So in C#, I have an ODBCDataReader that returns that it has rows, but when I try to access the data it returns an object not set to a reference of an object error. I tested the sql directly on the db and it does return a row without any nulls
OdbcDataReader results;
try
{
// Initialize & open odbc connection
using (OdbcConnection conn = new OdbcConnection(connectionString.ToString()))
{
conn.Open();
// Initialiaze odbc command object
using (OdbcCommand comm = new OdbcCommand(query.ToString(), conn))
{
results = comm.ExecuteReader();
}
}
}
catch
{
//detailed error messaging here (which does not get hit)
}
temp = results;
if (temp.HasRows == false)
{
//error messaging here does not get hit.
}
while (temp.Read())
{
try
{
//I attempted to access the data by creating an object array:
object [] objarray = new object[temp.FieldCount)
temp.GetValues(objarray); //this causes error
}
catch{ // error is caught here "object not set to a reference of an object" }
for (i = 0; i < temp.FieldCount; i++)
{
try
{
//I also attempted other ways to access the data including:
temp[i].ToString(); // this causes error
temp.GetInt32(i).ToString(); // this causes error
temp.GetName(i); //this causes error
}
catch
{
// error is caught here "object not set to a reference of an object"
}
}
}
You are using it outside the using blocks. Move the part where you use [results] inside the using blocks (immediately after the ExecuteReader() call) and you should be in a much better place.
I ran into this same issue. The problem in my case was that I was not binding my parameters correctly. I was binding using #:
SELECT * FROM MyTable WHERE MyField = #MyField
For some reason, this is valid in MySQL and doesn't produce an error, but will not return data. The solution was to bind using ?:
SELECT * FROM MyTable WHERE MyField = ?
Then in C# bind the parameters:
cmd.Parameters.AddWithValue("#MyField", myFieldValue);
Old question, but it's the first result on Google and unanswered. Hope it's helpful.
Related
I have created a simplified SQL Data class, and a class method for returning a ready to use resultset:
public SQL_Data(string database) {
string ConnectionString = GetConnectionString(database);
cn = new SqlConnection(ConnectionString);
try {
cn.Open();
} catch (Exception e) {
Log.Write(e);
throw;
}
}
public SqlDataReader DBReader(string query) {
try {
using (SqlCommand cmd = new SqlCommand(query, this.cn)) {
return cmd.ExecuteReader(CommandBehavior.CloseConnection);
}
} catch {
Log.Write("SQL Error with either Connection String:\n" + cn + " \nor Query:\n" + query);
throw;
}
}
(I catch any errors, log them, and then catch the error higher up the chain. Also, I did not include the ConnectionString() code for brevity. It just returns the requested connection string. That's all.)
This all works just fine, and with a single line of code, I'm ready to .Read() rows.
SqlDataReader rs = new SQL_Data("MyDatabase").DBReader(#"SELECT * FROM Employees");
while (rs.Read()) {
// code
}
rs.Close();
I want to expand this and add a .ColumnReader() method that I want to chain to .DBReader() like this:
string empID = new SQL_Data("MyDatabase").DBReader(#"SELECT * FROM Employees).ColumnReader("EmpID");
I attempted this by adding a .ColumnReader() method, but it ends up being a method of SQL_Data() class directly, not a member or extension of .DBReader(). I also tried adding the .ColumnReader() inside the .DBReader() (like a "closure"), but that didn't work either.
Can this be done?
This ended up working for me:
public static class SQLExtentions {
public static dynamic ColumnReader(this SqlDataReader rs, string colName) {
return rs[colName];
}
}
I will have to expand on it a bit to add some error checking, and perhaps return more than just the dynamic value - like return an object with the value and it's SQL data type. But Paul and Bagus' comments got me on the right track.
I have an SQL statement that checks to see if a value is in my database or not. I want to respond with the "happy path" if the value is not in the database.
I have found using DbDataReader (.NET) that if a SELECT query can't find the value it throws an exception - so my "happy path" ends up in the exception, not in the main try block.
