如果默认情况下字符串为空，为什么我无法将类型为“ System.DBNull”的对象转换为类型为“ System.String”的对象？

Question

I have seen similar threads like this where the solution is a tertiary if.

My question is, why even get such an error if Strings are nullable?

I am reading a value of a text column in access using ado.net. Whenever there is a row with an empty text column i get that error.

Culprit:

while (dr.Read())
{
      UserList.Add(new UserInfo()
      {
        .
          DestributionGroup = (string)dr["Destribution Group"]
        .
      }
}
class UserInfo
{
.
    public string DestributionGroup;
.
}

Edit:

So in other words I have to convert all of my strings that I am reading from the DB into a line similar to this?

return (accountNumber == DBNull.Value) ? string.Empty : accountNumber.ToString ()

No other way around it?

Answer 1

Because null != DBNull.Value .

But you can check if it the value in the DataReader is null with the IsDbNull method:

DestributionGroup = dr.IsDbNull("Destribution Group") ? "" : dr.GetString("Destribution Group");

Answer 2

如果您不介意在dr [“ Destribution Group”]为DBNull.Value时将DestributionGroup设置为null，则可以使用此方法：

DestributionGroup = dr["Destribution Group"] as string;