/// The example below shows how to use Floor.Create method to create a new structural floor (slab) on one level 
/// using a geometry profile and a floor type. 
/// In this sample, the geometry profile is a CurveLoop of lines, you can also use arcs, ellipses and splines.
Floor CreateFloor(Document document, Level level)
{
   // Get a floor type for floor creation
   ElementId floorTypeId = Floor.GetDefaultFloorType(document, false);

   // Build a floor profile for the floor creation
   XYZ first = new XYZ(0, 0, 0);
   XYZ second = new XYZ(20, 0, 0);
   XYZ third = new XYZ(20, 15, 0);
   XYZ fourth = new XYZ(0, 15, 0);
   CurveLoop profile = new CurveLoop();
   profile.Append(Line.CreateBound(first, second));
   profile.Append(Line.CreateBound(second, third));
   profile.Append(Line.CreateBound(third, fourth));
   profile.Append(Line.CreateBound(fourth, first));

   return Floor.Create(document, new List<CurveLoop> { profile }, floorTypeId, level.Id);
}

/// The example below shows how to use Floor.Create method to create a new Floor with specified elevation, on one level 
/// using a geometry profile and a floor type. 
/// It shows how to adapt your code that used NewFloor and NewSlab methods, which are obsolete since 2022.
/// In this sample, the geometry profile is a CurveLoop of lines, you can also use arcs, ellipses and splines.
Floor CreateFloorAtElevation(Document document, double elevation)
{
   // Get a floor type for floor creation
   // You must provide a valid floor type (unlike in now obsolete NewFloor and NewSlab methods).
   ElementId floorTypeId = Floor.GetDefaultFloorType(document, false);

   // Get a level
   // You must provide a valid level (unlike in now obsolete NewFloor and NewSlab methods).
   double offset;
   ElementId levelId = Level.GetNearestLevelId(document, elevation, out offset);

   // Build a floor profile for the floor creation
   XYZ first = new XYZ(0, 0, 0);
   XYZ second = new XYZ(20, 0, 0);
   XYZ third = new XYZ(20, 15, 0);
   XYZ fourth = new XYZ(0, 15, 0);
   CurveLoop profile = new CurveLoop();
   profile.Append(Line.CreateBound(first, second));
   profile.Append(Line.CreateBound(second, third));
   profile.Append(Line.CreateBound(third, fourth));
   profile.Append(Line.CreateBound(fourth, first));

   // The elevation of the curve loops is not taken into account (unlike in now obsolete NewFloor and NewSlab methods).
   // If the default elevation is not what you want, you need to set it explicitly.
   var floor = Floor.Create(document, new List<CurveLoop> { profile }, floorTypeId, levelId);
   Parameter param = floor.get_Parameter(BuiltInParameter.FLOOR_HEIGHTABOVELEVEL_PARAM);
   param.Set(offset);

   return floor;
}

Private Function CreateFloor(application As UIApplication, level As Level) As Floor
   ' Get the Revit document
   Dim document As Autodesk.Revit.DB.Document = application.ActiveUIDocument.Document

   ' Get the application creation object
   Dim appCreation As Autodesk.Revit.Creation.Application = application.Application.Create

   ' Get a floor type for floor creation
   Dim collector As New FilteredElementCollector(document)
   collector.OfClass(GetType(FloorType))
   Dim floorType As FloorType = TryCast(collector.FirstElement(), FloorType)

   ' Build a floor profile for the floor creation
   Dim first As New XYZ(0, 0, 0)
   Dim second As New XYZ(20, 0, 0)
   Dim third As New XYZ(20, 15, 0)
   Dim fourth As New XYZ(0, 15, 0)
   Dim profile As New CurveLoop()
   profile.Append(Line.CreateBound(first, second))
   profile.Append(Line.CreateBound(second, third))
   profile.Append(Line.CreateBound(third, fourth))
   profile.Append(Line.CreateBound(fourth, first))

   Return Floor.Create(document, New List(Of CurveLoop)({profile}), floorType.Id, level.Id)
End Function

Private Function CreateFloorAtElevation(ByVal document As Document, ByVal elevation As Double) As Floor
   ' Get a floor type for floor creation
   ' You must provide a valid floor type (unlike in now obsolete NewFloor and NewSlab methods).
   Dim floorTypeId As ElementId = Autodesk.Revit.DB.Floor.GetDefaultFloorType(document, False)

   ' Get a level
   ' You must provide a valid level (unlike in now obsolete NewFloor and NewSlab methods).
   Dim offset As Double
   Dim levelId = Level.GetNearestLevelId(document, elevation, offset)

   ' Build a floor profile for the floor creation
   Dim first As XYZ = New XYZ(0, 0, 0)
   Dim second As XYZ = New XYZ(20, 0, 0)
   Dim third As XYZ = New XYZ(20, 15, 0)
   Dim fourth As XYZ = New XYZ(0, 15, 0)
   Dim profile As CurveLoop = New CurveLoop()
   profile.Append(Line.CreateBound(first, second))
   profile.Append(Line.CreateBound(second, third))
   profile.Append(Line.CreateBound(third, fourth))
   profile.Append(Line.CreateBound(fourth, first))

   ' The elevation of the curve loops is not taken into account (unlike in now obsolete NewFloor and NewSlab methods).
   ' If the default elevation is not what you want, you need to set it explicitly.
   Dim floor = Autodesk.Revit.DB.Floor.Create(document, New List(Of CurveLoop) From {
             profile
         }, floorTypeId, levelId)
   Dim param As Parameter = floor.Parameter(BuiltInParameter.FLOOR_HEIGHTABOVELEVEL_PARAM)
   param.Set(offset)
   Return floor
End Function