)
{
Label label = new Label {Text = listControl.SelectedItem.Text, CssClass = "text"};
Replace(listControl, label);
}
else if (textControl != null)
{
Label label = new Label {Text = textControl.Text, CssClass = "text"};
Replace((Control) textControl, label);
}
else if (updatePanel != null)
{
// replace the update panel with a place holder
PlaceHolder holder = new PlaceHolder();
Control[] panelControls = new Control[updatePanel.ContentTemplateContainer.Controls.Count];
updatePanel.ContentTemplateContainer.Controls.CopyTo(panelControls, 0);
foreach (Control panelControl in panelControls)
{
holder.Controls.Add(panelControl);
}
ReplaceEditableControls(holder);
Replace(updatePanel, holder);
}
else if (control.HasControls())
{
Control[] controlsCopy = new Control[control.Controls.Count];
control.Controls.CopyTo(controlsCopy, 0);
foreach (Control controlCopy in controlsCopy)
{
ReplaceEditableControls(controlCopy);
}
}
}

There are a few things to note here.

• The check for ListControl is before IEditableTextControl because of the way it implements IEditableTextControl. ListControl.Text returns ListControl.SelectedValue, but ListControl.SelectedItem.Text makes more sense.

• UpdatePanels are a special case because of ContentTemplate. They are replaced with a PlaceHolder and then the method is recursively called on each child control.

• Finally, if the control has a control collection of its own, a recursive call is made on each child control.

• Notice that the control collection is copied to an array before making the recursive call. This is because the control collection is modified and you can’t modify a collection while iterating it. Well, you can, but you will have problems.

Now we can change the button handler to:

protected void btnSubmit_Click(object sender, EventArgs e)
{
divForm.ReplaceEditableControls();
}

Which will render the following HTML:

<div id="divForm">
<fieldset class="inputArea">
<legend>Contact</legend>
<label for="txtName">
Name</label>
<span id="txtName" class="text">John Rummell</span>
<label for="txtEmail">
Email</label>
<span id="txtEmail" class="text">jrummell@example.com</span>
<label for="txtWebsite">
Website</label>
<span id="txtWebsite" class="text">john.rummell.info</span>
<label for="txtComment">
Comment</label>
<span id="txtComment" class="text">Check out this new post!</span>
</fieldset>
</div>

To capture this as a string, just add a call to GetRenderedHtml:

protected void btnSubmit_Click(object sender, EventArgs e)
{
divForm.ReplaceEditableControls();
string html = divForm.GetRenderedHtml();
//TODO: send email
}

(The form style is a slight variation of Janko’s tutorial)