I can always say "NOT IN" but I don't want to return all of the rows in the database that don't have the value - as this would return many thousands of results where as all I want is just a "no it is not here" type response.
public void wristbandScan(string barcode)
{
string query = "SELECT ticket FROM tickets WHERE
linked_barcode='" + barcode + "'";
ValidTicketEventArgs args = new ValidTicketEventArgs();
try
{
var queryResult = _dbRunner.queryThis(query);
args.Result = false;
args.Message = "WB already linked";
args.Barcode = barcode;
OnValidTicketEvent(args);
}
catch (Exception e)
{
this.updateWristband(barcode);
this.updateValid();
args.Result = true;
args.Message = "WB linked";
args.Barcode = barcode;
OnValidTicketEvent(args);
}
}
It feels wrong to me to catch the happy path in an error statement, but I do not want the lag associated with reading in all the rows with the NOT IN statement.
Is there a better way to do this or is this approach acceptable best practice?
Well, you don't have to fetch all the records to the client; let's extract a method for this. Assuming that you work with MS Sql:
public bool hasScanCode(string barcode) {
if (string.IsNullOrWhiteSpace(barcode))
return false;
//DONE: paramterize queries
string query =
#"SELECT ticket
FROM tickets
WHERE linked_barcode = #prm_BarCode";
using (var conn = new SqlConnection(connection_string_here)) {
conn.Open();
using (var q = new SqlCommand(conn, query)) {
//TODO: q.Parameters.Add is a better choice
q.Parameters.AddWithValue("#prm_BarCode", barcode.Trim());
using (var reader = q.ExecuteReader()) {
// we read (fetch) at most 1 record
// if empty cursor - no record with given barcode
return reader.Read();
}
}
}
}
then we can use it:
public void wristbandScan(string barcode) {
bool result = hasScanCode(barcode);
ValidTicketEventArgs args = new ValidTicketEventArgs() {
Result = result,
Message = result ? "WB linked" : "WB already linked",
Barcode = barcode,
};
OnValidTicketEvent(args);
}
Please, remember - exceptions are for exceptional situations. Exceptions are very slow (stack unrolling wants resources); they are not readable - catch, in fact, works as a notorious goto; they are dangerous - in your current code you catch too many exceptions: e.g. AccessViolationException if it's thrown somewhere within dbRunner.queryThis will be efficiently masked.
Create and call a StoredProcedure which can to handle the empty situation and return no rows instead of an exception.
Then handle the no rows scenario outside the try/catch.
I am going through a YouTube video, to learn ASP.NET. I've gotten it to work for the most part, with a somewhat major caveat: I can't retrieve a value from a hidden field ID. Because of that, I don't have a value to send to a stored procedure to create or update.
The commented out line is the original statement. When I have that then execute
.ExecuteNonQuery, I get the following error:
Procedure or function 'ResourceCreateOrUpdate' expects parameter '#ResourceID', which was not supplied
When I try to display hfResourceID, I have nothing when trying to pass 0, for a create, or the ResourceID value, i.e. 1. That value however, doesn't get there. I know the stored procedure works because I can execute it in SQL Server Management.
I tried moving hfResourceID to a string, then a integer value, but I seem to be having problems creating the if/else: everything is marked as an error. When I hover over the lines, I get the following message, which pretty much leave me clueless:
"Embedded statement cannot be a declaration or labeled statement".
Would I be able to get any pointers on how to clear up my error, please? Thanks.
