I have following statement using using:
using (var reader = data.CreateCommand(sql).ExecuteDataReader())
in this case data is some object which internally holds SqlConnection. CreateCommand(sql) function returns SqlCommand and ExecuteDataReader returns SqlDataReader. Since SqlCommand and SqlDataReader are both IDisposable, will they both be disposed by this use of using statement?
For now I have done it like this:
using (var cmd = data.CreateCommand(sql))
using (var reader = cmd.ExecuteDataReader())
But I would like to know if it's possible to combine them as stated above?
"if it's possible to combine them" - two using are totally fine, because both needs to be disposed.
You can combine them by extracting into method:
void RunWithReader(string sql, Action<SQLDataReader> action)
{
using (var cmd = data.CreateCommand(sql))
using (var reader = cmd.ExecuteDataReader())
action(reader);
}
Then you can use lambda
RunWithReader("whatever command", reader =>
{
... // while(reader.Read() { ... } - this could also be extracted
});
Agree with Matthew Watson's comment. You need to have both of usings statemets.
Here is the related question with additional reasoning. SqlConnection SqlCommand SqlDataReader IDisposable
The way you have presented your code the inner IDisposable (IDbCommand) is not disposed.
You have two choices:
You can put it all in one using like this:
using (IDisposable cmd = data.CreateCommand(), reader = ((IDbCommand)cmd).ExecuteReader())
{
// code here
}
But that is rather cumbersome. The other option is nested using statements:
using (var cmd = data.CreateCommand())
{
using (var reader = cmd.ExecuteReader())
{
// code here
}
}
Other than that you can get a bit complicated and write an extension method to help (sort of) clean it up for you.
public static class Ex
{
public static void Using<T, R>(this T obj, Func<T, R> create, Action<T, R> use) where R : IDisposable
{
using (var d = create(obj))
{
use(obj, d);
}
}
}
Then you can do this:
data.Using(d => d.CreateCommand(), (d, c) => c.Using(c2 => c2.ExecuteReader(), (c3, r) =>
{
// code here
}));
But perhaps that's not much of an improvement.
In General
You should Dispose every class that implements IDisposable.
However, your question is specifically dealing with a scenario where all the resources are all related, which if my understanding is correct, the Garbage Collector will dispose these resources for you automatically.
The Finalize() Method ("Destructors" or "Finalizers")
The Finalize() method makes sure that resources are disposed when
the instance is no longer referenced by the application.
In concert together with the Dispose() method
The Dispose() method will "release all resources used by the component". Which would mean that this code block (taken from MSDN's ExecuteReader Method Example) will properly dispose of all the compenents used by it...
using (SqlConnection connection = new SqlConnection(connectionString))
{
connection.Open();
SqlCommand command = new SqlCommand(queryString, connection);
SqlDataReader reader = command.ExecuteReader();
while (reader.Read())
{
// ...
}
}
In this case, when disposing SqlConnection, it will properly
release all resources used by the component. Which means:
SqlCommand
SqlDataReader
** Note: This has always been my understanding...
Refactoring of the above code to the following:
using (SqlConnection connection = new SqlConnection(connectionString))
{
connection.Open();
using (SqlCommand command = new SqlCommand(queryString, connection))
{
using (SqlDataReader reader = command.ExecuteReader())
{
while (reader.Read())
{
// ...
}
}
}
}
The main advantage of manually disposing of each resource, would be
for a performance gain since we are not processing up to the
Finalize method (which is relatively a more expensive than doing Dispose().
However, that doesn't mean you want to Dispose() everything, as in some cases the Finalize() method is a better choice.
Assuming that I am still in good shape so far, and my understanding of the Garbage Collector's behavior is valid.
I would consider that SqlCommand (full code reference here), would then also dispose of SqlReader, which would mean that you could just get away with:
using (SqlCommand command = new SqlCommand(queryString, connection))
{
SqlDataReader reader = command.ExecuteReader();
while (reader.Read())
{
// ...
