I've got this code block:
using (SqlConnection con2 = new SqlConnection(str2))
{
using (SqlCommand cmd2 = new SqlCommand(#"SELECT * FROM VW_MOS_DPL_AccountValidation WHERE CUST_NUM = #CNum", con2))
{
con2.Open();
cmd2.Parameters.AddWithValue("#CNum", TBAccountNum.Text);
using (SqlDataReader DT2 = cmd2.ExecuteReader())
{
// If the SQL returns any records, process the info
if (DT2.HasRows)
{
// If there's a BusinessID (aka Business Type), fill it in
string BizID = (DT2["Business_ID"].ToString());
if (!string.IsNullOrEmpty(BizID))
{
DDLBustype.SelectedValue = BizID;
}
}
}
con2.Close();
}
}
When it gets to the line
string BizID = (DT2["Business_ID"].ToString());
it throws an error:
Invalid attempt to read when no data is present.
Why would it get past if (DT2.HasRows) if there was no data?
You need to call
if(DT2.Read())
....
before proceding to read data from a DataReader.
The HasRows tells you only that the SqlDataReader contains data, but the SqlDataReader loads one record at time from the connection. Thus every tentative to extract the data from the SqlDataReader should be preceded by a call to Read to position the SqlDataReader on the first record returned through the connection.
And, because the Read method returns true if the call has been able to read a record, you could replace the call to HasRows with something like this
using (SqlDataReader DT2 = cmd2.ExecuteReader())
{
// If the SQL returns any records, process the info
while(DT2.Read())
{
// If there's a BusinessID (aka Business Type), fill it in
string BizID = (DT2["Business_ID"].ToString());
if (!string.IsNullOrEmpty(BizID))
{
DDLBustype.SelectedValue = BizID;
}
}
}
By the way, if it is possible to have a NULL for BusinessID then you need a different test to avoid exception problems
int bizColIndex = DT2.GetOrdinal("Business_ID");
string BizID = (DT2.IsDBNull(bizColIndex) ? string.Empty : DT2.GetString(bizColIndex));
if (!string.IsNullOrEmpty(BizID))
{
DDLBustype.SelectedValue = BizID;
}
Related
Can someone tell my why my expectedNumber reader throws an error
The name reader does not exist in its current context
As far as I can see all this is doing is reading the first row and first column, don't understand why the reader is throwing a tantrum.
It doesn't like the line:
ExpectedNumber = reader.GetInt16(0);
The query is :
SELECT TOP (1) [ExpectedNumber]
FROM [dbo].[MyDatabase]
WHERE id = '{0}'
Code:
try
{
using (SqlCommand cmd = new SqlCommand(string.Format(Query, id), Connection))
{
Connection.Open();
using (SqlDataReader reader = cmd.ExecuteReader())
{
// Check is the reader has any rows at all before starting to read.
if (reader.HasRows)
{
int ExpectedNumber = 0;
// Read advances to the next row.
while (reader.Read() == true)
{
// To avoid unexpected bugs access columns by name.
ExpectedNumber = reader.GetInt16(0);
}
Connection.Close();
return ExpectedResult;
}
Assert.Fail("No results returned from expected result query");
return 0;
}
}
}
catch (Exception e)
{
Connection.Close();
throw;
}
You should escape your query parameters, otherwise your code is vulnerable to SQL injection attacks, also, by using command parameters as in the example below you can make sure you are using the right data type (it seems you are trying to pass an int id as a string).
You are just trying to get one value so you don't need to use a reader and can use ExecuteScalar instead.
Finally, you don't need to handle closing the connection if you enclose it in a using block so you can avoid the try catch block as well.
string query = "SELECT TOP (1) [ExpectedNumber] FROM [dbo].[MyDatabase] WHERE id = #id";
using (var connection = new SqlConnection("connStr"))
{
connection.Open();
using (var cmd = new SqlCommand(query, connection))
{
cmd.Parameters.Add("#id", SqlDbType.Int).Value = id;
object result = cmd.ExecuteScalar();
if (result != null && result.GetType() != typeof(DBNull))
{
return (int)result;
}
Assert.Fail("No Results Returned from Expected Result Query");
return 0;
}
}
Note: this code assumes you are using SQL Server, for other systems the format of the parameters in the connection string might change, e.g. for Oracle it should be :id instead of #id.
I am using a SqlDataReader to fetch data from a stored procedure. Even though the records are being fetched, the while (reader.Read()) gets executed only once, and so in my list only one row is added.
