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Instance failure when connection to DB through C# code

I am getting this error while connecting to database , though this error is not seen everytime.
The connection string in c# is as follows,
"Data Source=" + machineName + "\\" + instance + ";Initial
Catalog=dbName;Integrated Security=True";
When i narrowed down the exact place the exception is thrown, it happens to be when database connection open is attempted.
DBConnection.con.Open();
This issue is not reproducible consistenly.
I searched alot and was not able to figure out why this was happenend.I verified the suggestion that was shared,
Verified that instance being used and SQL service is up and running.
TCP/IP is enabled and remote connections are allowed.
Checked the SQL error logs and couldn't figure out anything wrong there.
The connection string has proper format of machinename\instance .
I also restarted the SQL service mutliple times but no luck with that too. I am stuck and do not know how to proceed forward.
Any suggestion is most welcome.

Try connecting to your database from Visual Studio (SQL Server Explorer). Once successful, right-click on the server instance, copy the connection string. Like this, you are sure about the syntax. A database connection is meant to be on a local network. If you connect to a database over internet, implement a webservice.
Have fun :)

web service - SQL connection string

I've built an .asmx web service which retrieves informations from a local sql server 2014 database.
Everything is working fine on localhost, but after publishing the web service to Azure i get the error:
An unhandled exception of type 'System.Web.Services.Protocols.SoapException'
occurred in System.Web.Services.dll
Additional information: Server was unable to process request. ---> A
network-related or instance-specific error occurred while
establishing a connection to SQL Server. The server was not found or
was not accessible. Verify that the instance name is correct and that
SQL Server is configured to allow remote connections. (provider: SQL
Network Interfaces, error: 26 - Error Locating Server/Instance
Specified)
SQL Server is configured to allow remote connections.
My connection string is like:
string con2 = #"Data Source=OfficePc\MSSQLSERVER2014;Initial Catalog=Database;Persist Security Info=True;User ID=Admin;Password=123456";
Is the error the result of something missing from the connection string, or am i missing some configurations changes?

As Paul mentioned in a comment under the question, your connection string is pointing to a local database resource (presumably on your dev machine). Even though you configured your local database server to support remote connections, the address OfficePc\MSSQLSERVER2014 isn't addressable, as that does not equate to a machine address (IP address).
Your app would need to connect to your database via an accessible IP address (which might require you to do some port-forwarding on your local network, or open ports on your firewall).
Alternatively, you can migrate your database to Azure (either with SQL Server in a VM or with the SQL Database service).
Keep in mind: If you are accessing a local (on-premises) database server from Azure, there will be latency added, as well as some outbound bandwidth costs.

It looks like SQL Server instance is not running or not accessible. Try connecting to the same database using SSMS and if you get the same error then the instance is not running.

Mostly the error occurs when the Database server was not found. Recheck if the server name (Data Source) is mentioned correctly. If you manually generated the connection string use .uld file to generate connection string.
To auto generate connection string using .udl file:
Create a sampe.txt file.
Rename it as sample.udl file.
Then double click on it, It will show you window entitled 'Data Link Properties'.
Configure the connection there.
Then Test the connection using test connection button.
Then open the file with notepad. It will show you the exact connection string.
For further reference check : MSDN

cannot connect sql server 2008 from another computer [duplicate]

This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
System.Data.SqlClient.SqlException A network-related or instance-specific error
i have winform application in c# which connect to my own db and its working fine my current sql connection string is :
SqlConnection con = new SqlConnection("Data Source=ITPL_PC1;Initial Catalog=Data_Project;Persist Security Info=True;user id=sa;Password=insforia");
but when i am taking this winform to another computer its not working .. then i am changing my connection string to :
SqlConnection con = new SqlConnection("Data Source=192.168.0.28\\ITPL_PC1;Initial Catalog=Data_Project;Persist Security Info=True;user id=sa;Password=insforia");
but its not working ...
i have changed all the settings in sql server of remote access but still it showing an error
Sql Exception was unhandled
A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)
is my connection string is wrong? what it should be?
pls help me out

Data Source=ITPL_PC1
Identifies a server called ITPL_PC1. You're connecting to the default instance on this server.
Data Source=192.168.0.28\ITPL_PC1
Identifies an instance called ITPL_PC1 running on a server with IP address 192.168.0.28. We don't know the name of this server.
If the instance you want to connect to is the default instance on ITPL_PC1, then the first version should always work (provided ITPL_PC1 can be resolved).

