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C# Update Connection String Visual Studio

I am having some connectivity issues with Visual Studio and SQL Server.
This is the connections string in my web.config file;
<connectionStrings>
<add name="WintipToysConnectionString" connectionString="Data
Source=localhost\sqlexpress;Initial Catalog=WINGTIPTOYS;Integrated
Security=True" providerName="System.Data.SqlClient" />
</connectionStrings>
According to my DB connection string found in Visual Studio, my connection string is;
Data Source=localhost\sqlexpress;Initial Catalog=WINGTIPTOYS;Integrated Security=True
Which seems to add up - but, I get this error when my program hits a certain line;
System.Data.SqlClient.SqlException: '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: 50
- Local Database Runtime error occurred. Cannot create an automatic instance. See the Windows Application event log for error details.
I am actually following the WingTipToys tutorial on the Microsoft website and I am struggling to get it to use the Database I attached to SSMS.
I can write queries inside VS which works fine. So I don't really understand why my program is having this error when running?
Thanks.

Admittedly haven't used SQLExpress in a looong time. It looks like the syntax you're using is for an "instance" - which is pointed out in the error message:
...Cannot create an automatic instance...
What you have does look like server\instance conn string syntax...
Connect to the "default" instance of SQL Express like so: Data Source=.\\SQLExpress
Refer to this:
SSE Connection string sample
Hth...

Service-Based Database Entity Framework Connection Error On released version

I am writing a Windows Forms application, where I want to save data inside the application. For the reason I used a serviced-based database with a .mdf database file, and I use Entity Framework.
So far everything is working fine until I released the file and opened it on another computer. The application opened and everything worked fine but when it comes to the database interaction, it throws a big error like this:
************ Exception Text ************
System.Data.Entity.Core.EntityException: The underlying provider failed on Open.
System.Data.SqlClient.SqlException: 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: 50 - Local Database Runtime error occurred. The specified LocalDB instance does not exist.)
Here a screenshot of the error: https://www.dropbox.com/s/gwban6ab97c6fya/22093543_10213995193571435_760919436_n.png?dl=0
In case you need the project its uploaded to here: https://www.dropbox.com/s/ubpc683ggtihh6k/usercontrol.zip?dl=0
I have tried in so many ways but nothing is working, same thing happens for the installer version as well.
Can anyone help me on this please?
Here are my connection strings:
<add name="DatabaseEntities"
connectionString="metadata=res://*/dbModel.csdl|res://*/dbModel.ssdl|res://*/dbModel.msl;provider=System.Data.SqlClient;provider connection string="data source=(LocalDB)\MSSQLLocalDB;attachdbfilename=|DataDirectory|\Database.mdf;integrated security=True;MultipleActiveResultSets=True;App=EntityFramework""
providerName="System.Data.EntityClient" />
<add name="OPLCheque.Properties.Settings.DatabaseConnectionString"
connectionString="Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=|DataDirectory|\Database.mdf;Integrated Security=True"
providerName="System.Data.SqlClient" />

I'm pretty sure this post would be flagged as duplicate since a similar question has been answered multiple times already, but here you go.
You are using LocalDB which means it cannot be shared since it runs on SQL Server Express.
If you simply want a LocalDB, see this answer to implement it after publishing your WinForms app.
Basically you want to publish your app with prerequisite components so you should publish those along your application.
If you want to use a single database to be shared between instances of your application, learn how to setup one using SQL Server here.
Afterwards, learn what connection string format you should use.
Sample to connect via IP Address:
connetionString="Data Source=IP_ADDRESS,PORT;
Network Library=DBMSSOCN;Initial Catalog=DatabaseName;
User ID=UserName;Password=Password"
See this for more.
If ever you plan on using a shared database, better make sure that the connection string that your database context should use is specified.
Config:
<add name="DefaultConnection" connectionString="" providerName="" />
Database Context:
public DbContext()
: base ("DefaultConnection")
{
}

How to connect to SQL Server Express using EntityFramework

I tried to find information about this on the web and Stack Overflow, but none of the responses could help me solving my problem.
I'm working with C# and SQL Server. I'm using code from an existing project, which project connects to an SQL Server instance to manage data. This project uses EntityFramework.
Now I want to use a local instance of SQL Server, precisely SQL Server Express, for development purpose. But I can't figure out how to connect to this local instance.
Connecting to the remote SQL Server works perfectly, using this connection string:
<add name="ProjectEntities" connectionString="metadata=res://*/Project.Project.csdl|res://*/Project.Project.ssdl|res://*/Project.Project.msl;provider=System.Data.SqlClient;provider connection string="data source=111.111.111.666\(local), 1433;initial catalog=PROJECT;persist security info=True;user id=user;password=password;MultipleActiveResultSets=True;App=EntityFramework"" providerName="System.Data.EntityClient" />
But if I modify it to use my local instance:
<add name="ProjectEntities" connectionString="metadata=res://*/Project.Project.csdl|res://*/Project.Project.ssdl|res://*/Project.Project.msl;provider=System.Data.SqlClient;provider connection string="data source=.\PROJECT, 1433;initial catalog=PROJECT;persist security info=True;user id=user;password=password;MultipleActiveResultSets=True;App=EntityFramework"" providerName="System.Data.EntityClient" />
it doesn't work.
I tried with putting the source in "MAC\PROJECT" also, it does not change anything. Using the udl file workaround to test connection parameters is totally OK both with "MAC\PROJECT" and ".\PROJECT" and the right credentials.
I can't figure out why my project connect to the database perfectly using the remote server but not the local one. Am I missing something?

