DataReader对象引用未设置错误
本文关键字：错误 设置 对象引用 DataReader | 更新日期: 2023-09-27 18:16:45
我做了一个自动发送一些文件的应用程序。现在的问题是，我在另一个数据库(sdf-file)中有电子邮件地址。
我想搜索"filenaam"的值，它可以是类似"q1869"的值。该变量必须在数据库(automail.sdf)中搜索。当里面有"q1869"的时候。然后在第二个字段中获取该行的值，在我的例子中是"email"。
现在我尝试了很多东西。但是我现在碰到了一个问题。
当我运行这段代码时，我在string emailaddress = getemail.ExecuteScalar().ToString();
System.Windows.Forms.Timer mytimer = new System.Windows.Forms.Timer();
//Automail mail = new Automail();
public Form1()
{
InitializeComponent();
//timer aanmaken, interval instellen en aanzetten + gegevens in datatable laden
mytimer.Tick +=mytimer_Tick;
mytimer.Interval = 10000;
Methods methods = new Methods();
Populate();
}
public OdbcConnection con = new OdbcConnection("Driver={iSeries Access ODBC Driver};uid=AYISHB;system=NLPROD;dbq=MMASHB MMASHB;dftpkglib=QGPL;languageid=ENU;pkg=QGPL/DEFAULT(IBM),2,0,1,0,512;qrystglmt=-1;signon=1;trace=2");
public iDB2Connection conn = new iDB2Connection("DataSource=NLPROD;UserID=AYISHB;Password=AYI;DataCompression=True;Default Collection=mmashb;");
public SqlCeConnection dbcon = new SqlCeConnection(@"Data Source=E:'Users'Ali'Documents'automail.sdf");
SmtpClient SmtpServer = new SmtpClient("smtp.dsv.com"); // smtp client maken
int aantalmails = 0;
public DataTable dt = new DataTable();
///////////////////////////methodes//////////////////////////
//Vul dataGridView1 methode
public void Populate()
{
dt.Clear();
OdbcCommand cm = new OdbcCommand("SELECT * FROM SELECTIE ORDER BY OMSCH",con); //querycommand om gegevens te openen
OdbcDataAdapter da = new OdbcDataAdapter();
da.SelectCommand = cm; // data adapter command aanmaken
da.Fill(dt); // datatable vullen met de data
}
public void sendMail()
{
while (true)
{
Populate();
if (dt.Rows.Count >= 1)
{
for (int x = dt.Rows.Count - 1; x >= 0; --x)
{
int i = x; //i is aantal rijen
int r = 1;
string query = dt.Rows[i][r].ToString();// zet de querynummer om in een string
string filenaam = query.Trim(); // haal de overtollige spaties uit de naam en filenaam is de querynmr
SqlCeCommand getemail = new SqlCeCommand("SELECT email" + " FROM Emails" + " WHERE (query LIKE @querynaam)" , dbcon);
getemail.Parameters.AddWithValue("@querynaam", filenaam);
string emailaddress = getemail.ExecuteScalar().ToString();
MessageBox.Show(emailaddress);
//try
//{
//}
//catch (Exception ex)
//{
// MessageBox.Show(ex.Message);
//}
System.Diagnostics.Process.Start(@"M:'dtf'" + filenaam + ".dtf"); // start de dtf bestand met de zelfde querynaam op de M schijf
Thread.Sleep(15000); // wacht 15 seconden
try
{
MailMessage mail = new MailMessage();//
mail.From = new MailAddress("ali.yilmaz@nl.dsv.com");
mail.To.Add(emailaddress.ToString());
mail.Subject = "Report " + filenaam + " DSV REPORT";
mail.Body = "Dear Customer,'nthis message is an automated mail sent by an unattended server. 'nThe attachment included in this mail is: " + filenaam + "'nPlease do not reply to this email 'n 'nThis mail has been sent on " + string.Format("{0:HH:mm:ss:tt yyyy-MM-dd}", DateTime.Now);
System.Net.Mail.Attachment attachment;
attachment = new System.Net.Mail.Attachment(@"M:/" + filenaam + ".xls");
mail.Attachments.Add(attachment);
SmtpServer.Credentials = new System.Net.NetworkCredential("ali.yilmaz@nl.dsv.com", "Uran1234");
SmtpServer.Send(mail);
mail.Dispose();
Thread.Sleep(10000);
string sourcefile = @"M:'" + filenaam + ".xls";
string destinationfile = @"M:'verzondenreports'" + filenaam + ".xls";
if (System.IO.File.Exists(@"M:'verzondenreports'" + filenaam + ".xls"))
{
System.IO.File.Delete(@"M:'verzondenreports'" + filenaam + ".xls");
System.IO.File.Move(sourcefile, destinationfile);
