When I tried to serialize a class with an Double? property as attribute, XMLSerializer died with its usual and oh-so-unhelpful cascade of InvalidOperationExceptions. The actual error message was:

Cannot serialize member ‘SomeValue’ of type System.Nullable`1[System.Double]. XmlAttribute/XmlText cannot be used to encode complex types.

Since XML attributes have very well-defined null semantics, I was pretty stumped, but soon I found the Allow override for XmlSerializer Nullable<T> attribute behavior feature request on Microsoft Connect:

Unfortunately, this change would be too large to take and would be a new feature rather than a bug fix. We are trying to minimize new feature work in the XmlSerializer, partly to reduce regression risk, and focus our effort on the next-generation serialization technology. Unfortunately, we have decided to resolve this as Won’t Fix.

So I went back the the trusty old *Specified property of XmlSerializer and hacked together a micro-pattern to support Nullable<T> for serialization to an XML attribute:

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