public void GetWallLayerMaterial(Autodesk.Revit.DB.Document document, Wall wall)
{
    // get WallType of wall
    WallType aWallType = wall.WallType;
    // Only Basic Wall has compoundStructure
    if (WallKind.Basic == aWallType.Kind)
    {
        // Get CompoundStructure
        CompoundStructure comStruct = aWallType.GetCompoundStructure();
        Categories allCategories = document.Settings.Categories;

        // Get the category OST_Walls default Material; 
        // use if that layer's default Material is <By Category>
        Category wallCategory = allCategories.get_Item(BuiltInCategory.OST_Walls);
        Autodesk.Revit.DB.Material wallMaterial = wallCategory.Material;

        foreach (CompoundStructureLayer structLayer in comStruct.GetLayers())
        {
            Autodesk.Revit.DB.Material layerMaterial = document.GetElement(structLayer.MaterialId) as Material;

            // If CompoundStructureLayer's Material is specified, use default
            // Material of its Category
            if (null == layerMaterial)
            {
                switch (structLayer.Function)
                {
                    case MaterialFunctionAssignment.Finish1:
                        layerMaterial = 
                            allCategories.get_Item(BuiltInCategory.OST_WallsFinish1).Material;
                        break;
                    case MaterialFunctionAssignment.Finish2:
                        layerMaterial = 
                            allCategories.get_Item(BuiltInCategory.OST_WallsFinish2).Material;
                        break;
                    case MaterialFunctionAssignment.Membrane:
                        layerMaterial = 
                            allCategories.get_Item(BuiltInCategory.OST_WallsMembrane).Material;
                        break;
                    case MaterialFunctionAssignment.Structure:
                        layerMaterial =  
                            allCategories.get_Item(BuiltInCategory.OST_WallsStructure).Material;
                        break;
                    case MaterialFunctionAssignment.Substrate:
                        layerMaterial = 
                            allCategories.get_Item(BuiltInCategory.OST_WallsSubstrate).Material;
                        break;
                    case MaterialFunctionAssignment.Insulation:
                        layerMaterial = 
                            allCategories.get_Item(BuiltInCategory.OST_WallsInsulation).Material;
                        break;
                    default:
                        // It is impossible to reach here
                        break;
                }
                if (null == layerMaterial)
                {
                    // CompoundStructureLayer's default Material is its SubCategory
                    layerMaterial = wallMaterial;
                }
            }
            TaskDialog.Show("Revit","Layer Material: " + layerMaterial);
        }
    }
}

Public Sub GetWallLayerMaterial(document As Autodesk.Revit.DB.Document, wall As Wall)
    ' get WallType of wall
    Dim aWallType As WallType = wall.WallType
    ' Only Basic Wall has compoundStructure
    If WallKind.Basic = aWallType.Kind Then
        ' Get CompoundStructure
        Dim comStruct As CompoundStructure = aWallType.GetCompoundStructure()
        Dim allCategories As Categories = document.Settings.Categories

        ' Get the category OST_Walls default Material; 
        ' use if that layer's default Material is <By Category>
        Dim wallCategory As Category = allCategories.Item(BuiltInCategory.OST_Walls)
        Dim wallMaterial As Autodesk.Revit.DB.Material = wallCategory.Material

        For Each structLayer As CompoundStructureLayer In comStruct.GetLayers()
            Dim layerMaterial As Autodesk.Revit.DB.Material = TryCast(document.GetElement(structLayer.MaterialId), Material)

            ' If CompoundStructureLayer's Material is specified, use default
            ' Material of its Category
            If layerMaterial Is Nothing Then
                Select Case structLayer.[Function]
                    Case MaterialFunctionAssignment.Finish1
                        layerMaterial = allCategories.Item(BuiltInCategory.OST_WallsFinish1).Material
                        Exit Select
                    Case MaterialFunctionAssignment.Finish2
                        layerMaterial = allCategories.Item(BuiltInCategory.OST_WallsFinish2).Material
                        Exit Select
                    Case MaterialFunctionAssignment.Membrane
                        layerMaterial = allCategories.Item(BuiltInCategory.OST_WallsMembrane).Material
                        Exit Select
                    Case MaterialFunctionAssignment.[Structure]
                        layerMaterial = allCategories.Item(BuiltInCategory.OST_WallsStructure).Material
                        Exit Select
                    Case MaterialFunctionAssignment.Substrate
                        layerMaterial = allCategories.Item(BuiltInCategory.OST_WallsSubstrate).Material
                        Exit Select
                    Case MaterialFunctionAssignment.Insulation
                        layerMaterial = allCategories.Item(BuiltInCategory.OST_WallsInsulation).Material
                        Exit Select
                    Case Else
                        ' It is impossible to reach here
                        Exit Select
                End Select
                If layerMaterial Is Nothing Then
                    ' CompoundStructureLayer's default Material is its SubCategory
                    layerMaterial = wallMaterial
                End If
            End If
            TaskDialog.Show("Revit", "Layer Material: " & layerMaterial.ToString())
        Next
    End If
End Sub