I try to use the code sample in DBDataAdapter.Update Method to clear a table in a database.
using (SqlConnection connection = new SqlConnection(connectionString))
{
SqlDataAdapter adapter = new SqlDataAdapter("SELECT * FROM WebCam", connection);
DataTable table = new DataTable();
adapter.Fill(table);
table.PrimaryKey = new DataColumn[] { table.Columns["Date"] };
//table.Rows[0]["Date"] = System.DateTime.Now; //It's OK to modify a row
table.Clear(); //But it is not working to clear the table
SqlCommandBuilder builder = new SqlCommandBuilder(adapter);
adapter.Update(table);
}
I can add new rows or modify existing rows, and the changes can be committed to the database, but if I try to empty the table, the change to 'table' can not be committed to the database, also, no exception is thrown.
Do I miss something?
u should get error on this line:
table.PrimaryKey = new DataColumn[] { table.Columns["Date"] };
this would mean that u have catched the exception somewhere else, maybe from where the function is being called, and not showing the error in a messege box.
Another reason might be because you are using words like Table, adapter etc ... my guess is that you might be overloading some reserved words functionalities.
Related
I'm trying to make a simple program that has a log-in part, with a local database just for testing.And i keep getting an error when I try to open the connection to the SQL database.
private void logInButton_Click(object sender, EventArgs e)
{
MainMenu openMainMenu = new MainMenu();
SqlConnection sqlcon = new SqlConnection(#"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=C: \Users\Nea Florin\Desktop\PlatformaTestare\PlatformaTestare\Server.mdf;Integrated Security=True;Connect Timeout=30");
sqlcon.Open();
SqlCommand cmd = new SqlCommand("Select * from Table Where username ='" + usernameTextBox.Text + "' and password = '" + passwrodTextBox.Text + "'");
SqlDataAdapter sda = new SqlDataAdapter(cmd);
DataTable dtbl = new DataTable();
sda.Fill(dtbl);
if (dtbl.Rows.Count > 0)
{
openMainMenu.Show();
this.Hide();
}
else
MessageBox.Show("Wrong username or password!");
}
I get the error at sqlcon.Open();, and it is: "An unhandled exception of type 'System.Data.SqlClient.SqlException' occurred in System.Data.dll
Additional information: An attempt to attach an auto-named database for file C: \Users\Nea Florin\Desktop\PlatformaTestare\PlatformaTestare\Server.mdf failed. A database with the same name exists, or specified file cannot be opened, or it is located on UNC share."
Well, the best advice I can give you is to google the error message. Keep in mind that if there is an error message it means that the problem is well known an as such it's a safe bet that someone have encountered it before you and managed to solve it. The first 4 results of this search are on stackoverflow and at least two of them have accepted answers, so I believe a little reasearch would have saved you a long time.
This is the best advice because it streaches far beyond your current problem. I firmly believe that good searching skills is the most important and most powerfull tools of a sotfware developer. I can assure you, no matter how much time you are developing software, almost every exception you get, someone else have already solved and posted the solution somewhere, you only need to find it.
Now, as for the code it self - You have some major problems other then the exception you are asking about:
Concatenating strings into sql statements instead of using parameters expose your code to SQL injection attacks. This is a very serious threat that is extremely easy to fix.
Using insntances of classes that implements the IDisposable interface without properly disposing them may lead to memory leak. Read about the using statement and make it a habit to use it every time it's possible.
Exception handling. Currently, if your database can't be reached, you get an exception and your program crash. You should use a try...catch block anywhere you can't control in code to let your program end gracefuly instead. (Don't ever use try...catch for things you can do in code such as validate user input or checking division by zero - only for things that are beyon your control such as database availability.)
Having said all that, your code should look something like this:
private void logInButton_Click(object sender, EventArgs e)
{
using (var sqlcon = new SqlConnection(#"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=|DataDirectory|C:\Users\Nea Florin\Desktop\PlatformaTestare\PlatformaTestare\Server.mdf;Integrated Security=True;Connect Timeout=30"))
{
sqlcon.Open();
using (var cmd = new SqlCommand("Select 1 from Table Where username = #userName and password = #password"))
{
cmd.Parameters.Add("#userName", SqlDbType.NVarChar).Value = usernameTextBox.Text;
cmd.Parameters.Add("#password", SqlDbType.NVarChar).Value = passwrodTextBox.Text;
using (var dtbl = new DataTable())
{
using (var sda = new SqlDataAdapter(cmd))
{
sda.Fill(dtbl);
}
if (dtbl.Rows.Count > 0)
{
var openMainMenu = new MainMenu();
openMainMenu.Show();
this.Hide();
}
}
else
{
MessageBox.Show("Wrong username or password!");
}
}
}
I'm executing same command in every 2 seconds. I think following code creates multiple connections:
[System.Web.Services.WebMethod]
public static int getActivity()
{
using (SqlConnection connection = new SqlConnection(ConfigurationManager.ConnectionStrings["dbconnection"].ToString()))
{
connection.Open();
using (var cmd = new SqlCommand("SELECT TOP 1 ValueX FROM TABLE WHERE ID= 2 AND EVENTID = 2 ORDER BY DATE DESC", connection))
{
var x = cmd.ExecuteScalar();
int Result;
if (x != null)
{
Result = int.Parse(x.ToString());
Console.WriteLine("USER ACTIVITY : " + Result);
}
else
{
Result = -999;
}
connection.Close();
return Result;
}
}
}
If I call this method several time Does following code make multi connection Or single connection ?
using (SqlConnection connection = new SqlConnection(ConfigurationManager.ConnectionStrings["dbconnection"].ToString()))
Can someone explain whether I need to modify this code or Is this good one ?
