I'm trying to return a single row from a database:
using (connection = new SqlConnection(ConfigurationManager.AppSettings["connection"]))
{
using (command = new SqlCommand(#"select top 1 col_1, col_2 from table1", connection))
{
connection.Open();
using (reader = command.ExecuteReader())
{
reader.Read();
return reader["col_1"];
}
}
}
But I'm getting the following error message:
Compiler Error Message: CS0266: Cannot implicitly convert type 'object' to 'string'. An explicit conversion exists (are you missing a
cast?)
Line 90: return reader["col_1"];
I'm sure I am making a really obvious mistake, but I can't seem to find any single row examples, all I examples I find are for multiple returned rows using a while loop.
reader["col_1"] returns object.
You want something like reader.GetString(reader.GetOrdinal("col_1")).
Edit -> I just wanted to add a note here that, in addition to the concerns others have raised, a SELECT TOP without an ORDER BY can give you random results based on schema changes and/or merry-go-round scans.
This is how I would style (and fix) the code:
using (var connection = new SqlConnection(ConfigurationManager.AppSettings["connection"]))
using (var command = new SqlCommand(#"select top 1 col_1, col_2 from table1", connection))
{
connection.Open();
using (var reader = command.ExecuteReader())
{
if (reader.Read()) // Don't assume we have any rows.
{
int ord = reader.GetOrdinal("col_1");
return reader.GetString(ord); // Handles nulls and empty strings.
}
return null;
}
}
Using the index reader[] will give you object types, these need casting. However, I hardly touch that style and always favour the slightly more verbose, but more robust use of ordinals and asking for types in a strongly-typed manner.
If you only need the value in the first column of the first row, you can use ExecuteScalar instead, again this returns an object that can be cast and doesn't need a reader:
using (var connection = new SqlConnection(ConfigurationManager.AppSettings["connection"]))
using (var command = new SqlCommand(#"select top 1 col_1, col_2 from table1", connection))
{
connection.Open();
var result = command.ExecuteScalar();
return result == null ? "" : (string)result;
}
The problem is the return type. The method you are in is expecting you to return a string, but reader["col_1"] is an object. I suggest returning reader["col_1"].ToString() or Convert.ToString(reader["col_1"]).
To me it seems, you don't want a single row, only a single value:
SqlConnection sqlConnection = new SqlConnection("Your Connection String");
SqlCommand cmd = new SqlCommand();
Object returnValue;
cmd.CommandText = "SELECT TOP 1 col_name FROM Customers";
cmd.CommandType = CommandType.Text;
cmd.Connection = sqlConnection1;
sqlConnection.Open();
returnValue = cmd.ExecuteScalar();
sqlConnection.Close();
return returnValue.ToString(); //Note you have to cast it to your desired data type
Instead of:
using (reader = command.ExecuteReader())
{
reader.Read();
return reader["col_1"];
}
You need to cast the reader["col_1"] to string, either reader["col_1"].ToString() or reader.GetString(0) like:
return reader.GetString(0);
reader["col_1"] returns an object. I assume your function has a return type of string, which is where the error is coming from, it cannot implicitly convert the object to a string.
You probably expect a string returned from col_1 so you can just cast it: (string)reader["col_1"].
You can use an if statement if your query only returns one value
[...]
string x = string.Empty;
if(reader.Read()) {
// make sure the value is not DBNull
if(DBNull.Value != reader["col_1"]) {
x = reader.GetString(0);
}
}
[...]
First of all you can use the cast (string)reader["col_1"]. You are probably expecting a string and reader["col_1"] is an object.
the reader returns object which you should cast it to what you need, in this case a string.
you can use any of this codes :
return reader.GetString(0);
return reader["col_1"].ToString();
return Convert.ToString(reader["col_1"]);
return reader["col_1"] as string;
but dont forget to close the connection and reader before leaving the function.
string ret = reader.GetString(0);
reader.Close();
connection.Close();
return ret;
Follow the following steps to select a single colume, and display them.