}
}
which in your case would translate to:
using (var cmd = data.CreateCommand(sql))
{
var reader = cmd.ExecuteDataReader();
// ...
}
Related
I am using npgsql to connect to a CockroachDb cluster, in code with each command I want to execute I create a new connection and open it! I wonder if creating a private field variable holding the connection and opening it then closing it is much better than my current approach?
static async Task Task1()
{
using (var conn = new NpgsqlConnection(connString))
{
conn.Open();
new NpgsqlCommand("SQL Query here", conn).ExecuteNonQuery();
conn.Close();
}
}
static async Task Task2()
{
using (var conn = new NpgsqlConnection(connString))
{
conn.Open();
new NpgsqlCommand("SQL Query here", conn).ExecuteNonQuery();
conn.Close();
}
}and so on ....
Not really: although possible, it's a bit of an anti pattern.
NpgsqlConnection inherits from DbConnection. And DbConnection is a general Microsoft abstract class that represents a database connection, which is IDisposable.
So when putting the conn in a using block, you implicitly call Dispose as soon as it goes out of scope. This will close the connection and clean up other stuff. You don't have to separately call Close.
What you could do, is make a general wrapper like
public static async Task NonQueryCommand(string command)
{
using (var conn = new NpgsqlConnection(connString))
{
conn.Open();
using (var cmd = new NpgsqlCommand(command, conn) // Also IDisposable
{
cmd.ExecuteNonQuery();
}
}
}
But you could lose all flexibility that way (the repository pattern can be bad if not used correctly).
Also check the official resources
This question already has answers here:
Dispose object that has been instantiated as method parameter c#
(2 answers)
Closed 3 years ago.
PREAMBLE
I understand that normally most SQL calls run thusly
using(var cnn = new DbConnection("YOURCONNECTIONSTRINGHERE")
{
cnn.Open(); //Open the connection.
using(var cmd = new DbCommand("YourSQL", cnn)
{
cmd.ExecuteScalar; //Or whatever type of execution you want.
}
}
This will properly dispose of both the connection and the command.
My Question: Will this code properly dispose of both objects?
using(var cmd = new SqlCommand("YourSQL", new Connection("YOURCONNECTIONSTRINGHERE"))
{
cmd.ExecuteScalar; //Or whatever type of execution you want.
}
In reality I'm using a method that provides and opens the connection.
public SqlConnection Connection()
{
var product = new SQLConnection("ConnectionString");
product.Open();
return product;
}
So at the end of the day the call looks like this:
using(var cmd = new SqlCommand("YourSQL", Connection())
{
cmd.ExecuteScalar; //Or whatever type of execution you want.
}
I know the SqlCommand object will be disposed of but will the SQLConnection, created within the using parameter declaration, be disposed of? I've tried running some simple unit tests but it seems inconclusive.
Will this code properly dispose of both objects?
using(var cmd = new SqlCommand("YourSQL", new Connection("YOURCONNECTIONSTRINGHERE"))
{
cmd.ExecuteScalar; //Or whatever type of execution you want.
}
The above code does not call Dispose() on the connection. The using block ensures that cmd.Dispose() is called immediately after the execution of the block, but the connection remains open. Since the connection has no object referencing it, it will be eventually closed/disposed by the Garbage Collector.
If you want to dispose the Command and Connection immediately, then try:
using (var con = Connection()) // <-- GetConnection() would be a better name
using (var cmd = new SqlCommand(con)
{
cmd.ExecuteScalar;
}
I've noticed something curious about reading from an IDataReader within a using statement that I can't comprehend. Though I'm sure the answer is simple.
