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C# What is the point of the using statement? [duplicate]

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Why do we need Dispose() method on some object? Why doesn't the garbage collector do this work?
(2 answers)
What are the uses of "using" in C#?
(29 answers)
Closed 10 months ago.
I'm not talking about references to assemblies, rather the using statement within the code.
For example what is the difference between this:
using (DataReader dr = .... )
{
...stuff involving data reader...
}
and this:
{
DataReader dr = ...
...stuff involving data reader...
}
Surely the DataReader is cleaned up by the garbage collector when it goes out of scope anyway?

The point of a using statement is that the object you create with the statement is implicitly disposed at the end of the block. In your second code snippet, the data reader never gets closed. It's a way to ensure that disposable resources are not held onto any longer than required and will be released even if an exception is thrown. This:
using (var obj = new SomeDisposableType())
{
// use obj here.
}
is functionally equivalent to this:
var obj = new SomeDisposableType();
try
{
// use obj here.
}
finally
{
obj.Dispose();
}
You can use the same scope for multiple disposable objects like so:
using (var obj1 = new SomeDisposableType())
using (var obj2 = new SomeOtherDisposableType())
{
// use obj1 and obj2 here.
}
You only need to nest using blocks if you need to interleave other code, e.g.
var table = new DataTable();
using (var connection = new SqlConnection("connection string here"))
using (var command = new SqlCommand("SQL query here", connection))
{
connection.Open();
using (var reader = command.ExecuteReader()
{
table.Load(reader);
}
}

Such using statement automatically disposes the object obtained at the end of scope.
You may refer to this documentation for further details.

How to close a Data Reader

I continue to receive the error that "There is already an open DataReader associated with this Command which must be closed first." when I execute following in ASP.NET MVC 4.5
foreach (var store in db.Stores)
{
var productsInStore = store.Products.ToList();
}
In my solution there a class called Store and in this particular example, I am trying to convert the Products inside each store into a new variable.
How can this be achieved?

You can force EF to load all entities immediately. To do so call ToList():
foreach (var store in db.Stores.ToList())
{
var productsInStore = store.Products.ToList();
}

Reading and writing data into sql server simultaneously

I have a service which continuously writes data in a separate thread into SQL database.Now from the same service if i am trying to read from the same table, since i already am writing into it,I get this exception : There is already an open DataReader associated with this Command which must be closed first.
So can anyone help me how to do this simultaneously?
Here s my code for reading data:
public Collection ReadData(string query)
{
{
_result = new Collection<string[]>();
string[] tempResult;
SqlDataReader _readerRead;
using (_command = new SqlCommand(query, _readConnection))
{
_readerRead = _command.ExecuteReader();
while (_readerRead.Read())
{
tempResult = new string[4];
tempResult[0] = _reader[0].ToString();
tempResult[1] = _reader[1].ToString();
tempResult[2] = _reader[2].ToString();
tempResult[3] = _reader[3].ToString();
_result.Add(tempResult);
//Console.WriteLine("Name : {0} Type : {1} Value : {2} timestamp : {3}", _reader[0], _reader[1], _reader[2], _reader[3]);
}
if (_readerRead != null)
{
_readerRead.Close();
}
_readConnection.Close();
return _result;
}
}
}
and here it is for writing to it :
public void WriteData(Collection<TagInfo> tagInfoList)
{
int i = 0;
for (i = 0; i < tagInfoList.Count; i++)
{
using( _command = new SqlCommand(insert statement here)
{
_command.Parameters.AddWithValue("Name", tagInfoList[i].Name);
_command.Parameters.AddWithValue("Type", tagInfoList[i].TagType);
_command.Parameters.AddWithValue("Value", tagInfoList[i].Value);
_reader = _command.ExecuteReader();
if (_reader != null)
{
_reader.Close();
}
}
}
}

You need a different SQLConnection to the database for your writer. You cannot use the same db connection for both.

Although its possible to do, using a separate connection I would question why you need to do this.
If you are reading and writing data to one table in the same service you will be placing unnecessary load on one SQL table, and depending on the number of queries you intend to make this could cause you problems. If you already have this data (in a different thread) why not Marshall the data from the background thread to where you need it as you write it into the database, and you don't need to read the data anymore.
However.... it is difficult to give an fair answer without seeing the code/what you are looking to achieve.

C# - ICertView2::OpenConnection Method

Iam using the OpenConnection from CertAdm.dll to open a connection.
Like this:
CERTADMINLib.CCertView connection = new CERTADMINLib.CCertViewClass();
I was wondering how can I close this connection when Iam done with it? I havent found anything about closing the connection.
Thnx in advance.

It's good practice to use "using" pattern for those kind of connections:
using (CERTADMINLib.CCertView connection = new CERTADMINLib.CCertViewClass())
{
// do something ...
}
after the last brace connections are disposed.

Close the connection so:
ICertView2 certView = null;
IEnumCERTVIEWROW row = null;
try
{
certView = new CCertView();
certView.OpenConnection( _strCAConfig );
certView.SetResultColumnCount( 1 );
certView.SetResultColumn( certView.GetColumnIndex( 0, "RequestID" ) );
row = certView.OpenView();
row.Next();
return row.GetMaxIndex();
}
finally
{
Marshal.ReleaseComObject( row );
Marshal.ReleaseComObject( certView );
}

Storing reader information in C#

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();
}