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SQL Server connection string specified without a computer name? How can that be?

I'm working from a code base I downloaded from a repository, and it is likely that I'm missing a system or local setting.
In Web.Config, I have this connection string:
<add name="Context"
connectionString="Data Source=InstanceName;
Initial Catalog=MyProduct;
Integrated Security=True;
Connect Timeout=15;
Encrypt=False;
TrustServerCertificate=False"
providerName="System.Data.SqlClient" />
(indentation mine)
Normally I would have expected the Data Source to be \\ComputerName\InstanceName or at least .\InstanceName if the SQL Server is on the same host. But here, nothing. The initially uploaded project had a local database, on the developer's machine. I can get the connection to work if I add .\, but I don't understand how only specifying the instance name can work. So, how can it?

The instance name is needed only if you want to connect to a named instance.
If your install of Sql Server hasn't created a named instance then the default for the instance is MSSQLSERVER and you DON'T need to specify that part on the connection string.
However, the computer name part is required but it could be expressed in various form
a point to mean the local computer
an IP address (local or not)
a server name recognized by the DNS system of your lan
the special string (LOCAL)
More info on the Data Source key could be found on MSDN docs for ConnectionString

Could it be that the InstanceName is the name of an ODBC Data Source that already has the target server configured, and the other employees have a corresponding ODBC data source set up?
The other option is that the connectionstring is modified before being passed to a data connector, so "MyMachineName" + connectionstring is happening somewhere (perhaps to separate production and development environments?
Also, double check the App_Data folder to make sure some sort of file-based database isn't being accessed.

In addition to what #Steve has already mentioned local or current machine can also be referred by a special string localhost. Please refer to below post:
What is the sql connection string I need to use to access localhost\SQLEXPRESS with Windows Authentication or SQL Authentication?

Unable to connect to SQL Server locally

I am getting the following exception when I am trying to connect to a web service
System.Data.SqlClient.SqlException: An attempt to attach an auto-named database for file DatabaseName.mdb failed. A database with the same name exists, or specified file cannot be opened, or it is located on UNC share.
The connection string I am using is:
Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|WS_Assignment.mdb;uid=Admin;pwd="
providerName="System.Data.OleDb"
I am trying to access SQL Server on local system.
I have gone through a lot of articles and followed all the different methods. But I got no solution.
Can anyone help me out?

Before Question was EDITED:
are you deploying on a web server , instead of your localhost. If thats the case you need to publish the sql script in Server DB as servers dont allow attachDB file in your deployment and if thats the case then your problem is solved.
After Update
you can always try with fullpath and yes you need OLEDB call not sql make sure thats not the case with your .cs code
Correction in your question:
i am trying to access SQL server On local system
:
i dont see how you can do that with .mdb file; even if thats not the case make sure sql services are running properly in your system go to->start button->program files->microsoft sql server yourversion-> configuration manager-> check running services.

In your Solution Explorer, click on "Show All Files". then go to your App_Data folder and delete WS_assignment.mdb and then run your application.
Also, your provider is wrong
providerName="System.Data.SqlClient" is the right one.
Secondly, your database name should end in .mdf or .sdf for SQL Server.
So, your connection string will become:
<connectionStrings>
<add name="ConnectionStringName"
connectionString="Data Source=|DataDirectory|WS_Assignment.sdf"
providerName="System.Data.SqlClient"/>
</connectionStrings>

Check this.
http://msdn.microsoft.com/en-IN/library/5ybdbtte%28v=vs.71%29.aspx
using Access requires System.Data.OleDb library

Is there a universal connection string for different computers to connect to a local database using my program developed by vs2010

newbie here.
I have a local db in my program. Whilst I was developing the program I used the SQL
Connection string :
SqlConnection sconn = new SqlConnection(#"Data Source=.\SQLEXPRESS;AttachDbFilename=C:\Users\leemk_000\Documents\......Integrated Security=True;User Instance=True;");
Now If I want to load this program onto a different computer I am sure that this connection will no longer work simply because it will still be looking for Users\Lee_000\
I have tried to remove Lee_000 but I get this following error:
An attempt to attach an auto-named database for file C:\Users\Documents..... failed. A database with the same name exists, or specified file cannot be opened, or it is located on UNC share.
What can I do to get a connection string to work on different computers.
With many thanks

The whole User Instance and AttachDbFileName= approach is flawed - at best - especially when you want to share your database amongst multiple clients!
When running your app in Visual Studio, it will be copying around the .mdf file (from your App_Data directory to the output directory - typically .\bin\debug - where you app runs) and most likely, your INSERT works just fine - but you're just looking at the wrong .mdf file in the end!
If you want to stick with this approach, then try putting a breakpoint on the myConnection.Close() call - and then inspect the .mdf file with SQL Server Mgmt Studio Express - I'm almost certain your data is there.
The real solution in my opinion would be to
install SQL Server Express (and you've already done that anyway)
install SQL Server Management Studio Express
create your database in SSMS Express, give it a logical name (e.g. YourDatabase)
connect to it using its logical database name (given when you create it on the server) - and don't mess around with physical database files and user instances. In that case, your connection string would be something like:
Data Source=.\\SQLEXPRESS;Database=YourDatabase;Integrated Security=True
and everything else is exactly the same as before...