//create a connection
SqlConnection sqlConnection = new SqlConnection("Your Connection String");
SqlCommand cmd = new SqlCommand();
cmd.Connection = sqlConnection;
//open the connection
sqlConnection.Open();
//Your command query string
cmd.CommandType = CommandType.Text;
cmd.CommandText = "SELECT TOP 1 col_name FROM Customers";
//Execute the reader
SqlDataReader result = cmd.ExecuteReader();
result.Read();
//close the connection
sqlConnection.Close();
return result["coiumn_name"].ToString();
Related
I'm trying to get the MAX value returned but it keeps returning 0.
string stateValue = "CA";
SqlCommand cmd = new SqlCommand("SELECT MAX(Population) FROM TestData WHERE State=" + stateValue);
cmd.Parameters.Add("#Population", SqlDbType.Int).Direction = ParameterDirection.ReturnValue;
DBConnection.Instance.execNonQuery(cmd);
int population = (int)cmd.Parameters["#Population"].Value;
In DBConnection class this is the execNonQuery function:
public int execNonQuery(string sCmd, CommandType cmdType)
{
SqlCommand cmd = new SqlCommand(sCmd, m_sqlDataBase);
cmd.CommandType = cmdType;
try
{
m_sqlDataBase.Open();
}
catch { }
return cmd.ExecuteNonQuery();
}
Parameter direction is used with Stored Procedures. Here you are simply executing a single query. You need to use SqlCommand.ExecuteScalar method, since you will only get back one result. It returns an object so you have to convert it to int before using it.
Also your code is using string concatenation to create SQL Query and it is prone to SQL Injection. Also consider using using statement with your Command and Connection.
using (SqlCommand cmd = new SqlCommand("SELECT MAX(Popluation) FROM TestData WHERE State=#state"))
{
//Associate connection with your command an open it
cmd.Parameters.AddWithValue("#state", stateValue);
int populuation = (int)cmd.ExecuteScalar();
}
The ExecuteNonQuery call does not return the result of executing your query.
You can use ExecuteScalar or Execute to get the value back.
public int execQuery(string sCmd, CommandType cmdType)
{
SqlCommand cmd = new SqlCommand(sCmd, m_sqlDataBase);
cmd.CommandType = cmdType;
try
{
m_sqlDataBase.Open();
return Convert.ToInt32(cmd.ExecuteScalar());
}
catch {
// handle your error but don't trap it here..
throw;
}
}
ExecuteScalar is a short-circuit for getting the first value, or the first result set. You can use it to return a single value out of a query, such as yours.
Another option is to use the Execute method to obtain a result set and then use that to get the value you're after:
public int execQuery(string sCmd, CommandType cmdType)
{
SqlCommand cmd = new SqlCommand(sCmd, m_sqlDataBase);
cmd.CommandType = cmdType;
try
{
m_sqlDataBase.Open();
using(var dataReader = cmd.Execute())
{
if (dataReader.Read())
{
return Convert.ToInt32(dataReader[0]);
}
}
}
catch {
// handle your error but don't trap it here..
throw;
}
}
How 'bout
var sql = string.Format("SELECT MAX(Popluation) FROM TestData WHERE State='{0}'", stateValue);
SqlCommand cmd = new SqlCommand(sql);
(The important part was adding the single quotes. The string.Format was to make it look pretty)
I want to retrieve some specific data from tblBranches based to the choice of the user in the combobox (cboBranch.Text), one is the class_name but when I tried to run the program the messagebox shows MySql.Data.MySqlClient.MySqlDataReader, so how can I properly retrieve the data in my database?
query = "SELECT class_name FROM tblBranches WHERE branch_name=#branch";
MySqlCommand cmd = new MySqlCommand(query, con);
cmd.Parameters.Add("#branch", MySqlDbType.VarChar, 30).Value = _order.cboBranch.Text;
MySqlDataReader dr;
con.Open();
dr = cmd.ExecuteReader();
string class_name = dr.ToString();
MessageBox.Show(class_name);
con.Close();
Rather than call the ToString() method, you need to to call the GetString() method passing the zero-based index of the ordinal position of the column in your query, zero in this case because there is only one column in your query.
Before that, you must call the Read() method to advance the reader onto the first or next record and you also need to check the return value because it will return a bool to indicate if another record was found.
So replace this line...
string class_name = dr.ToString();
With
string class_name = dr.Read() ? dr.GetString(0) : "Nothing Found";
Or if there could be more than one record returned...
string class_names = string.Empty;
while (dr.Read())
class_names = dr.GetString(0) + "\n";
Just an addition: isn't is easier to use ExecuteScalar in case there's only 1 row to expect as a result?
Example:
MySqlCommand cmd = new MySqlCommand(query, con);
cmd.Parameters.Add("#branch", MySqlDbType.VarChar, 30).Value = _order.cboBranch.Text;
var class_name = cmd.ExecuteScalar();
if (class_name != null)
{
//DoSomething with your result here.