The Windows security might be preventing the access. You can try the below.
1.Go to the service management console of windows by typing services.msc in the run window.
2. In the window displayed you can see a list of services. From the list find out Base Filtering Engine.
3.Right click and stop it. Its done
I have fixed the same issue like this after lot of googling.(For me it was not working even after enabling TCP/IP from sql configuration manager)

First you open SQL managnebt studio and connect to another pc sql server :
Please follow bolew step :
Strat -> program -> window sql server 2008 -> configuration tools
--> sql server Configuration manager -->
Start sql server browser service
-----------
Next --->Sql serverNetework COnfiguration --> Enable all Protocls and
also Sql Native Client.. --> Enable all Protocls then
--------------
reatsrt all service and connect to sql
-------------------------------------------------
If you get connection then SQL Database to connect database from visual studio from there to find connection string and copy and paste to connection string.

SQL Server connectivity issues even with valid connection string

This has been plaguing me for the last few hours, I'm recieving the following error message on an ASP.NET web page:
A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)
In the past to fix this I've found it was a simple connection string error, however I'm using Idera's Connection String generator tool. It seems to be able to connect to the server just fine when I test the string. I can connect with SQL Management Studio, as well as I can create a blank .udl file, fill in the details, and it connects without issue.
I've double checked, and the server is allowing remote connections, I turned the firewall off at this point (just trying to get this to work) and still no go.
Do you guys think I'm missing something ? Any help is GREATLY appreciated!

And when you try adding a connection from Visual Studio ?
You could also use the connection string that is generated by VS while doing this.
It seems as if you are missing the instance name or something like that.
Edit : Yes, as explained in the comment : Right click the connection, then copy the Connection string.
But you might have to replace the "name" by the IP address, if the resolution of the host name is not being performed correctly

From the web servercan you resolve the name of the server using in the connection string - i.e. is it a DNS issue?

SQL Error : error: 40 - Could not open a connection to SQL Server

I Get this error message whenever I tried to up my aspx page.
An error has occurred while establishing a connection to the server. When connecting to SQL Server 2005, this failure may be caused by the fact that under the default settings SQL Server does not allow remote connections. (provider: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server)
whic is connected in this connection string
SqlConnection conn = new SqlConnection("Data Source=192.168.xxx.xxx;Initial Catalog=DBSample;User ID=dev;Password=pass;Integrated Security=SSPI;"))
The weird thing is that the server that I'm connecting has already hosting some aspx page. I don't knnow if there's missing in my connectiong string Thanks. and I know the server that I'm connecting to is already allowed remote connection since it's already hosted some aspx websites. :(
Thanks!

Do you need to add an instance name to your connection string? Do you have the SQLBrowser service running on the target machine, or do you have to specify a port for the instance?
You also get that very same error when the database doesn't exist at the location that you are trying to connect to. Have tried looking at the connection strings of the aspx pages that are successfully connecting?
edited: Specifying Integrated Security=SSPI means you will be using Windows authentication to login to the database. What user is your aspx page running as (check your app pool)? Does it have the rights to log in to the database? This could also explain why it works on one server but not another.

Are you trying to connect to a hosted SQL Server over TCP/IP?
The reason I ask is some firewalls block traffic over Port 1433.
If not then it is simply a case of validating the connection string details and ensuring the SQL Server Engine is actually running...
Have you EVER been able to connect to this database from the PC you are currently attempting this connection on?