FOR XML support for the user-defined data types (UDT)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
FOR XML doesn't support common language runtime (CLR) user-defined data types (UDTs).
To use FOR XML with CLR user-defined data types, make sure that the data type has an XML serialization, and use an explicit cast to XML in the FOR XML select clause.
See also
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