}else{
//Item not found, handle it here
}
I am trying to use a SqlDataReader to run a query and then display the results in a messagebox, but I keep getting the error
Invalid attempt to read when no data is present.
Here is my code.
public void button1_Click(object sender, EventArgs e)
{
string results = "";
using (SqlConnection cs = new SqlConnection(#"Server=100-nurex-x-001.acds.net;Database=Report;User Id=reports;Password=mypassword"))
{
cs.Open();
string query = "select stationipaddress from station where stationname = #name";
using (SqlCommand cmd = new SqlCommand(query, cs))
{
// Add the parameter and set its value --
cmd.Parameters.AddWithValue("#name", textBox1.Text);
using (SqlDataReader dr = cmd.ExecuteReader())
{
while (dr.Read())
{
label3.Text = dr.GetSqlValue(0).ToString();
results = dr.GetValue(0).ToString();
//MessageBox.Show(dr.GetValue(0).ToString());
//MessageBox.Show(results);
}
MessageBox.Show(results);
}
}
}
}
That's correct.
When you exit from the while loop the DataReader has reached the end of the loaded data and thus cannot be used to get the value of a non-existant current record.
The Read method advances the SqlDataReader (dr) to the next record and it returns true if there are more rows, otherwise false.
If you have only one record you could use the results variable in this way
MessageBox.Show(results);
Now, this will work because you have a TOP 1 in your sql statement, but, if you have more than one record, it will show only the value of the last record.
Also as noted by marc_s in its comment, if your table is empty, your code doesn't fall inside the while loop, so probably you could initialize the results variable with a message like:
results = "No data found";
EDIT: Seeing your comment below then you should change your code in this way
.....
// Use parameters **ALWAYS** -- **NEVER** cancatenate/substitute strings
string query = "select stationipaddress from station where stationname = #name";
using (SqlCommand cmd = new SqlCommand(query, cs))
{
// Add the parameter and set its value --
cmd.Parameters.AddWithValue("#name", textBox1.Text);
using (SqlDataReader dr = cmd.ExecuteReader())
{
while (dr.Read())
{
label3.Text = dr.GetSqlValue(0).ToString();
results = dr.GetValue(0).ToString();
}
}
}
.....
I ran into a similar issue trying to get a GUID I knew was there - I could run the same SQL directly in SQL Management Studio and get my result. So instead of trying to bring it back as a GUID (it was saved in a char(35) field, even though it really was a GUID!), I brought it back as a string, instead:
SqlConnection sqlConn = null;
string projId = String.Empty;
string queryString = "SELECT * FROM project WHERE project_name='My Project'";
try
{
sqlConn = new SqlConnection(connString);
SqlCommand cmd = new SqlCommand(queryString, sqlConn);
sqlConn.Open();
SqlDataReader reader = cmd.ExecuteReader();
if (reader.HasRows)
{
while (reader.Read())
{
projId = reader.GetSqlValue(0).ToString(); // <-- safest way I found to get the first column's parameter -- increment the index if it is another column in your result
}
}
reader.Close();
sqlConn.Close();
return projId;
}
catch (SqlException ex)
{
// handle error
return projId;
}
catch (Exception ex)
{
// handle error
return projId;
}
finally
{
sqlConn.Close();
}
I'm wondering how to return the result from a SELECT COUNT statement in C#.
I have a sql statement that returns the count of 15.
Currently, I'm returning the datareader. Can I somehow return the result of that as a string?
static public SqlDataReader FillDataReader(string sql, SqlParameter[] parms)
{
SqlConnection conn = new SqlConnection(ConnectionString);
SqlCommand cmd = new SqlCommand(sql, conn);
SqlDataReader dr = null;
conn.Open();
cmd.CommandTimeout = 120; //120 seconds for the query to finish executing
foreach (SqlParameter p in parms)
{
cmd.Parameters.Add(p);
}
try
{
dr = cmd.ExecuteReader(CommandBehavior.CloseConnection);
}
catch (SqlException ex)
{
if (dr != null)
{
dr.Close();
}
conn.Close();
//DBUtilExceptionHandler(ex, sql);
throw ex;
}
finally
{
}
return dr; //This could be null...be sure to test for that when you use it
}
Or I could use a different method. I just don't know what it should be.
Any help is appreciated.
