public void ApplyPaintByMaterialParameter(Document document, Extrusion extrusion, Material material)
{
   Face geoFace = null;

   using (Transaction transaction = new Transaction(document, "Painting"))
   {
      transaction.Start();

      // create a new type and material parameter in the document.
      FamilyManager famMgr = document.FamilyManager;
      if (famMgr.Types.Size == 0)
      {
         FamilyType createdType = famMgr.NewType("New_type");
         famMgr.CurrentType = createdType;
      }

      FamilyParameter materialParam =
         famMgr.AddParameter("Material_Para", BuiltInParameterGroup.PG_MATERIALS, ParameterType.Material, true);

      famMgr.Set(materialParam, material.Id);

      // Before acquiring the geometry, make sure the detail level is set to 'Fine'
      Options geoOptions = new Options();
      geoOptions.DetailLevel = ViewDetailLevel.Fine;

      // Find the first geometry face of the given extrusion object
      GeometryElement geoElem = extrusion.get_Geometry(geoOptions);
      IEnumerator<GeometryObject> geoObjectItor = geoElem.GetEnumerator();
      while (geoObjectItor.MoveNext())
      {
         // need to find a solid first
         Solid theSolid = geoObjectItor.Current as Solid;
         if (null != theSolid)
         {
            foreach (Face face in theSolid.Faces)
            {
               geoFace = face;
               break;
            }
         }
      }

      if (null == geoFace)
      {
         TaskDialog.Show("Failure", "Could not find a face to paint.");
         transaction.RollBack();
         return;
      }

      // Paint a material family parameter to the extrusion face.
      document.Paint(extrusion.Id, geoFace, materialParam);
      transaction.Commit();
   }

   // For illustration purposes only, check if the painted material indeed got applied

   bool isPainted = document.IsPainted(extrusion.Id, geoFace);
   if (isPainted)
   {
      ElementId paintedMatId = document.GetPaintedMaterial(extrusion.Id, geoFace);
      if (paintedMatId == material.Id)
      {
         TaskDialog.Show("Painting material", "Face painted successfully.");
      }
   }
}

Public Sub ApplyPaintByMaterialParameter(document As Document, extrusion As Extrusion, material As Material)
    Dim geoFace As Face = Nothing

    Using transaction As New Transaction(document, "Painting")
        transaction.Start()

        ' create a new type and material parameter in the document.
        Dim famMgr As FamilyManager = document.FamilyManager
        If famMgr.Types.Size = 0 Then
            Dim createdType As FamilyType = famMgr.NewType("New_type")
            famMgr.CurrentType = createdType
        End If

        Dim materialParam As FamilyParameter = famMgr.AddParameter("Material_Para", BuiltInParameterGroup.PG_MATERIALS, ParameterType.Material, True)

        famMgr.[Set](materialParam, material.Id)

        ' Before acquiring the geometry, make sure the detail level is set to 'Fine'
        Dim geoOptions As New Options()
        geoOptions.DetailLevel = ViewDetailLevel.Fine

        ' Find the first geometry face of the given extrusion object
        Dim geoElem As GeometryElement = extrusion.Geometry(geoOptions)
        Dim geoObjectItor As IEnumerator(Of GeometryObject) = geoElem.GetEnumerator()
        While geoObjectItor.MoveNext()
            ' need to find a solid first
            Dim theSolid As Solid = TryCast(geoObjectItor.Current, Solid)
            If theSolid IsNot Nothing Then
                For Each face As Face In theSolid.Faces
                    geoFace = face
                    Exit For
                Next
            End If
        End While

        If geoFace Is Nothing Then
            TaskDialog.Show("Failure", "Could not find a face to paint.")
            transaction.RollBack()
            Return
        End If

        ' Paint a material family parameter to the extrusion face.
        document.Paint(extrusion.Id, geoFace, materialParam)
        transaction.Commit()
    End Using

    ' For illustration purposes only, check if the painted material indeed got applied


    Dim isPainted As Boolean = document.IsPainted(extrusion.Id, geoFace)
    If isPainted Then
        Dim paintedMatId As ElementId = document.GetPaintedMaterial(extrusion.Id, geoFace)
        If paintedMatId = material.Id Then
            TaskDialog.Show("Painting material", "Face painted successfully.")
        End If
    End If
End Sub