List<Student> tablelist = new List<Student>();
using (SqlConnection con = new SqlConnection(connectionString))
{
using (SqlCommand cmd = new SqlCommand("SP_ReadPromotedStudents"))
{
cmd.Connection = con;
cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.Add("#Name", SqlDbType.VarChar).Value = Data[0];
cmd.Parameters.Add("#Email", SqlDbType.VarChar).Value = Data[1];
cmd.Parameters.Add("#Class", SqlDbType.VarChar).Value = Data[2];
con.Open();
using (SqlDataReader reader = cmd.ExecuteReader())
{
while (reader.HasRows)
{
while (reader.Read())
{
tablelist.Add(new Student
{
Name = (string)(reader[0]),
Email = (string)(reader[1]),
Class = (string)(reader[2]),
});
reader.NextResult();
}
}
}
}
}
return tablelist;
My Student class:
public class Student
{
public string Name { get; set; }
public string Email { get; set; }
public string Class { get; set; }
}
I have about 46 records being fetched. But in the list only one record gets added. What is the mistake here?
You need to move your call to NextResult outside the reader.Read() loop. Otherwise after the first read the code encounters the NextResult call and tries to load a second sets of data returned by the stored procedure.
Also the loop over HasRows is an infinite loop. If the property reader.HasRows is true it will be true also when you finish to read the rows.
using (SqlDataReader reader = cmd.ExecuteReader())
{
while (reader.Read())
{
tablelist.Add(new Student
{
Name = (string)(reader[0]),
Email = (string)(reader[1]),
Class = (string)(reader[2]),
});
}
// This should be called only if your stored procedure returns
// two or more sets of data otherwise you can remove everything
reader.NextResult();
// If there is another set of data then you can read it with a
// second while loop with
while(reader.Read())
{
.....
}
}
The ideal scenario would to have a new sql statement to get just what you want instead the get a list and need just the first access. Imagine if you have a table with millions of records, would you need to execute a query to get all and read just the first one? No, you execute a query to get the you need.
The NextResult method from DataReader moves the pointer to the next result if you have it on the result. Remove it.
After you chanfge the sql statement to get what you need, you are looping the result set. You could read just the first line (changing the while to if):
if (reader.Read())
{
tablelist.Add(new Student
{
Name = (string)(reader[0]),
Email = (string)(reader[1]),
Class = (string)(reader[2]),
});
}
As mentioned above:
I'm using C# to connect to a MySQL database and I want to read JSON data type.
I use the method MySqlCommand.ExecuteReader:
using (MySqlConnection sconn = new MySqlConnection(sqlConnectString))
{
sconn.Open();
String sql_Command = #"SELECT `id` FROM orders.jsontest;";
using (MySqlCommand scmd = new MySqlCommand(sql_Command, sconn))
{
**MySqlDataReader sdr = scmd.ExecuteReader();** // fatal error
DataTable datatable = new DataTable();
// ...
}
}
Can it be that I cannot use ExecuteReader here?
I know this question is old, but just in case you do not yet have a solution to your problem or anyone else encounters a similar problem, the following code should work for you:
private IEnumerable<int> GetIds()
{
using (MySqlConnection connection = new MySqlConnection(connectionString))
{
connection.Open();
string commandText = #"SELECT id FROM jsontest"; // Assuming that `orders` is your database, then you do not need to specify it here.
using (MySqlCommand command = new MySqlCommand(commandText, connection))
{
MySqlDataReader reader = command.ExecuteReader();
while (reader.Read())
{
yield return reader.GetInt32(0);
}
}
}
}
Now what you should pay attention to is this line
while (reader.Read())
which fetches results from the jsontest table as long as the MySqlDataReader can still read valid results and this line
yield return reader.GetInt32(0);
which instructs the reader to get and return each record of the fetched table one at a time as an Int32 (int). You need to change this if your tables column type is not INT.
Since you selected just one column (i.e. "SELECT id"), the parameter is 0, because your fetched resul table consists of one column only.
Additionally, in your code you seem to want to get the results as a DataTable; if so, you should use MySqlDataAdapter instead of the MySqlDataReader as follows:
DataTable resultTable = new DataTable("ResultTable");
MySqlDataAdapter adapter = new MySqlDataAdapter(command);
adapter.Fill(table);
Correct your sql command
String sql_Command = #"SELECT id FROM orders.jsontest";
I have a simple database that I am using. It contains two entries for users which is a user with UserID 1 and IsAdmin 0 and another with UserID 3041234567 and IsAdmin of 1. The only fields in the database is a string UserID and a bit IsAdmin. I am reading from the database with the following code:
SqlConnection conn = new SqlConnection(Properties.Settings.Default.Conn);
conn.Open();
SqlCommand cmd = new SqlCommand("SELECT * FROM Users WHERE UserID = " + t.Text.ToString(), conn);
SqlDataReader reader = cmd.ExecuteReader();
if (reader.HasRows)
{
while (reader.Read())
{
user.UserID = reader["UserID"].ToString();
user.IsAdmin = Convert.ToBoolean(reader["IsAdmin"]);
}
}
conn.Close();
If I enter the number 3041234567 as the UserID everything works perfectly, but If I enter the number 1 I get an exception saying that "The conversion of the nvarchar value '3041234567' overflowed an int column."