This is my select statement:
select count(LeadListID) from LeadLists WHERE SalesPersonID = 1
AND LeadListDateCreated BETWEEN '9/1/11' AND '10/1/11 23:59:59'
Sure - just use:
int count = (int) query.ExecuteScalar();
// TODO: Decide the right culture to use etc
return count.ToString();
Notes:
Use using statements instead of manual try/catch/finally blocks
You should close the connection whether or not there was an error
Given that the natural result of the query is an integer, I would change it to return an int, not a string. Let the caller make that conversion if they want to
If there's an error, you should almost certainly let the exception bubble up, rather than returning null
I would write the code as:
public static int ExecuteScalarInt32(string sql, SqlParameter[] parms)
{
using (SqlConnection conn = new SqlConnection(ConnectionString))
using (SqlCommand command = new SqlCommand(sql, conn) { Parameters = parms })
{
conn.Open();
command.CommandTimeout = 120;
return (int) command.ExecuteScalar();
}
}
If you really needed a version to work on an arbitrary data reader, you could write it as:
public static T ExecuteQuery<T>(string sql, SqlParameter[] parms,
Func<SqlDataReader, T> projection)
{
using (SqlConnection conn = new SqlConnection(ConnectionString))
using (SqlCommand command = new SqlCommand(sql, conn) { Parameters = parms })
{
conn.Open();
command.CommandTimeout = 120;
return projection(command.ExecuteReader());
}
}
And then call it with:
int count = ExecuteQuery<int>(sql, parms, reader => {
if (!reader.MoveNext()) {
throw new SomeGoodExceptionType("No data");
}
return reader.GetInt32(0);
});
Sure, you can always call the ToString() method in .NET on the single field when reading it:
dr[0].ToString()
Are there many records that you want to concat as a string? Then you loop through each row, grab the value as a string, and create a master string in a for loop fashion.
Since you're expecting only a single value, a better alternative to ExecuteReader is the ExecuteScalar method:
try
{
var count = cmd.ExecuteScalar().ToString();
}
Either cast it as a varchar in your sql:
select cast(count(LeadListID) as varchar(10))
from LeadLists
WHERE SalesPersonID = 1
AND LeadListDateCreated BETWEEN '9/1/11' AND '10/1/11 23:59:59'
or just call .ToString() on the result, as shown in other answers.
Additionally, I'm not a fan of relying on CommandBehavior.CloseConnection for DataReaders. I much prefer code like this:
static public IEnumerable<IDataRecord> GetDataReader(string sql, SqlParameter[] parms)
{
using (var conn = new SqlConnection(ConnectionString))
using (var cmd = new SqlCommand(sql, conn))
{
cmd.CommandTimeout = 120; //120 seconds for the query to finish executing
foreach (SqlParameter p in parms)
{
cmd.Parameters.Add(p);
}
conn.Open();
using (var dr= cmd.ExecuteReader())
{
while (dr.Read())
{
yield return dr;
}
}
}
}
Use ExecuteScalar instead, that will return the first field from the first record of the returned recordset, which is what you want. That will return to you an object that is really an integer. Add a ToString to that and you should be good.
I need to execute the following command and pass the result to a label. I don't know how can i do it using Reader. Someone can give me a hand?
String sql = "SELECT * FROM learer WHERE learer.id = " + index;
SqlCommand cmd = new SqlCommand(sql,conn);
learerLabel.Text = (String) cmd.ExecuteReader();
As you can see i create the SQL statement and i execute it, but it does not work. Why?
The console says:
Cannot implicitly SqlDataReader to
String...
How can i get the desired results as String so the label can display it properly.
using (var conn = new SqlConnection(SomeConnectionString))
using (var cmd = conn.CreateCommand())
{
conn.Open();
cmd.CommandText = "SELECT * FROM learer WHERE id = #id";
cmd.Parameters.AddWithValue("#id", index);
using (var reader = cmd.ExecuteReader())
{
if (reader.Read())
{
learerLabel.Text = reader.GetString(reader.GetOrdinal("somecolumn"))
}
}
}
It is not recommended to use DataReader and Command.ExecuteReader to get just one value from the database. Instead, you should use Command.ExecuteScalar as following:
String sql = "SELECT ColumnNumber FROM learer WHERE learer.id = " + index;
SqlCommand cmd = new SqlCommand(sql,conn);
learerLabel.Text = (String) cmd.ExecuteScalar();
Here is more information about Connecting to database and managing data.
ExecuteScalar() is what you need here
Duplicate question which basically says use ExecuteScalar() instead.