怎么赋空值的有关问题

如何赋空值的问题
相关代码如下：
 

在Models中定义如下：
public Nullable<double> WATERDEW_POINT {
get { return wATERDEW_POINT; }
set { wATERDEW_POINT = value; }
}

现在我txtWaterDew_point中不录入值则报错
 Models.CHECKDATA_GASELEMENT mygas = new Models.CHECKDATA_GASELEMENT();

mygas.WATERDEW_POINT = txtWaterDew_point.Text.Trim() == "" ? Convert.ToDouble("") : Convert.ToDouble(txtWaterDew_point.Text.Trim());



[解决办法]
这样写
mygas.WATERDEW_POINT = txtWaterDew_point.Text.Trim() == "" ? (double?)null : Convert.ToDouble(txtWaterDew_point.Text.Trim());