Right after posting this question I found the answer.... The old connection string gives a port number, which is not needed when connecting to the local instance.

cannot deploy c# app with entity framework

i have finished a desktop c# project in which i make use of Entity framework 5 and SQL server 2008
in my machine it's working well, but i created a virtual machine and tried to deploy it there and I have problems.
I get this error : System.Data.EntityException: The underlying provider failed on Open. --->
System.Data.SqlClient.SqlException: 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.
I used installshield for the setup and I have installed sql express in the visrual machine, during the setup I create the database and all the tables.
my connection string is this :name="BussinessContainer" connectionString="metadata=.\Bussiness.csdl|.\Bussiness.ssdl|.\Bussiness.msl;provider=System.Data.SqlClient;provider connection string="data source=localhost;initial catalog=yingyangDB;User ID=username;Password=password;integrated security=True;MultipleActiveResultSets=True;App=EntityFramework"" providerName="System.Data.EntityClient"
Should I change the connection string?
I searched a lot before posting this, but nothing worked..any help?

SQL Express (normally) doesn't create as a "Default instance" - i.e. connecting by localhost isn't sufficient to connect to the local SQL express instance.
You may need to use localhost\SQLExpress to connect to the local SQL Express instance (the instance name may be different, but usually it is not.
Additionally, you specify an Initial Catalog in your connection string, which I assume is pointing at your new DB. However you say that you create the DB during the installation - so that DB may not actually exist in your Instance yet. That may be causing it to fail out. Try taking it out of the string and see if that lets you connect.

SQL connection failure in LINQ-to-SQL

I'm cloning a local C#-SQL Windows Forms application to a new laptop. I've successfully cloned the C# project and back-up-and-restored the SQL database to the new machine, but the new project is unable to read/query any data from the new, cloned SQL database.
Since the SQL DB is local, the LINQ-to-SQL addressing is slightly different on account of the computer having a different name. So, I deleted the SQL table representations from the existing LINQ .dbml's graphical interface, removed the old server connection from the Server Explorer, added the new server connection, and click-and-dragged the table objects from the Server Explorer's new Data Connection object to the now-empty .dbml file.
Upon rebuilding and running the application in debug mode, I found that the dropdown menus which pull from the SQL DB are empty. Upon trying a button function which pulls from the database, I get the following error:
SqlException 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)
(The cloned SQL server is configured to allow remote connections, TCP/IP is enabled in SQL, and the original C#-SQL application works without a hitch.)

Check in your Web.Config and make sure that your Connection String is set up correctly. Also check to make sure that when you initialize your DataContext that you are using the correct Connection string from your Web.Config.
Sometimes when you remove dbs from the dbml, it doesn't completely clear the web.config of the old entries. Look for a line like this in your designer.cs file:
public SomeDbDataContext() :
base(global::System.Configuration.ConfigurationManager.ConnectionStrings["SomeDbConnectionString"].ConnectionString, mappingSource)
{
OnCreated();
}
Compare it with the connectionStrings in your web.config
<connectionStrings>
<add name="ApplicationServices" connectionString="data source=.\SQLEXPRESS;Integrated Security=SSPI;AttachDBFilename=|DataDirectory|aspnetdb.mdf;User Instance=true" providerName="System.Data.SqlClient" />
<add name="someDBConnectionString" connectionString="Data Source=SomeServer;Initial Catalog=SomeDB;Integrated Security=True" providerName="System.Data.SqlClient" />
<add name="theOLDDBConnectionString" connectionString="Data Source=SomeServer;Initial Catalog=OldDB;Integrated Security=True" providerName="System.Data.SqlClient" />
</connectionStrings>

I would check a few things:
1) Is SQL Server started on the new machine?
2) Is the firewall configured to allow SQL connections?
3) Is the client connection mechanism (named pipes, tcp/ip) also configured in SQL Server.
4) Is there a valid network connection in the new machine (we used to have a lot of problems on laptops when they were not connected to the network).
You might have much better luck if you change the SQL Server machine name to . or (local) if you are always going to have the DB installed on the same machine as the app.