ElementId currentMaterialId = ElementId.InvalidElementId;
Color currentColor = new Color(0,0,0);
double currentTransparency = 0;
Asset currentAppearance = null;

/// <summary>
/// This code demonstrates how to process material information
/// </summary>
/// <remarks>
/// OnMaterial method can be invoked for every single out-coming mesh
/// even when the material has not actually changed. Thus it is usually
/// beneficial to store the current material and only get its attributes
/// when the material actually changes.
/// </remarks>
public void OnMaterial(MaterialNode node)
{
   // acquire properties of the material if it is different
   // than the material we have set as the currently applicable
   if (currentMaterialId != node.MaterialId)
   {
      if (node.MaterialId != ElementId.InvalidElementId)
      {
         currentColor = node.Color;
         currentTransparency = node.Transparency;
      }
      else
      {
         // the default material is being used in this case
      }

      // Appearance Asset is mainly for Revit internal use. However, if it is utilized 
      // in the export context, it needs to be checked whether or not the asset of the 
      // material is overridden by some local modification (e.g. by applying a decal)

      if (node.HasOverriddenAppearance)
      {
         currentAppearance = node.GetAppearanceOverride();
      }
      else
      {
         currentAppearance = node.GetAppearance();
      }
   }
}

Private currentMaterialId As ElementId = ElementId.InvalidElementId
Private currentColor As New Color(0, 0, 0)
Private currentTransparency As Double = 0
Private currentAppearance As Asset = Nothing

' <summary>
' This code demonstrates how to process material information
' </summary>
' <remarks>
' OnMaterial method can be invoked for every single out-coming mesh
' even when the material has not actually changed. Thus it is usually
' beneficial to store the current material and only get its attributes
' when the material actually changes.
' </remarks>
Public Sub OnMaterial(node As MaterialNode) Implements IExportContext.OnMaterial
    ' acquire properties of the material if it is different
    ' than the material we have set as the currently applicable
    If currentMaterialId <> node.MaterialId Then
        If node.MaterialId <> ElementId.InvalidElementId Then
            currentColor = node.Color
            currentTransparency = node.Transparency
            ' the default material is being used in this case
        Else
        End If

        ' Appearance Asset is mainly for Revit internal use. However, if it is utilized 
        ' in the export context, it needs to be checked whether or not the asset of the 
        ' material is overridden by some local modification (e.g. by applying a decal)


        If node.HasOverriddenAppearance Then
            currentAppearance = node.GetAppearanceOverride()
        Else
            currentAppearance = node.GetAppearance()
        End If
    End If
End Sub