C# SqlDataReader.GetString 和 sqlnullvalueexception
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SqlDataReader.GetString and sqlnullvalueexception
提问by Rejeev Divakaran
I am new to C#. I was executing some select queries from database tables using System.Data.SqlClient classes. I got sqlnullvalueexception while executing some select query. On googling I come to know that if the value is null in the database, SqlDataReader.GetString (or it's variants) will throw sqlnullvalueexception. What is the best coding practice for this?
我是 C# 的新手。我正在使用 System.Data.SqlClient 类从数据库表中执行一些选择查询。我在执行一些选择查询时遇到了 sqlnullvalueexception。在谷歌搜索中，我知道如果数据库中的值为 null，SqlDataReader.GetString（或其变体）将抛出 sqlnullvalueexception。最好的编码实践是什么？
if (!sqlDataReader.IsDBNull(n)) value = r.GetString(n);
Any better way of coding?
有没有更好的编码方式？
采纳答案by Tommy Carlier
If you don't want to repeat this a lot, just create a helper function, like this:
如果您不想重复很多，只需创建一个辅助函数，如下所示：
public static class DataReaderExtensions
{
public static string GetStringOrNull(this IDataReader reader, int ordinal)
{
return reader.IsDBNull(ordinal) ? null : reader.GetString(ordinal);
}
public static string GetStringOrNull(this IDataReader reader, string columnName)
{
return reader.GetStringOrNull(reader.GetOrdinal(columnName));
}
}
Which you can call like this:
你可以这样调用：
value = reader.GetStringOrNull(n);
回答by Andrew Hare
That really is the best way to go about it if you wish to avoid any exceptions. You need to decide whether or not a null field represents an exceptional situation in your code - if it doesn't then use this method. If it does then I would suggest that you either allow the exception to be thrown or catch the exception and wrap it in a more meaniful exception and throw that one.
如果您希望避免任何异常，那确实是最好的方法。您需要确定 null 字段是否表示代码中的异常情况 - 如果没有，则使用此方法。如果是这样，那么我建议您要么允许抛出异常，要么捕获异常并将其包装在一个更有意义的异常中并抛出该异常。
But the main thing to know is that this is the standard way to retrieve values from a data reader when a null field does not represent an exceptional situation in the application domain.
但要知道的主要事情是，当空字段不代表应用程序域中的异常情况时，这是从数据读取器检索值的标准方法。
回答by Thomas Levesque
The code you posted is fine. You could also do something like that :
你贴的代码没问题。你也可以这样做：
value = r[n] as string;
If the value in the database is null, r[n]will return DBNull.Value, and the cast to stringwill return null.
如果数据库中的值为空，r[n]将返回 DBNull.Value，并且强制转换string将返回null。
回答by drzymala
This worked for me:
这对我有用：
value = reader.GetValue(n).ToString();