2017-10-13 # 10:38: code updated
<asp:HiddenField ID="hfResourceID" runat="server" />
protected void btnSave_Click(object sender, EventArgs e)
{
int intResourceID = 0;
bool boolIDHasValue = true;
try
{
intResourceID = Convert.ToInt32(hfResourceID.Value);
}
catch (Exception ex)
{
lblErrorMessage.Text = ex.Message;
boolIDHasValue = false;
}
if (boolIDHasValue)
{
if (sqlconnODRConnection.State == System.Data.ConnectionState.Closed)
sqlconnODRConnection.Open();
SqlCommand sqlcmdCreateOrUpdate = new SqlCommand("ResourceCreateOrUpdate", sqlconnODRConnection);
sqlcmdCreateOrUpdate.Parameters.AddWithValue("#ResourceID", intResourceID);
sqlcmdCreateOrUpdate.Parameters.AddWithValue("#Status", txtStatus.Text.Trim());
sqlcmdCreateOrUpdate.Parameters.AddWithValue("#FirstName", txtFirstName.Text.Trim());
sqlcmdCreateOrUpdate.Parameters.AddWithValue("#MiddleName", txtMiddleName.Text.Trim());
sqlcmdCreateOrUpdate.Parameters.AddWithValue("#LastName", txtLastName.Text.Trim());
sqlcmdCreateOrUpdate.Parameters.AddWithValue("#NickName", txtNickName.Text.Trim());
sqlcmdCreateOrUpdate.Parameters.AddWithValue("#Gender", txtGender.Text.Trim());
sqlcmdCreateOrUpdate.Parameters.AddWithValue("#USCitizen", txtUSCitizen.Text.Trim());
sqlcmdCreateOrUpdate.ExecuteNonQuery();
sqlconnODRConnection.Close();
string strResourceID = hfResourceID.Value;
Clear();
if (strResourceID == "")
lblSuccessMessage.Text = "Saved Successfully";
else
lblSuccessMessage.Text = "Updated Successfully";
FillGridView();
}
}
There are a few issues with the code you copied from that video. But here a snippet as to how it should be done. I've added 3 ways to convert from the HiddenField value to an actual int. Which one you use can depend on how you want to handle errors, 0 values etc. Not included in the snippet, but I like to check for IsNullOrEmpty while using Trim(), that gets rid of spaces that might make the value non-convertible if (!string.IsNullOrEmpty(hfResourceID.Value.Trim())).
int intResourceID = 0;
//this will try to convert but you won't see exeptions when failed
Int32.TryParse(hfResourceID.Value, out intResourceID);
//checks if there is a value in the hiddenfield, but throws yellow screen if not convertible
if (!string.IsNullOrEmpty(hfResourceID.Value))
{
intResourceID = Convert.ToInt32(hfResourceID.Value);
}
//catch an error when the value is not convertible, can be wrapped with !string.IsNullOrEmpty(hfResourceID.Value)
try
{
intResourceID = Convert.ToInt32(hfResourceID.Value);
}
catch (Exception ex)
{
//handle the error, can be seen with ex.Message
}
//if the hidden value is still 0 (for whatever reason) you might not want to execute the query
//so the next part will return and stop executing the rest of the code
if (intResourceID == 0)
{
return;
}
//update the database, using 'using' will ensure proper closure of the connection and disposing of any objects
using (SqlConnection connection = new SqlConnection("myConnectionString"))
using (SqlCommand command = new SqlCommand("ResourceCreateOrUpdate", connection))
{
//set the command type and add the parameters
command.CommandType = CommandType.StoredProcedure;
command.Parameters.Add("#ResourceID", SqlDbType.Int).Value = intResourceID;
try
{
//open the database connection and execute the command
connection.Open();
command.ExecuteNonQuery();
}
catch (Exception ex)
{
//there was an error opening the database connection or with the command, can be viewed with ex.Message
}
}
Your error regarding the embedded statement is because you a declaring
if (strResourceID == "")
int intResourceID = 0;
else
int intResourceID = (Convert.ToInt32(hfResourceID.Value));
When decalring a variable directly after an If or else then you need your curly brackets. So...
if (strResourceID == "")
{
int intResourceID = 0;
}
else
{
int intResourceID = (Convert.ToInt32(hfResourceID.Value));
}
As for your other issues I would need to see your client side code.
Take a look on ASP code, specifically the way you are setting hidden field value. I guess it is properly marked as runat=server but maybe some problem is going on in your asp.net code, try to debug client code by using: console.log function and see the output in your console browser.
In short I am writing a class handler to handle to database integration of some software I am writing for myself, however as there is not always a connection to the remote database I thought I would use SQLCE to create a local database buffer so when a connection is made the changes can be synchronized.