If I set a breakpoint and watch the while(reader.read()) loop the loop iterates through fine and sets the user.UserID = 1 and the user.IsAdmin = false. The exception is thrown when the loop begins to iterate a second time. I guess I have a couple of questions:
Why is the loop iterating a second time?
How is the ID 3041234567 being returned with the sql command "SELECT * FROM Users WHERE UserID = 1"
What is the int column that is being overflowed?
Well, since
3041234567 > int.MaxValue ( == 2147483647)
you've got an overflow; if you want some kind of integer value, however, try long (which is 64 bit long):
long value = Convert.ToInt64(reader["UserID"]);
Something like this:
// Wrap IDisposable into using
using (SqlConnection conn = new SqlConnection(Properties.Settings.Default.Conn)) {
conn.Open();
// Make sql
// 1. Readable
// 2. Parametrized
// 3. Avoid * in select
String sql =
#"select UserID,
IsAdmin
from Users
where UserID = #prm_UserId";
// Wrap IDisposable into using
using (SqlCommand cmd = new SqlCommand(sql, conn)) {
// Explicit data type will be better here (Add Parameter with type)
// but I don't know it
cmd.Parameters.AddWidthValue("prm_UserId", t.Text);
// Wrap IDisposable into using
using (SqlDataReader reader = cmd.ExecuteReader()) {
// You don't want to iterate the whole cursor, but the first record
if (reader.Read()) {
//TODO: Make UserID being "long"
user.UserID = Convert.ToInt64(reader["UserID"]);
user.IsAdmin = Convert.ToBoolean(reader["IsAdmin"]);
}
}
}
}
I ran into another issue again. I was trying to get data from the database using DataReader but I got the error when i was testing my code. Can anyone help me out? The error occurred at this line:
chkAssess = readAssess[columnName].ToString();
Below is the code snippet:
public string CheckAssess(string emailAddress, string columnName)
{
string chkAssess = "";
SqlDataReader readAssess;
//readAssess = new SqlDataReader();
string MgrAssessQry = "SELECT '"+columnName+"' FROM tblAllUsers";
//MgrAssessQry += " WHERE email ='" + emailAddress + "'";
SqlCommand cmdReadAssess = new SqlCommand(MgrAssessQry, cn);
cn.Open();
readAssess = cmdReadAssess.ExecuteReader();
while(readAssess.Read())
{
// Add the rows
chkAssess = readAssess[columnName].ToString();
}
return chkAssess;
}
try to use column name without ''
select something from table
instead of
select 'something' from table
for security reasons, don't create sql queries in that way (by concatenating strings) - use #parameters instead
2. close the reader at the end
Try this:
public string CheckAssess(string emailAddress, string columnName)
{
string chkAssess = "";
SqlDataReader readAssess;
//readAssess = new SqlDataReader();
string MgrAssessQry = "SELECT #Column_Name FROM tblAllUsers";
SqlCommand cmdReadAssess = new SqlCommand(MgrAssessQry, cn);
cmdReadAssess.Parameters.AddWithValue(new SqlParameter("Column_Name", columnName));
cn.Open();
readAssess = cmdReadAssess.ExecuteReader();
while(readAssess.Read())
{
// Add the rows
chkAssess = readAssess.GetString(0);
}
return chkAssess;
}
You have got several problems here.
Check whether your readAssess has rows like below.
if(readAssess.HasRows)
If it doesn't have rows then trying
chkAssess = readAssess.GetString(0);
would throw this error, as Arrays are index-based.
So your code should be like below
if(readAssess.HasRows)
{
while(readAssess.Read())
{
chkAssess = readAssess.GetString(0);
}
}
Other problem is you need to close both the reader & the connection afterwards.
readAssess.Close();
cn.Close();
Also your code is potentially vulnerable to SQL Injection.
if (reader.HasRows)
{
while (reader.Read())
{
int result = Convert.ToInt32(reader.GetString(0));
Console.WriteLine(result);
}
}
The most important thing is check the query first by executing in SQL Server and see if any result is coming or not.
Secondly based on the type of output you are receiving cast it to that particular data type (important).Mostly everyone is saving the data in varchar so.