private void ReadMaterialAppearanceProp(Document document, Material material)
{
   ElementId appearanceId = material.AppearanceAssetId;
   AppearanceAssetElement appearanceElem = document.GetElement(appearanceId) as AppearanceAssetElement;
   Asset theAsset = appearanceElem.GetRenderingAsset();
   string libraryName = theAsset.LibraryName;
   string title = theAsset.Title;

   // The predefined asset can be empty. at this time, get the system appearance asset instead.
   if (theAsset.Size == 0)
   {
      AssetSet systemAppearanceAssets = document.Application.get_Assets(AssetType.Appearance);
      foreach (Asset systemAsset in systemAppearanceAssets)
      {
         if(theAsset.LibraryName == systemAsset.LibraryName
            && theAsset.Name == systemAsset.Name)
         {
            // Read the Asset information here.
            break;
         }
      }
   }
}

Private Sub ReadMaterialAppearanceProp(document As Document, material As Material)
    Dim appearanceId As ElementId = material.AppearanceAssetId
    Dim appearanceElem As AppearanceAssetElement = TryCast(document.GetElement(appearanceId), AppearanceAssetElement)
    Dim theAsset As Asset = appearanceElem.GetRenderingAsset()
    Dim libraryName As String = theAsset.LibraryName
    Dim title As String = theAsset.Title

    ' The predefined asset can be empty. at this time, get the system appearance asset instead.
    If theAsset.Size = 0 Then
        Dim systemAppearanceAssets As AssetSet = document.Application.Assets(AssetType.Appearance)
        For Each systemAsset As Asset In systemAppearanceAssets
            If theAsset.LibraryName = systemAsset.LibraryName AndAlso theAsset.Name = systemAsset.Name Then
                ' Read the Asset information here.
                Exit For
            End If
        Next
    End If
End Sub