Why is it that whilst inside the using (SqlDataReader rd) { ... } if I directly perform a yield return the reader stays open for the duration of the read. But if I perform a direct return calling a SqlDataReader extension method (outlined below) that the reader closes before the enumerable can be actualized?
public static IEnumerable<T> Enumerate<T>(this SqlDataReader rd)
{
while (rd.Read())
yield return rd.ConvertTo<T>(); //extension method wrapping FastMember
rd.NextResult();
}
To be absolutely clear of what I'm asking, I'm unsure why the following are fundamentally different:
A fleshed out example, as per #TimSchmelter's request:
/*
* contrived methods
*/
public IEnumerable<T> ReadSomeProc<T>() {
using (var db = new SqlConnection("connection string"))
{
var cmd = new SqlCommand("dbo.someProc", db);
using(var rd = cmd.ExecuteReader())
{
while(rd.Read())
yield return rd.ConvertTo<T>(); //extension method wrapping FastMember
}
}
}
//vs
public IEnumerable<T> ReadSomeProcExt<T>() {
using (var db = new SqlConnection("connection string"))
{
var cmd = new SqlCommand("dbo.someProc", db);
using(var rd = cmd.ExecuteReader())
{
return rd.Enumerate<T>(); //outlined above
}
}
}
/*
* usage
*/
var lst = ReadSomeProc<SomeObect>();
foreach(var l in lst){
//this works
}
//vs
var lst2 = ReadSomeProcExt<SomeObect>();
foreach(var l in list){
//throws exception, invalid attempt to read when reader is closed
}
Summary: Both versions of the method defer, but because ReadSomeProcExt doesn't defer execution, the reader is disposed before execution is passed back to the caller (i.e. before Enumerate<T> can run). ReadSomeProc, on the other hand, doesn't create the reader until it's been passed back to the caller, so it doesn't dispose the container until all its values have been read.
When your method uses yield return, the compiler actually changes the compiled code to return an IEnumerable<>, and the code in your method will not run until other code starts iterating over the returned IEnumerable<>.
That means that the code below doesn't even run the first line of your Enumerate method before it disposes the reader and returns a value. By the time someone else starts iterating over your returned IEnumerable<>, the reader has already been disposed.
using(SqlDataReader rd = cmd.ExecuteReader()){
return rd.Enumerate<T>();
}
But this code would execute the entire Enumerate() method in order to produce a List<> of results prior to returning:
using(SqlDataReader rd = cmd.ExecuteReader()){
return rd.Enumerate<T>().ToList();
}
On the other hand, whoever's calling the method with this code doesn't actually execute the method until the result is evaluated:
using(SqlDataReader rd = cmd.ExecuteReader()){
while(rd.Read())
yield return rd.ConvertTo<T>(); //extension method wrapping FastMember
}
But the moment they execute the returned IEnumerable<>, the using block opens up, and it doesn't Dispose() until the IEnumerable<> finishes its iterations, at which point you will have already read everything you need from the data reader.
It's because the "yield return" will return one element and continue the iteration, while the "normal" return will finish the invocation.
I'm looking to figure out the best way to execute a database query using the least amount of boilerplate code. The method suggested in the SqlCommand documentation:
private static void ReadOrderData(string connectionString)
{
string queryString = "SELECT OrderID, CustomerID FROM dbo.Orders;";
using (SqlConnection connection = new SqlConnection(connectionString))
{
SqlCommand command = new SqlCommand(queryString, connection);
connection.Open();
SqlDataReader reader = command.ExecuteReader();
try
{
while (reader.Read())
{
Console.WriteLine(String.Format("{0}, {1}", reader[0], reader[1]));
}
}
finally
{
reader.Close();
}
}
}
mostly consists of code that would have to be repeated in every method that interacts with the database.
I'm already in the habit of factoring out the establishment of a connection, which would yield code more like the following. (I'm also modifying it so that it returns data, in order to make the example a bit less trivial.)
private SQLConnection CreateConnection()
{
var connection = new SqlConnection(_connectionString);
connection.Open();
return connection;
}
private List<int> ReadOrderData()
{
using(var connection = CreateConnection())
using(var command = connection.CreateCommand())
{
command.CommandText = "SELECT OrderID FROM dbo.Orders;";
using(var reader = command.ExecuteReader())
{
var results = new List<int>();
while(reader.Read()) results.Add(reader.GetInt32(0));
return results;
}
}
}
That's an improvement, but there's still enough boilerplate to nag at me. Can this be reduced further? In particular, I'd like to do something about the first two lines of the procedure. I don't feel like the method should be in charge of creating the SqlCommand. It's a tiny piece of repetition as it is in the example, but it seems to grow if transactions are being managed manually or timeouts are being altered or anything like that.
edit: Assume, at least hypothetically, there's going to have to be a bunch of different types of data being returned. And consequently the solution can't be just one one-size-fits-all method, there will have to be a few different ones depending, at minimum, on whether ExecuteNonQuery, ExecuteScalar, ExecuteReader, ExecuteReaderAsync, or any of the others are being called. I'd like to cut down on the repetition among those.
