How do I have optional XML elements using LINQ lamda syntax?

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C#

LINQ

syntax

lambda

This is a follow-on to another question I asked here: https://www.codeproject.com/Answers/5162098/How-do-I-convert-this-LINQ-query-to-function-synta#answer1

I am generating the XML I require using the following code:

bool includeDetails = true;
XElement breakdown = new XElement
( "Breakdown",
	new XElement( "DetailsIncluded", includeDetails ),
	new XElement
	( "Sections", estimateBreakdown.Sections.Select( x => new XElement
		( "Section",
			new XAttribute( "SectionNumber", x.SectionNumber ),
			new XAttribute( "SectionName", x.SectionName ),
			x.CostItems.Select( c => new XElement
			( "CostItem",
				new XElement( "Name", c.Name ),
				new XElement( "Units", c.Units.ToString() ),
				( !includeDetails ) ? null : new XElement( "UnitCost", c.UnitCost.ToString() )
			))
		)
	))
);
return breakdown.ToString();

Which generates the following XML when includeDetails is true:

<Breakdown>
  <DetailsIncluded>true</DetailsIncluded>
  <Sections>
    <Section SectionNumber="1" SectionName="Design">
      <CostItem>
        <Name>Site Survey</Name>
        <Units>NotSpecified</Units>
        <UnitCost>0</UnitCost>
      </CostItem>
      <CostItem>
        <Name>Concept Design</Name>
        <Units>NotSpecified</Units>
        <UnitCost>0</UnitCost>
      </CostItem>
    </Section>
  </Sections>
</Breakdown>

And the following XML when includeDetails is false:

<Breakdown>
  <DetailsIncluded>false</DetailsIncluded>
  <Sections>
    <Section SectionNumber="1" SectionName="Design">
      <CostItem>
        <Name>Site Survey</Name>
        <Units>NotSpecified</Units>
      </CostItem>
      <CostItem>
        <Name>Concept Design</Name>
        <Units>NotSpecified</Units>
      </CostItem>
    </Section>
  </Sections>
</Breakdown>

All good so far. My problem is that, when includeDetails is true, I actually need to add two XElement objects to the XML. This shows what I mean, I think, however this compiles but does not work:

( "CostItem",
	new XElement( "Name", c.Name ),
	new XElement( "Units", c.Units.ToString() ),
	( includeDetails ) ? new XElement( "UnitCost", c.UnitCost.ToString() ) : null,
	( includeDetails ) ? new XElement( "Quantity", c.Quantity.ToString() ) : null
))

Apart from the obvious inefficiency that I am testing the flag twice, I get nothing in my XML, which I suspect means I am getting an exception in the XElement constructor.

Can anyone show me how to do this?

What I have tried:

I am guessing that I need to generate a List<XElement> containing the two extra nodes that need to be added when includeDetails is true and then iterate over it but I can't work out the syntax.