I’ve found myself writing code like this quite a bit lately:

var orderElement 
    = document.Element(OrderXml.OrderElementName);
if (orderElement == null)
{
    var message
        = string.Format(
            "Didn't find expected child element <{0}>",
            OrderXml.OrderElementName);
    throw new InvalidOperationException(message);
}

The intent is to find a specified child element, throwing an error if the element is not found.

After writing the same error checking code one or two times too many, I wrote some extension methods to make the task simpler. With the extension method RequiredElement, the above code simplifies quite a lot:

var orderElement 
    = document.RequiredElement(OrderXml.OrderElementName);

Here’s the definition for RequiredElement - as you can see, the only significant difference is in the type of exception thrown.

public static XElement RequiredElement(
    this XElement parent, 
    XName child)
{
    parent.MustNotBeNull(
        "parent",
        "Parent element must not be null");
    var result = parent.Element(child);
    if (result == null)
    {
        throw new XmlFormatException(
            parent,
            string.Format(
                "Didn't find required child element {0}", 
                child.LocalName));
    }

    return result;
}

The related extension methods, RequiredAtribute and RequiredElements (plural) - included in the attachment - are entirely similar.

XmlFormatException is a pretty vanilla custom exception type, except for the detail of taking an Xelement as one of the parameters to its constructors - this is promptly case to IXmlLineInfo allowing the exception to properly report the location within the Xml file where the problem was detected, useful for other developers and for end users trying to solve a problem.

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