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/Program.cs

using System;
using System.Xml;
using System.Xml.Linq;
using System.Linq;
					
public class Program
{
	public static void Main()
	{
		var xaml = @"<StackPanel>
	<TextBlock Text='not example'/>
	<!-- Some other stuff... -->
	<GridViewColumn Header='example'>
		<!-- Some other stuff... -->
	</GridViewColumn>
</StackPanel>";
		var templateXaml = @"<DataTemplate>
           <TextBlock Text='example'/>
       </DataTemplate>";
		var doc = XDocument.Parse(xaml);
		var template = XElement.Parse(templateXaml);
		//XNamespace d = "http://schemas.microsoft.com/winfx/2006/xaml/presentation";
		foreach (var attr in doc.Descendants().Attributes("Text").Where(o => o.Value != "example"))
		{
			attr.Value = "example";
		}
		foreach (var element in doc.Descendants().Where(o => o.Attribute("Header") != null))
		{
			element.DescendantNodes().Remove();
			element.Add(new XElement(element.Name.LocalName + ".HeaderTemplate", template));
		}
		Console.WriteLine(doc.ToString());
	}
}