Tried Dapper?
Granted this doesn't get you a DataReader but you might just prefer it this way once you've tried it.
It's about the lightest-weight an ORM can be while still being called an ORM. No more methods to map between DataReader and strong types for me.
Used right here on all the StackExchange sites.
using (var conn = new SqlConnection(cs))
{
var dogs = connection.Query("select name, age from dogs");
foreach (dynamic dog in dogs)
{
Console.WriteLine("{0} age {1}", dog.name, dog.age);
}
}
or
using (var conn = new SqlConnection(cs))
{
var dogs = connection.Query<Dog>("select Name, Age from dogs");
foreach (Dog dog in dogs)
{
Console.WriteLine("{0} age {1}", dog.Name, dog.Age);
}
}
class Dog
{
public string Name { get; set; }
public int Age { get; set; }
}
If you want to roll data access on your own, this pattern of help methods could be one way to remove duplication:
private List<int> ReadOrderData()
{
return ExecuteList<int>("SELECT OrderID FROM dbo.Orders;",
x => x.GetInt32("orderId")).ToList();
}
private IEnumerable<T> ExecuteList(string query,
Func<IDataRecord, T> entityCreator)
{
using(var connection = CreateConnection())
using(var command = connection.CreateCommand())
{
command.CommandText = query;
connection.Open();
using(var reader = command.ExecuteReader())
{
while(reader.Read())
yield return entityCreator(reader);
}
}
}
You'll have to add support for parameters and this might not compile, but the pattern is what I'm trying to illustrate.
What I typically do is use a custom class that I wrote a while back that accepts a SQL string, and optionally a list of parameters and it returns a DataTable.
Since the thing that changes between invocations is typically just the SQL that is optimal IMHO.
If you truly do need to use a DataReader you can do something like this:
public void ExecuteWithDataReader(string sql, Action<DataReader> stuffToDo) {
using (SqlConnection connection = new SqlConnection(connectionString)) {
using (SqlCommand command = new SqlCommand(sql, connection)) {
connection.Open();
using (SqlDataReader reader = command.ExecuteReader()) {
try {
while (reader.Read()) {
stuffToDo(reader);
}
}
finally {
reader.Close();
}
}
}
}
}
private static void ReadOrderData(string connectionString) {
string sql = "SELECT OrderID, CustomerID FROM dbo.Orders;";
ExecuteWithDataReader(sql, r => Console.WriteLine(String.Format("{0}, {1}", r[0], r[1])));
}
The first two line are the most important thing you need...
but if you still wish to do it, you can turn them to a database handler class, yes it will become more of code, but in refactoring concept, every thing will move to the related topic...
try to write a singleton class, that receive a command and do action, so return result of type SqlDataReader reader...
Doing this in comments was too much.
I would suggest that the boilerplate code around
using(conn = new sqlconnection)
using(cmd = new sqlcommand) {
// blah blah blah
}
isn't something to be lightly removed and instead would encourage that you keep it exactly where it's at. Resources, especially unmanaged ones, should be opened and released at the closest point to execution as possible IMHO.
In no small part due to the ease with which other developers will fail to follow the appropriate clean up conventions.
If you do something like
private SQLConnection CreateConnection()
{
var connection = new SqlConnection(_connectionString);
connection.Open();
return connection;
}
Then you are inviting another programmer to call this method and completely fail to release the resource as soon as the query is executed. I don't know what kind of app you are building, but in a web app such a thing will lead to memory / connection / resource errors of types that are difficult to debug, unless you've been through it before.