If it's a local db you should be placing it within the app folder and carry it with the app right?
Put the database in the App_data folder of your app and use that in your connection string
<add name="YourConnectionString" connectionString="Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\yourfile.mdf;Integrated Security=True;User Instance=True" providerName="System.Data.SqlClient"/>

You need to use a database server and let your users use it via your connection string like this;
Server=myServerAddress;Database=myDataBase;User Id=myUsername;Password=myPassword;
"myServerAddress" should be the ip adress of your server machine.

Database Error while launching application

I deployed C# application and created installer. Installer gets created successfully but when tried to launch the application it throws up following error:
An attempt to attach an auto-named database for file CampusPointe.mdf
failed. A database with the same name exists, or specified file cannot
be opened, or it is located on UNC share.
connection string in app.config is as follows:
<connectionStrings>
<add name="App_Key_Management.Properties.Settings.VendorKeyConnectionString1"
connectionString="Data Source=.\SQLEXPRESS;AttachDbFilename="C:\Program Files\Vendor Key Management\VendorKey.mdf";Integrated Security=True;Connect Timeout=30;User Instance=True"
providerName="System.Data.SqlClient" />
</connectionStrings>
Any help would be appreciated.
Regards,
Sri

Sri,
First of all, you need to ensure a couple of things:
1. You have installed SQL Server
2. You need to deal with the creation or replacement of the file you created. I think this is your issue ... If the .mdf file exists then you need to either update the existing file or drop and replace it (USE [database] GO DROP TABLE... GO CRATE TABLE ... GO). You will also need to consider step 4 below.
3. If you are accessing a a file located in a database you need a fully formed name. You also need to add the database user to the connection string in ASP.NET Web.Config or App.config (depending on the app). I looks like you have started this ...
4. Make sure the user account that you are using in the connection string has the necessary privileges on the database side. If this is a full fledged database in SQL SERVER then the user account will need update, delete, create privileges. If you are replacing or creating databases on the fly then the user will need CREATE DATABASE or DROP / CREATE privileges.
I think to best answer your question we would need more information on the architecture of the application. My guess is that your code creates the database fine the first time and then when you try to run it the second time it fails because the database is already there. If this is the case you also need to address your code (with IF statements or SWITCH CASE ... your call) to handle situations where the table (1) does not exist and needs to be created; (2) exists and needs to be updated; and / or (3) exists and needs to be replaced. The correct answer again depends on your code and what you are trying to do.

How to make the connection string of an Windows form application in C#.net,Computer independent?

As I transfer my Windows form application in C#.net from one computer to another, I have to change the connection string every time as the location of the Database file has changed.
How can I can I prevent this,so that I don't have to change the connection string again and again?

If the service you need to connect is always running on the local machine, you might use the localhost as the server name...
By the way localhost is mapped to the ip 127.0.0.1 in the hosts file.

Have your DB file in the same location of your application exe and then you can use
Application.StartupPath()
to get the path.
*I am assuming that this is a Windows Forms Application.

How has the location of the database changed? Is it not in a central location that all computers/users can access?
If not, you could store the connection information in settings and create a form that allows you to update those as needed. The form could be launched as part of the installer or on first run of the application.
A little more information about what you're doing would be helpful in presenting a real solution.

It's tricky to tell without an example of the type of connection string you're using (or which database you're accessing) but can't you use a relative path and assume that the database is somewhere relative to your app?
If you set your connection sting up like this...
<connectionStrings>
<add name="ConsoleApplication1.Properties.Settings.Database1ConnectionString"
connectionString="Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\Database1.mdf;Integrated Security=True;User Instance=True"
providerName="System.Data.SqlClient" />
</connectionStrings>
... then |DataDirectory| will by default resolve to the application's folder. You can, however, change DataDirectory by calling the AppDomain.SetData method. See the following for how to change it...
http://social.msdn.microsoft.com/Forums/en-US/adodotnetdataproviders/thread/805bfd29-d864-4400-b353-bea13167f046
I've shown how you'd set the connection string in config, but the same holds true if you're setting it in code. If you are building a connection string in code, then may I suggest you look at using a connection string builder...
http://msdn.microsoft.com/en-us/library/ms254947.aspx
I am, of course, assuming a file path in your connection string, but if it's a database then won't localhost also work?