So far it is going well except for the parameters. The function I am looking to call is shown below however this function is complaining about invalid arguments.
public Object run(string query, List<Object> dbparams = null)
{
if (MyDB.isConnected())
{
return MyDB.run(query, dbparams);
}
else
{
SqlCeCommand sql = _OfflineConnection.CreateCommand();
sql.CommandText = query;
if (dbparams.Count > 0)
{
sql.Parameters.AddRange(dbparams.ToArray());
}
return sql;
}
}
MyDB.run is the exact same code as in the else statement except for mysql, the line that it is moaning about is the return mydb.run as the mydb class is expecting the dbparams list to be of mysqlparameters.
Does anyone know how I can achieve this? I attempted to use LINQ to do a convert but that failed miserably.
EDIT
At present I have the following working but I am sure there is a cleaner solution
public Object run(string query, List<Object> dbparams = null)
{
if (MyDB.isConnected())
{
List<MySqlParameter> mydbparams = null;
for (int i = 0; i < dbparams.Count; i++)
{
mydbparams.Add((MySqlParameter)dbparams[i]);
}
return MyDB.run(query, mydbparams);
}
else
{
SqlCeCommand sql = _OfflineConnection.CreateCommand();
sql.CommandText = query;
if (dbparams.Count > 0)
{
sql.Parameters.AddRange(dbparams.ToArray());
}
return sql;
}
}
A bit cleaner solution would be
mydbparams = dbparams.Cast<MySqlParameters>().ToList();
Also, you should check for and handle the null condition of dbparams.
I know what I asking might not make a lot of sense for C# experts but I'll explain what I want to do and then you can suggest me how to do it in a better way if you want ok?
I have a C# class called DatabaseManager that deals with different MySQL queries (ado.net NET connector, not linq or any kind of ActiveRecord-ish library).
I am doing something like
categories = db_manager.getCategories();
The list of categories is quite small (10 items) so I'd like to know what's the best way of accessing the retrieved information without a lot of additional code.
Right now I'm using a Struct to store the information but I'm sure there's a better way of doing this.
Here's my code:
public struct Category
{
public string name;
}
internal ArrayList getCategories()
{
ArrayList categories = new ArrayList();
MySqlDataReader reader;
Category category_info;
try
{
conn.Open();
reader = category_query.ExecuteReader();
while (reader.Read())
{
category_info = new Category();
category_info.name = reader["name"].ToString();
categories.Add(category_info);
}
reader.Close();
conn.Close();
}
catch (MySqlException e)
{
Console.WriteLine("ERROR " + e.ToString());
}
return categories;
}
Example:
public IEnumerable<Category> GetCategories()
{
using (var connection = new MySqlConnection("CONNECTION STRING"))
using (var command = new MySqlCommand("SELECT name FROM categories", connection))
{
connection.Open();
using (var reader = command.ExecuteReader())
{
while (reader.Read())
{
yield return new Category { name = reader.GetString(0) };
}
}
}
}
Remarks:
Let ADO.NET connection pooling do the right work for you (avoid storing connections in static fields, etc...)
Always make sure to properly dispose unmanaged resources (using "using" in C#)
Always return the lowest interface in the hierarchy from your public methods (in this case IEnumerable<Category>).
Leave the callers handle exceptions and logging. These are crosscutting concerns and should not be mixed with your DB access code.
The first thing I would do is to replace you use of ArrayList with List that will provide compile-time type checkig for your use of the category list (so you will not have to type cast it when using it in your code).
There's nothing wrong with returning them in an like this. However, a few things stand out:
Your catch block logs the error but
then returns either an empty array or
a partially populated array. This
probably isn't a good idea
If an exception is thrown in the try
block you won't close the connection
or dispose of the reader. Consider
the using() statement.
You should use the generic types
(List<>) instead of ArrayList.
From your code I guess you are using .NET 1.1, becuase you are not using the power of generics.
1) Using a struct that only contains a string is an overkill. Just create an arraylist of strings (or with generics a List )
2) When an exception occurs in your try block, you leave your connection and reader open... Use this instead:
try
{
conn.open();
//more code
}
catch (MySqlException e) { // code
}
finally {
conn.close()
if (reader != null)
reader.close();
}