[英]how to check if a datareader is null or empty
I have a datareader that return a lsit of records from a sql server database. 我有一个datareader从sql server数据库返回一个lsit记录。 I have a field in the database called "Additional".
我在数据库中有一个名为“Additional”的字段。 This field is 50% of the time empty or null.
此字段为空或空的50%。
I am trying to write code that checks if this field isnull. 我正在尝试编写检查此字段是否为空的代码。 The logic behind this is: If the field "Additional" contains text then display the info otherwise hide the field.
这背后的逻辑是:如果字段“附加”包含文本,则显示信息,否则隐藏字段。
I have tried: 我试过了:
if (myReader["Additional"] != null)
{
ltlAdditional.Text = "contains data";
}
else
{
ltlAdditional.Text = "is null";
}
The above code gives me this error: 上面的代码给了我这个错误:
Exception Details: System.IndexOutOfRangeException: Additional 异常详细信息: System.IndexOutOfRangeException:Additional
Any help would be greatly appreciated... 任何帮助将不胜感激...
Check for column name in a SqlDataReader object
检查SqlDataReader对象中的列名称
if (myReader["Additional"] != DBNull.Value)
{
ltlAdditional.Text = "contains data";
}
else
{
ltlAdditional.Text = "is null";
}
if (myReader.HasRows) //The key Word is **.HasRows**
{
ltlAdditional.Text = "Contains data";
}
else
{
ltlAdditional.Text = "Is null Or Empty";
}
I haven't used DataReaders for 3+ years, so I wanted to confirm my memory and found this. 我没有使用DataReaders 3年多,所以我想确认我的记忆并找到了这个。 Anyway, for anyone who happens upon this post like I did and wants a method to test IsDBNull using the column name instead of ordinal number, and you are using VS 2008+ (& .NET 3.5 I think), you can write an extension method so that you can pass the column name in:
无论如何,对于像我这样发帖的人,想要一个方法来测试使用列名而不是序号的IsDBNull,并且你使用的是VS 2008+(我认为是.NET 3.5),你可以编写一个扩展方法这样您就可以传递列名:
public static class DataReaderExtensions
{
public static bool IsDBNull( this IDataReader dataReader, string columnName )
{
return dataReader[columnName] == DBNull.Value;
}
}
Kevin 凯文
This is the correct and tested solution 这是正确且经过测试的解决方案
if (myReader.Read())
{
ltlAdditional.Text = "Contains data";
}
else
{
ltlAdditional.Text = "Is null";
}
I also use OleDbDataReader.IsDBNull() 我也使用OleDbDataReader.IsDBNull()
if ( myReader.IsDBNull(colNum) ) { retrievedValue = ""; }
else { retrievedValue = myReader.GetString(colNum); }
First of all, you probably want to check for a DBNull
not a regular Null
. 首先,您可能想要检查
DBNull
而不是常规Null
。
Or you could look at the IsDBNull
method 或者您可以查看
IsDBNull
方法
@Joe Philllips @Joe Philllips
SQlDataReader.IsDBNull(int index) requires the ordinal number of the column. SQlDataReader.IsDBNull(int index)需要列的序号。 Is there a way to check for nulls using Column Name, and not it's Ordinal Number?
有没有办法使用列名检查空值,而不是它的序号?
In addition to the suggestions given, you can do this directly from your query like this - 除了给出的建议,您可以直接从您的查询中执行此操作 -
SELECT ISNULL([Additional], -1) AS [Additional]
This way you can write the condition to check whether the field value is < 0 or >= 0. 这样您就可以编写条件来检查字段值是否为<0或> = 0。
This 这个
Example: 例:
objCar.StrDescription = (objSqlDataReader["fieldDescription"].GetType() != typeof(DBNull)) ? (String)objSqlDataReader["fieldDescription"] : "";
试试这个更简单的等价语法:
ltlAdditional.Text = (myReader["Additional"] == DBNull.Value) ? "is null" : "contains data";
AMG - Sorry all, was having a blond moment. AMG - 对不起,有一个金发碧眼的时刻。 The field "Additional" was added to the database after I had initially designed the database.
在我最初设计数据库之后,“Additional”字段被添加到数据库中。
I updated all my code to use this new field, however I forgot to update the actual datareader code that was making the call to select the database fields, therefore it wasn't calling "Additional" 我更新了所有代码以使用这个新字段,但是我忘了更新正在调用的实际datareader代码来选择数据库字段,因此它没有调用“Additional”
I also experiencing this kind of problem but mine, i'm using DbDataReader as my generic reader (for SQL, Oracle, OleDb, etc.). 我也遇到过这种问题,但我的,我正在使用DbDataReader作为我的通用阅读器(对于SQL,Oracle,OleDb等)。 If using DataTable, DataTable has this method:
如果使用DataTable,DataTable有这个方法:
DataTable dt = new DataTable();
dt.Rows[0].Table.Columns.Contains("SampleColumn");
using this I can determine if that column is existing in the result set that my query has. 使用这个我可以确定该列是否存在于我的查询所具有的结果集中。 I'm also looking if DbDataReader has this capability.
我也在寻找DbDataReader是否具备此功能。
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