Constructor of a XmlSerializer serializing a List throws an InvalidOperationException when used with XmlAttributeOverrides

Question

Summary

When using the XmlSerializer class, serializing a List (where T can be serialized with XmlSerialize

Answer 1

Your problem is that you are trying to use overrides to attach [XmlArray] and [XmlArrayItem] to the type List<T>. However, as shown in the docs, [XmlArray] cannot be used in this manner:

[AttributeUsageAttribute(AttributeTargets.Property | AttributeTargets.Field 
                        | AttributeTargets.Parameter | AttributeTargets.ReturnValue, 
    AllowMultiple = false)]
public class XmlArrayAttribute : Attribute

Notice there is no AttributeTargets.Class included? That means that [XmlArray] cannot be applied directly to a type and so attempting to do so via XML overrides rightly throws an exception.

But as to why that exception message states,

System.InvalidOperationException: XmlRoot and XmlType attributes may not be specified for the type System.Collections.Generic.List`1...

Er, well, that message is simply wrong. It would appear to be a minor bug in XmlSerializer. You could even report it to Microsoft if you want.

What you need to do instead is:

• Override the [XmlRoot] attribute of List<T> to specify the desired name, in this case "texparams", AND

• Override the [XmlType] attribute of T and set XmlTypeAttribute.TypeName to be the desired collection element name. In the absence of an [XmlArrayItem(name)] override this is what controls the element names of collections whose items are of type T.

Thus your code should look like:

static XmlSerializer MakeListSerializer<T>(string rootName, string elementName)
{
    xmls.XmlAttributeOverrides attributeOverrides = new xmls.XmlAttributeOverrides();
    attributeOverrides.Add(typeof(List<T>), new xmls.XmlAttributes()
    {
        XmlRoot = new xmls.XmlRootAttribute(rootName),
    });
    attributeOverrides.Add(typeof(T), new xmls.XmlAttributes()
    {
        XmlType = new xmls.XmlTypeAttribute(elementName),
    });

    return new XmlSerializer(typeof(List<T>), attributeOverrides);
}

Sample fiddle.

Note that when constructing an XmlSerializer using XmlAttributeOverrides you must cache the serializer for later reuse to avoid a severe memory leak, for reasons explained here.