如何使用C＃在ms访问中插入空值 [英] How to insert null values in ms access using C#
问题描述
使用c＃和Msaccess我只是添加像这样的Mc访问列数据类型的值（文本，数字 - 双精度，文本，数字 - 百分比）
但我插入这个值（john，， ，）这一个值没有插入
这个错误
因为我只有名单listdata
帮帮我
i需要插入空值或提交buttonclick创建新的空行
类型'System.NullReferenceException'的第一次机会异常发生在biodata.exe
附加信息：对象引用未设置为对象的实例。
我尝试过：
private void button1_Click（object sender，EventArgs e）
{
con.Open（）;
cmd.CommandText =插入hhh（name，answerinfloat，subject，avg）VALUES（'+ a.Text +'，'+ b.Text +'，'+ c.Text + '，'+ d.Text +'）;
cmd.CommandType = CommandType.Text;
cmd.Parameters.AddWithValue（name，a.Text）;
cmd.Parameters.AddWithValue（answerinfloat，b.Text）;
cmd.Parameters.AddWithValue（subject，c.Text）;
cmd.Parameters.AddWithValue（avg，d.Text）;
int n = cmd.ExecuteNonQuery（）;
if（n> 0）
{
MessageBox.Show（Record Submitted，Congrats）;
}
else
MessageBox.Show（插入失败）;
con.Close（）;
}
对于初学者，不要这样做！永远不要连接字符串来构建SQL命令。它让您对意外或故意的SQL注入攻击持开放态度，这可能会破坏您的整个数据库。改为使用参数化查询。
连接字符串时会导致问题，因为SQL会收到如下命令：
SELECT * FROM MyTable WHERE StreetAddress = ' Baker' s Wood '就SQL而言，用户添加的引号会终止字符串，并且您会遇到问题。但情况可能更糟。如果我来并改为输入：x'; DROP TABLE MyTable; - 然后SQL收到一个非常不同的命令：
SELECT * FROM MyTable WHERE StreetAddress = ' x'; DROP 表 MyTable; - '哪个SQL看作三个单独的命令：
SELECT * FROM MyTable WHERE StreetAddress = ' x';完全有效的SELECT
DROP TABLE MyTable;完全有效的删除表格通讯和
- '其他一切都是评论。
所以它确实：选择任何匹配的行，从数据库中删除表，并忽略其他任何内容。
所以总是使用参数化查询！或者准备好经常从备份中恢复数据库。你经常定期备份，不是吗？
修复这个问题的可能性 - 在整个代码中 - 也可以解决你的其他问题...
use c# and Msaccess i just add value like this Mc access column data type (text,numeric-double,text,numeric-percentage)
but i insert this values(john,"","","")this one value value not inserted
this error
because i have only name listdata
help me
i need insert null values or submit buttonclick create new empty row
A first chance exception of type 'System.NullReferenceException' occurred in biodata.exe
Additional information: Object reference not set to an instance of an object.
What I have tried:
private void button1_Click(object sender, EventArgs e)
{
con.Open();
cmd.CommandText = "insert into hhh (name,answerinfloat,subject,avg)VALUES('" + a.Text + "','" + b.Text + "','" + c.Text + "','" + d.Text + "')";
cmd.CommandType = CommandType.Text;
cmd.Parameters.AddWithValue("name", a.Text);
cmd.Parameters.AddWithValue("answerinfloat", b.Text);
cmd.Parameters.AddWithValue("subject", c.Text);
cmd.Parameters.AddWithValue("avg", d.Text);
int n = cmd.ExecuteNonQuery();
if (n > 0)
{
MessageBox.Show("Record Submitted", "Congrats");
}
else
MessageBox.Show("insertion failed");
con.Close();
}For starters, don't do it like that! Never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Use Parametrized queries instead.
When you concatenate strings, you cause problems because SQL receives commands like:
SELECT * FROM MyTable WHERE StreetAddress = 'Baker's Wood'The quote the user added terminates the string as far as SQL is concerned and you get problems. But it could be worse. If I come along and type this instead: "x';DROP TABLE MyTable;--" Then SQL receives a very different command:
SELECT * FROM MyTable WHERE StreetAddress = 'x';DROP TABLE MyTable;--'Which SQL sees as three separate commands:
SELECT * FROM MyTable WHERE StreetAddress = 'x';A perfectly valid SELECT
DROP TABLE MyTable;A perfectly valid "delete the table" command
--'And everything else is a comment.
So it does: selects any matching rows, deletes the table from the DB, and ignores anything else.
So ALWAYS use parameterized queries! Or be prepared to restore your DB from backup frequently. You do take backups regularly, don't you?
The chances are that fixing that - throughout the whole of your code - will get rid of your other problem as well...
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