这是一个后续问题.Net C＃的string.join如何输出零，而不是空字符串，如果元素值为null？如果答案建议使用 ?? 运营商定义自定义的空值，但更换从未被触发。

This is a follow-up question to .Net C# String.Join how to output "null" instead of empty string if element value is null? where the answer suggested to use the ?? operator to define a custom null value, but the replacement never got triggered.

DataSet myDataSet = new DataSet();
mySqlDataAdapter.Fill(myDataSet);
DataTable rotationData = myDataSet.Tables["Table"];

rotationValues = string.Join(", ",
    from r in rotationData.Rows.OfType<DataRow>()
    select r[5] ?? "null");

当我改变code到：

rotationValues = string.Join(", ",
    from r in rotationData.Rows.OfType<DataRow>()
    select r[5].GetType());

我可以观察到，对于具有有效数据在他们的元素数据类型是 System.Double ，而对于它们是空的元素数据类型是 System.DBNull 。请问 ?? 不是 System.DBNull ？

I can observe that the data type for elements that have valid data in them is System.Double whereas the data type for elements which are NULL is System.DBNull. Does ?? not operate on System.DBNull?

推荐答案

没有的DBNull 和空不是一样。 但是你可以这样写：

No DBNull and null is not the same thing. But you can write this instead:

select r[5] == DBNull.Value ? "null" : r[5]);

另一种可能性是使用字段扩展方法，它会给你指定行中每个列值的强类型的访问，而 ?? 操作员工作。

Another possibility is to use Field extension method, it will give you strongly-typed access to each of the column values in the specified row, and the ?? operator will work.

select r.Field<string>(5) ?? "null";