Documentation error?

For the four-parameter overload, you state an XmlException is thrown if "The localName or ns is null."

Yet in your example you are passing null as ns, and indeed it does not throw an exception.

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* ID: e399c951-620f-6fe0-d81f-399661484c11
* Version Independent ID: cd7d83f9-159d-da2f-4d41-909b24309a9c
* Content: [XmlWriter.WriteAttributeString Method (System.Xml)](https://docs.microsoft.com/en-us/dotnet/api/system.xml.xmlwriter.writeattributestring?view=netframework-4.7.2#System_Xml_XmlWriter_WriteAttributeString_System_String_System_String_System_String_System_String_)
* Content Source: [xml/System.Xml/XmlWriter.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Xml/XmlWriter.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**