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How to filter on an optional element (LINQ to XML)

Sometimes you want to filter for an element even though you're not sure it exists in your XML document. You can write your search so that, even if the particular element doesn't have the child element, you don't trigger a null reference exception by filtering for it.

Example: Search that doesn't trigger an exception when the target element doesn't exist

In the following example, the Child5 element doesn't have a Type child element, but the query still executes correctly. The example uses the Elements extension method.

XElement root = XElement.Parse(@"<Root>
  <Child1>
    <Text>Child One Text</Text>
    <Type Value=""Yes""/>
  </Child1>
  <Child2>
    <Text>Child Two Text</Text>
    <Type Value=""Yes""/>
  </Child2>
  <Child3>
    <Text>Child Three Text</Text>
    <Type Value=""No""/>
  </Child3>
  <Child4>
    <Text>Child Four Text</Text>
    <Type Value=""Yes""/>
  </Child4>
  <Child5>
    <Text>Child Five Text</Text>
  </Child5>
</Root>");
var cList =
    from typeElement in root.Elements().Elements("Type")
    where (string)typeElement.Attribute("Value") == "Yes"
    select (string)typeElement.Parent.Element("Text");
foreach(string str in cList)
    Console.WriteLine(str);

Dim root As XElement = _
    <Root>
        <Child1>
            <Text>Child One Text</Text>
            <Type Value="Yes"/>
        </Child1>
        <Child2>
            <Text>Child Two Text</Text>
            <Type Value="Yes"/>
        </Child2>
        <Child3>
            <Text>Child Three Text</Text>
            <Type Value="No"/>
        </Child3>
        <Child4>
            <Text>Child Four Text</Text>
            <Type Value="Yes"/>
        </Child4>
        <Child5>
            <Text>Child Five Text</Text>
        </Child5>
    </Root>
Dim cList As IEnumerable(Of String) = _
    From typeElement In root.Elements().<Type> _
    Where typeElement.@Value = "Yes" _
    Select typeElement.Parent.<Text>.Value
Dim str As String
For Each str In cList
    Console.WriteLine(str)
Next

This example produces the following output:

Child One Text
Child Two Text
Child Four Text

Example: Same search but for XML in a namespace

The following example is the same query but for XML that's in a namespace. For more information, see Namespaces overview.

XElement root = XElement.Parse(@"<Root xmlns='http://www.adatum.com'>
  <Child1>
    <Text>Child One Text</Text>
    <Type Value=""Yes""/>
  </Child1>
  <Child2>
    <Text>Child Two Text</Text>
    <Type Value=""Yes""/>
  </Child2>
  <Child3>
    <Text>Child Three Text</Text>
    <Type Value=""No""/>
  </Child3>
  <Child4>
    <Text>Child Four Text</Text>
    <Type Value=""Yes""/>
  </Child4>
  <Child5>
    <Text>Child Five Text</Text>
  </Child5>
</Root>");
XNamespace ad = "http://www.adatum.com";
var cList =
    from typeElement in root.Elements().Elements(ad + "Type")
    where (string)typeElement.Attribute("Value") == "Yes"
    select (string)typeElement.Parent.Element(ad + "Text");
foreach (string str in cList)
    Console.WriteLine(str);

Imports <xmlns='http://www.adatum.com'>

Module Module1
    Sub Main()
        Dim root As XElement = _
            <Root>
                <Child1>
                    <Text>Child One Text</Text>
                    <Type Value="Yes"/>
                </Child1>
                <Child2>
                    <Text>Child Two Text</Text>
                    <Type Value="Yes"/>
                </Child2>
                <Child3>
                    <Text>Child Three Text</Text>
                    <Type Value="No"/>
                </Child3>
                <Child4>
                    <Text>Child Four Text</Text>
                    <Type Value="Yes"/>
                </Child4>
                <Child5>
                    <Text>Child Five Text</Text>
                </Child5>
            </Root>
        Dim cList As IEnumerable(Of String) = _
            From typeElement In root.Elements().<Type> _
            Where typeElement.@Value = "Yes" _
            Select typeElement.Parent.<Text>.Value
        Dim str As String
        For Each str In cList
            Console.WriteLine(str)
        Next
    End Sub
End Module

This example produces the following output:

Child One Text
Child Two Text
Child Four Text

See also