Thanks.
Since you are using the using statement clause so once you are done with the method the resources are freed and the connection is closed. So everytime when you call the same method a new connection will be made. When you are using the using clause then it is equivalent to the below code:
SqlConnection connection = null;
try
{
connection = new SqlConnection(connectionString);
}
finally
{
if(connection != null)
((IDisposable)connection).Dispose();
}
Also note that you dont need to explicitly call the connection.Close(); in your method as using statement will take care of it.
Your method is fine, you just don't need connection.Close() as described by Rahul. Using statement when dealing with SQL objects is good practice.
What you should keep in mind, is that ADO.NET connection pooling, takes care of handling new objects referring to the same connection string, thus minimizing the time needed to open a connection.
More about connection pooling can be found Here
SqlCommand cmd = new SqlCommand("SELECT * FROM Products",con);
SqlDataReader reader = cmd.ExecuteReader();
while (reader.Read())
{
if (reader["ProductName"].ToString() == "dsd")
{
SqlDataAdapter da = new SqlDataAdapter("SELECT * FROM Products", con);
DataSet ds = new DataSet();
da.Fill(ds);
ListView1.DataSource = ds;
ListView1.DataBind();
}
}
reader.Close();
Error :
Already an open DataReader associated with this Command which must be closed first
I am getting error on this line da.fill(ds). And I used two commands one SqlCommand to read the record and second SqlDataAdapter to show record.
If I understand your problem correctly, you need to bind a listview control with all the products by name "dsd".
If this is correct, you can change the SQL query like this:
SELECT * FROM Products WHERE ProductName = 'dsd'
You can read the response from the DataReader and prepare the List to bind to your listview control. I don't think you need DataSet to bind to listview control.
You need two connection objects. One for your open data reader and a second to use with the SqlDataAdapter.
(Or you can use MARS, but really it is far better to just open a second connection.)
I'm looking for some examples on how to best use SqlDataAdapter to access and update data in my application.
Right now I have something like this:
SqlDataAdapter adapter;
DataSet myData = MyDataAccessClass.GetData("Select * from Students", ref adapter);
// change some data here and save changes
adapter.Update();
All of this occurs in code behind, and I dont really like it at all.
So, I'm trying to find a way to do something like this:
DataSet myData = MyDataAccessClass.GetStudents();
// change some data and save changes
MyDataAccessClass.SaveStudents(myData);
Where SaveStudents method still uses SqlDataAdapter to update db.
Any ideas on how to make this work or some pointers to best practices of doing
something like this are highly appreciated. Thank you.
That seems like a fairly basic Data Access Layer implementation, to me. Generally, I do it something like this:
public class MyDataAccessClass
{
private string ConnString;
public MyDataAccessClass()
{ //Get connection string from configuration file }
public MyDataAccessClass(string connString)
{ ConnString = connString; }
public DataSet GetAllStudents()
{
//your SQL Adapter code here...
}
}
One note that I'd make is that with so many ORM solutions (including just Entity Framework and Linq2Sql) available, you may want to consider using collections of objects instead of data-sets for your Data Representations. Then you can have a method like:
public void CreateUpdateStudent(Student student)
{
//update database
}
That's fairly subjective, I'll admit, but I find it preferable to using straight DataSets.
If you want to get update data using the sql-data-adapter then you could use these
Using System.Data.SqlClient;
SqlConnection con = new SqlConnection("Data Source=abcd-pc;Initial Catalog=user_info;Integrated Security=True");
SqlDataAdapter da = new SqlDataAdapter();
try
{
da.UpdateCommand = new SqlCommand("Update logindemo set password=#pswd where username=#uname",con);
da.UpdateCommand.Parameters.Add("#pswd", SqlDbType.VarChar).Value = txtpass.Text;
da.UpdateCommand.Parameters.Add("#uname", SqlDbType.VarChar).Value = txtusername.Text;
con.Open();
da.UpdateCommand.ExecuteNonQuery();
Label1.Text = "Data Updated";
con.Close();
}
catch
{
Label1.Text = "Unable To Connect";
}
I hope you understand how to update the data easily. It just like the example. You can use these type of example in Inserting into the Data, and Deleting the Data with using specific the command and sql query as it required.
I am working on a DAL that is getting a DataReader Asynchronously.
I would like to write a single method for transforming the DataReader into a DataSet. It needs to handle different schema so that this one method will handle all of my fetch needs.
P.S. I am populating the SQLDataReader Asynchronously, please don't give answers that are getting rid of the DataReader.
DataTable.load() can be used for a generic approach.
do {
var table = new DataTable();
table.Load(reader);
dataset.Tables.Add(table);
} while(!reader.IsClosed);
Try DataSet.Load(). It has several overloads taking an IDataReader.
If for some reason the Load method fails, here is a manual way to do it:
DataTable dt = new DataTable();
dt = sdr.GetSchemaTable();
//dt.Constraints.Clear();
//dt.PrimaryKey = null;
//dt.BeginLoadData();
if (sdr.HasRows)
{
DataRow row;
while (sdr.Read())
{
row = dt.NewRow();
sdr.GetValues(row.ItemArray);
dt.Rows.Add(row);
}
Another way is to use SqlTableAdapter:
var adapter = new SqlDataAdapter(command);
DataSet ds = new DataSet();
adapter.Fill(ds);