Instead, I'd suggest you look into a lightweight ORM such as Dapper.net or similar to see how they approached it. I don't use dapper, but I hear it's pretty good. The reason I don't use it is simply that we don't allow inline sql to be executed against our databases (but that's a very different conversation).
Here's our standard:
public static DataTable StatisticsGet( Guid tenantId ) {
DataTable result = new DataTable();
result.Locale = CultureInfo.CurrentCulture;
Database db = DatabaseFactory.CreateDatabase(DatabaseType.Clients.ToString());
using (DbCommand dbCommand = db.GetStoredProcCommand("reg.StatsGet")) {
db.AddInParameter(dbCommand, "TenantId", DbType.Guid, tenantId);
result.Load(db.ExecuteReader(dbCommand));
} // using dbCommand
return result;
} // method::StatisticsGet
We make heavy use of Enterprise Library. It's short, simple and to the point and very well tested. This method just returns a datatable but you could easily have it return an object collection.. or nothing.
Which pattern is better for SqlConnection object? Which is better in performance?
Do you offer any other pattern?
class DataAccess1 : IDisposable
{
private SqlConnection connection;
public DataAccess1(string connectionString)
{
connection = new SqlConnection(connectionString);
}
public void Execute(string query)
{
using (SqlCommand command = connection.CreateCommand())
{
command.CommandText = query;
command.CommandType = CommandType.Text;
// ...
command.Connection.Open();
command.ExecuteNonQuery();
command.Connection.Close();
}
}
public void Dispose()
{
connection.Dispose();
}
}
VS
class DataAccess2 : IDisposable
{
private string connectionString;
public DataAccess2(string connectionString)
{
this.connectionString = connectionString;
}
public void Execute(string query)
{
using (SqlConnection connection = new SqlConnection(connectionString))
{
SqlCommand command = connection.CreateCommand();
command.CommandText = query;
command.CommandType = CommandType.Text;
// ...
command.Connection.Open();
command.ExecuteNonQuery();
command.Connection.Close();
}
}
public void Dispose()
{
}
}
There's no real way to answer this question. The short, canonical answer is that the connection should stay alive for the lifetime of your unit of work. Because we have no way of knowing how DataAccess is used (does it exist for the lifetime of your application, or do you instantiate it and dispose it whenever you do something?), it's impossible to give a concrete answer.
That being said, I would recommend the first pattern, but instantiate and dispose of your DataAccess object as needed; don't keep it around longer than necessary.
Suggest going with DataAccess2. It's a personal preference though. Some might even suggest your class be static. It'd be difficult to say that one is more performant than the other. You're on the path of IDisposable, which is great.
I'd be happy to read and maintain both styles shown above in your question.
Consider having your DAL be able to read the connection string from a .config as well, rather than exclusively allowing the value to be passed in the constructor.
public DataAccess2(string connStr)
{
this.connectionString = connStr;
}
public DataAccess2()
{
this.connectionString =
ConfigurationManager.ConnectionStrings["foo"].ConnectionString;
}
Consider wrapping your SqlCommand in a using as well.
using (var conn = new SqlConnection(connectionString))
{
using(var cmd = conn.CreateCommand())
{
}
}
I think it depends on how your DataAccess object is intended to be used, if it's used within a 'using' clause then the connection is guaranteed to be disposed of after it's done.
But in general I prefer the second pattern as the sql connection is created and disposed of within the Execute method so it's less likely to be left open when you forget to dispose of your DataAccess object.
Considering that sql connection can be a scarse resource I think every attempt should be made to ensure that they're not wasted.
The first will result in errors if you make concurrent calls.
The second will ensure you use a clean connection for each command resulting in more connections being made.
I agree with the statements above that it depends on the scenario for use, to get over the problem related to the first I have a wrapper that needs to use such a pattern so I set a field value boolean to show that a command is being executed on the connection already then "queue" the next command for execution.
There will of course be situations where you may prefer to use multiple connections ...