运算符“||”不能应用于string和bool类型的操作数

我在运行下面的代码段时遇到问题,并给出错误:

运算符“||”不能应用于“string”和“bool”类型的操作数

    var qisg = new QuoteItemSectionGroup
    {
        SectionGroup = db.SectionGroups.Where(x => x.Name == "Ali Bottom Rail" && x.Section == TruckSection.FrontEndRequirments).First(),
        StockItem = quoteItem.BodyType.Name == "Royal Corrugated" ? db.StockItems.Where(x => x.StockCode == "AEX165").First() : null,
    };
    qisg.Quantity = qisg.StockItem == null ? 0 : 1;
    //Error in the line below
    qisg.Length = qisg.StockItem == null ? 0 : (double)quoteItem.ExternalWidth + quoteItem.BodyType.Name = "Royal Corrugated" || quoteItem.BodyType.Name == "Royal Smooth Glued" || quoteItem.BodyType.Name == "Smooth Folded" || quoteItem.BodyType.Name == "Royal Smooth Riveted" ? -0.136 : -0.020;
    quoteItem.SectionGroups.Add(qisg);

我怎样才能用不同于使用 '||' 操作人员