https://stackoverflow.com/questions/48260849/linq-xml-finding-nodes-returns-null

stack.48260849.cs

// Rely on http://xmltocsharp.azurewebsites.net/ for xml to C# mappings
void Main()
{
	var xml = @"
		<SalesQuotes xmlns:xsd='http://www.w3.org/2001/XMLSchema' xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' xmlns='http://api.some.com/version/1'>
		  <Pagination>
		    <NumberOfItems>2380</NumberOfItems>
		    <PageSize>200</PageSize>
		    <PageNumber>1</PageNumber>
		    <NumberOfPages>12</NumberOfPages>
		  </Pagination>
		  <SalesQuote>
		    <Guid>825634b9-28f5-4aa7-98e7-5e4a4ed6bc6a</Guid>
		    <LastModifiedOn>2018-01-09T12:23:56.6133445</LastModifiedOn>
		    <Comments>Please note:
		installation is not included in this quote
		    </Comments>
		  </SalesQuote>
		</SalesQuotes>";

	var processor = new XmlProcessor();
	processor.Run(xml);
}

public class XmlProcessor
{
	public void Run(string xml)
	{
		// Load
		var document = XDocument.Parse(xml);
		var xmlns = document.Root.GetDefaultNamespace();
		document.Dump("Xml");

		// Find
		var query = from element in document
						.Descendants(xmlns + "SalesQuote")
						.Elements(xmlns + "Guid")
					where element.Value == "825634b9-28f5-4aa7-98e7-5e4a4ed6bc6a"
					select element;
					
		query.Dump("Node");

	}
}

[XmlRoot(ElementName = "Pagination")]
public class Pagination
{
	[XmlElement(ElementName = "NumberOfItems")]
	public string NumberOfItems { get; set; }
	[XmlElement(ElementName = "PageSize")]
	public string PageSize { get; set; }
	[XmlElement(ElementName = "PageNumber")]
	public string PageNumber { get; set; }
	[XmlElement(ElementName = "NumberOfPages")]
	public string NumberOfPages { get; set; }
}

[XmlRoot(ElementName = "SalesQuote")]
public class SalesQuote
{
	[XmlElement(ElementName = "Guid")]
	public string Guid { get; set; }
	[XmlElement(ElementName = "LastModifiedOn")]
	public string LastModifiedOn { get; set; }
	[XmlElement(ElementName = "Comments")]
	public string Comments { get; set; }
}

[XmlRoot(ElementName = "SalesQuotes")]
public class SalesQuotes
{
	[XmlElement(ElementName = "Pagination")]
	public Pagination Pagination { get; set; }
	[XmlElement(ElementName = "SalesQuote")]
	public SalesQuote SalesQuote { get; set; }
	[XmlAttribute(AttributeName = "xsd")]
	public string Xsd { get; set; }
	[XmlAttribute(AttributeName = "xsi")]
	public string Xsi { get; set; }
	[XmlAttribute(AttributeName = "xmlns")]
	public string Xmlns { get; set; }
}