C# check XmlNode value attribute

Question

I wrote this code to check if a XmlNode has a value, but when I run it crash always on the !=null. It is strange because this solution is well known.

private static void TraverseNodes(XmlNodeList nodes)
{
    foreach (XmlNode node in nodes)
    {

        if (!node.HasChildNodes)
        {
            Console.WriteLine(node.Name + " " + node.Attributes["id"].Value);
        }
        if (node.Attributes["SplitCombinationOperator"].Value != null)
        {
            Console.WriteLine(node.Name + " " + node.Attributes["SplitCombinationOperator"].Value);
        }
        else
        {
            Console.WriteLine(node.Name);
        }

        TraverseNodes(node.ChildNodes);
    }
}

The error is the following: Object reference not set to an instance of an object.

The SplitCombinationOperator may not exist, while your code assumes the "Value" property will be available, in any case. — Veverke
– Veverke, Commented Jul 14, 2015 at 15:36

Andrey Korneyev · Accepted Answer · 2015-07-14 15:34:46Z

1

You should check for null values something like this:

node.Attributes["SplitCombinationOperator"] != null &&
node.Attributes["SplitCombinationOperator"].Value != null

Otherwise you will get NullReferenceException when node.Attributes["SplitCombinationOperator"] is null while trying to access its Value property.

answered Jul 14, 2015 at 15:34

Andrey Korneyev

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1 Comment

user840718 Over a year ago

Good, now I understand.

Jon Skeet · Accepted Answer · 2015-07-14 15:41:26Z

1

You only need to check whether the attribute indexer itself returns null:

if (node.Attributes["SplitCombinationOperator"] != null)

Currently, that's returning null, and you're dereferencing it for the Value property - hence the exception. Note that you're also assuming there'd an id property, which may not be a good idea.

(You don't need to check whether Value itself is null - if the attribute exists, the value is non-null. Even if it's empty, you'll get an empty string rather than a null reference.)

edited Jul 14, 2015 at 15:41

answered Jul 14, 2015 at 15:36

Jon Skeet

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1 Comment

user840718 Over a year ago

I was figuring out why it was not working, but this explanation made it very clear.

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