我不知道氟，但我想你可以通过：

(myDouble ?? Double.NaN)

这个运算式的型别是double，而不是double?，而且如果myDouble是null，它就会是NaN。

我们只是遇到了同样的问题。我们的解决方案是修改Fluorine代码来写对象。
在文件AMFWriter的第1367行中，在调用WriteAMF3Data(memberValue)之前，我添加了以下代码：

//Mapping null double?s to NaN when writing data.
if (memberValue == null)
{
    System.Reflection.PropertyInfo p = type.GetProperty(classMember.Name);
    if (p != null)
    {
        Type t = p.PropertyType; // t will be System.String
        if (t.IsEquivalentTo(typeof(Nullable<Double>)))
            memberValue = Double.NaN;
    }
}

到目前为止，它看起来是可行的。但是，我通常不使用.NET编写代码，所以可能有更好的方法来实现这一点。

看起来Fluorine有一个配置部分，定义了如何转换空值。我还没有测试过。
从http://www.fluorinefx.com/docs/fluorine/nullable.html复制

FluorineFx.NET  
Null values
The <nullable> configuration section allows the use of special value of the given value type as the null value. 

Use this solution only when you can identify a value which is unused.

<nullable>
    <type name="System.Int32" assembly="MinValue"/>
    <type name="System.Double" assembly="MinValue"/>
    <type name="System.DateTime" assembly="MinValue"/>
    <type name="System.Guid" assembly="Empty"/>
</nullable>

The name attribute is the fully qualified type name, the value attribute is a static member of the type (such as "MinValue") or a parseable value (0 for System.Int32 for example).

The acceptNullValueTypes option
Fluorine will accept null values sent from client for value-types if configured accordingly

    <acceptNullValueTypes>false</acceptNullValueTypes>

If acceptNullValueTypes = true (the default is false if not specified) any value-type that is not explicitly initialized with a value will contain the default value for that object type (0 for numeric types, false for Boolean, DateTime.Min for DateTime)