C#操作WORD设置底纹无效

Question

 if (doc.Application.Selection.Font.Shading.Texture == Word.WdTextureIndex.wdTextureNone)
            {
                doc.Application.Selection.Font.Shading.Texture = Word.WdTextureIndex.wdTexture20Percent;
                doc.Application.Selection.Font.Shading.ForegroundPatternColor = Word.WdColor.wdColorBlack;
                doc.Application.Selection.Font.Shading.BackgroundPatternColor = Word.WdColor.wdColorWhite;
            }
            else
            {
                doc.Application.Selection.Font.Shading.Texture = Word.WdTextureIndex.wdTextureNone;
            }
            doc.Application.Selection.Range.Text += "感言：\n";
录制的VBA,设置底纹无效