sqlfacet problem with clr funtion

Hello, I have a litte problem with the sqlfacet attribute. See function below. The problem is, that it does not compile. It marks the sqlfacet attribute as error. The error message is the following (translated from german)

"The attribute SQLFacetAttribute can not be used on Hourdiff, because the attribute is not valid for this type of declaration"

Though it all works with datatype double, I would like to use SQLDecimal, but everytime I call this function without sqlfacet, it will round the return values.

<Microsoft.SqlServer.Server.SqlFunction(), SqlFacet(Precision:=10, Scale:=2)> _
Public Shared Function HourDiff(ByVal [Date1] As SqlDateTime, ByVal [Date2] As SqlDateTime) As SqlDecimal
  Dim d As SqlDecimal
     If Date1.IsNull Or Date2.IsNull Then
        d = 0
     Else
        Try
           d = CType((CType(Date2, DateTime) - CType(Date1, DateTime)).TotalHours, SqlDecimal)
        Catch ex As Exception
           Throw ex
        Finally
           If d < 0 Then
               Throw New Exception("Function HourDiff has detected a negative time span")
           End If
        End Try
     End If
     Return d
End Function

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