}
else
{
System.IO.File.Move(sourcefile, destinationfile);
}
}
catch (Exception ex)
{
MessageBox.Show(ex.Message);
}
Thread.Sleep(2000);
try
{
iDB2DataAdapter data = new iDB2DataAdapter("SELECT FROM SELECTIE ORDER BY OMSCH", conn);
data.DeleteCommand = new iDB2Command("DELETE FROM SELECTIE WHERE" + filenaam, conn);
data.DeleteCommand.ExecuteNonQuery();
}
catch (Exception ex)
{
MessageBox.Show(ex.Message);
}
if (dt.Rows.Count > 0)
{
dt.Rows[i].Delete();
}
else
{
return;
}
dt.AcceptChanges();
filenaam = "";
}
dt.Clear();
}
else
{
dt.Clear();
Populate();
}
}
}
///////////////////////////methodes//////////////////////////
//wanneer form word opgestart
private void Form1_Shown(object sender, EventArgs e)
{
try
{
//open connecties
con.Open();
conn.Open();
dbcon.Open();
//vul de tabel met de gegevens
Populate();
}
catch(iDB2Exception ex)
{
// als connectie niet lukt weergeef foutmelding
MessageBox.Show(ex.Message);
}
}
private void dataGridView1_CellContentClick(object sender, DataGridViewCellEventArgs e)
{
}
private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
con.Close();
conn.Close();
}
private void button1_Click(object sender, EventArgs e)
{
label2.Text = ("Running...");
mytimer.Enabled = true;
Thread t = GetT();
t.Start();
//Populate();
//if (dataGridView1.Rows.Count > 1)
//{
// int count = dataGridView1.Rows.Count;
// for (int x = 0; x <= dataGridView1.Rows.Count; x++)
// {
// int i = 0;
// int r = 1;
// string number = dataGridView1.Rows[i].Cells[r].Value.ToString();
// string filenaam = number.Trim();
// textBox1.Text = filenaam;
// System.Diagnostics.Process.Start(@"M:'dtf'" + filenaam + ".dtf");
// i = i++;
// Thread.Sleep(100);
// }
// //OdbcCommand cm = new OdbcCommand("DELETE FROM SELECTIE WHERE *", con);
// //OdbcDataAdapter da = new OdbcDataAdapter();
// //da.DeleteCommand = cm;
// iDB2DataAdapter data = new iDB2DataAdapter("SELECT FROM SELECTIE ORDER BY OMSCH", conn);
// conn.Open();
// data.DeleteCommand = new iDB2Command("DELETE FROM SELECTIE", conn);
// data.DeleteCommand.ExecuteNonQuery();
//}
//else
//{
// Populate();
//}
//conn.Close();
//Populate();
}
private Thread GetT()
{
Thread t = new Thread(new ThreadStart(sendMail));
return t;
}
void mytimer_Tick(object sender, EventArgs e)
{
//sendMail();
}
private void btnStop_Click(object sender, EventArgs e)
{
Thread t = GetT();
t.Abort();
label2.Text = ("Stopped...");
mytimer.Enabled = false;
}
private void label2_Click(object sender, EventArgs e)
{
}
private void Form1_Load(object sender, EventArgs e)
{
}
如果没有结果，getemail.ExecuteScalar()将返回null，在这种情况下，getemail.ExecuteScalar().ToString()将抛出空引用异常。
你需要检查，像这样:
var result = getemail.ExecuteScalar();
if (result != null)
{
string emailaddress = result.ToString();
// Etc
如果您执行var result = getemail.ExecuteScalar();，您将看到result是null，因此您不能在其上调用ToString()。您必须检查返回值，看看它是否可用。
您的SQL查询可能是错误的，因此，ExecuteScalar返回null。你不能在null上使用ToString方法。验证您的查询将返回一个结果，我建议您在应用ToString之前从ExecuteScalar测试您的结果为null。
可以这样使用:
var emailaddress = getemail.ExecuteScalar();
if (emailaddress != null) {
result = emailaddress.ToString();
}
getemail.ExecuteScalar()返回null。因此，需要在if块中检查条件
您可以按如下操作
emailaddress = Convert.ToString(getemail.ExecuteScalar());
你可以继续
if(!String.IsNullOrEmpty(emailaddress))
{
// do something with email
}
SqlCeCommand getemail = new SqlCeCommand("SELECT email FROM Emails WHERE query LIKE @querynaam" , dbcon);
getemail.Parameters.AddWithValue("@querynaam","%" + filenaam + "%");
试试下面这行代码来代替getemail.ExecuteScalar().ToString()
Convert.ToString(getemail.ExecuteScalar())
Convert.ToString()方法将为你处理null，如果值是非空值，它将返回字符串，如果它返回null，它将简单地返回一个空字符串。
谢谢