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Insert data to a SQL database in .NET Core 3.1 using C# receives an error

I have created a SQL DataBase (DatabaseTest.mdf) in Visual Studio 2019 Preview (.NET Core 3.1, Windows Form Application).
It is my first time, I am trying to do this. I run the database locally on my computer.
The database consists of 4 columns:
First Name
Last Name
PhoneNumber
Salary
Now, I am trying to use C# to programatically ADD a row with information to this database.
The code is the below:
using System.Data.SqlClient;
private void button1_Click(object sender, EventArgs e)
{
string connectionString = GetConnectionString();
SqlConnection con = new SqlConnection(connectionString);
SqlCommand cmd = new SqlCommand("sp_insert", con);
cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.AddWithValue("#First Name", "Peter");
cmd.Parameters.AddWithValue("#Last Name", "Smith");
cmd.Parameters.AddWithValue("#PhoneNumber", "5548945667");
cmd.Parameters.AddWithValue("#Salary", 50000);
con.Open();
int i = cmd.ExecuteNonQuery();
con.Close();
if (i != 0)
{
MessageBox.Show(i + "Data Saved");
}
}
static private string GetConnectionString()
{
return "Data Source=(LocalDB)/MSSQLLocalDB;AttachDbFilename=C:/Users/andre/source/repos/TestDatabaseCreation/DatabaseTest.mdf;Integrated Security=True";
}
However, when I now run this code by clicking on the button. I receive this error:
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: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server)'
Win32Exception: Network path not found.
I have copied this connection string from the DatabaseTest.mdf properties exactly. So the path to the DatabaseTest.mdf below is correct etc
"Data Source=(LocalDB)/MSSQLLocalDB;AttachDbFilename=C:/Users/andre/source/repos/TestDatabaseCreation/DatabaseTest.mdf;Integrated Security=True"
I wonder what the problem is that I get this error message?
(I attach a screenshot on the link below also from Visual Studio 2019 Preview .NET Core 3.1)
Image of the error in C# code behind in Visual Studio 2019 Preview

First, you can ensure SQL Server Express is installed and working by the following steps
Open Visual Studio Installer >> Modify
Expand .Net desktop development
Scroll down and check SQL Server Express is installed or not
[
if it is installed go to step 2
Open Server Explorer
Connect to Database
Choose Data Source >> Microsoft SQL Server Database File
Add Connection >> Browse to your database file, and make sure to Test Connection, then OK
After that, inside Server Explorer will display the connections, right click on that >> properties
Then you can get the Connection String
Second, with Connection String got above, you can check it work or not by
SqlConnection cnn = new SqlConnection(**connectionString**);
cnn.Open();
// not do anything yet
cnn.Close();
Then run to make sure you can open the connection to the database
UPDATE :
Check which Stored Procedures you want is existed or not?

Connect to a SQL Server database without a set file path

I am attempting to connect to a local SQL Server database in C#.
I am currently using the following connection string:
connectionString = #"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=C:\Users\User\source\repos\majorWork\majorWork\gameStats.mdf;Integrated Security=True";
However, I do not want to use a hardcoded file path, as I wish to be able to use the application across multiple computers, where the file path will be different. How should I go about doing this?

Best way is set this connection in Web.Config file.
<Database>
<ConnectionString name="connection">Server=servername; Initial Catalog=dbname; Persist Security Info=False; User ID=username; Password=password; MultipleActiveResultSets=False; Encrypt=True; TrustServerCertificate=False; Connection Timeout=30;;</ConnectionString>
</Database>
Then add Add System.Configuration as a reference.
in C# you can call this
string constring = ConfigurationManager.ConnectionStrings["connection"].ConnectionString;
After that you can create new connection instance by passing this to
SqlConnection con = new SqlConnection(constring)

If u install SQL server express using the default instance, then you can connect using . as your server name anyone can use it with default instance as well.
1.then in visual studio, click on solution explorer
2. Connect database, follow the instruction for SQL server
3. When it gets to server name use . to connect and choose your database name which you have created already in ms SQl, then test your connection
4. After it successful, u can now click on the database name showing under solution explorer,
5.after u click the database name, at the button right corner, there will be a connection string, copy it and use
This will be declared publicly or globally
Sqlconnection con = new sqlconnection("paste the connection string");
And to use
Sqlcommand cmd = new sqlcommand("insert into......",con);
Con.open ();
Cmd.executenonquery();
Con.close();

c# windows, local database(sql server), connection string not working properly

This is my connection string
SqlConnection conn = new SqlConnection(#"Data Source=C:\Users\admin\documents\visual studio 2010\Projects\WFA1\WFA1\database.sdf");
I copied it from the property, I added # in front but didn't change anything. There are no errors during Build and Run. But when I click insert button, the below error shows up.
"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 tried editing connection string and using datadirectory instead of the entire path, but the same error occurs, I am new to .net so I am unable to figure out what could be the cause. I tried google but I can't tell whether the solutions there are related to my problem.
Thanks in advance!

You're using the wrong type of SQL connection. You should be using the SqlCeConnection class:
SqlCeConnection conn = new SqlCeConnection(#"Data Source=C:\Users\admin\documents\visual studio 2010\Projects\WFA1\WFA1\database.sdf");
Here is a really good reference for SQL CE connection strings.

The data source for SQL Server is a machine name, not a file name.
You should add a reference to the SQL CE assembly, and do this:
SqlCeConnection conn = new SqlCeConnection(#"Data Source=C:\Users\admin\documents\visual studio 2010\Projects\WFA1\WFA1\database.sdf");

For MSSQL Server Compact Edition, you should use the SqlCeConnection class.
So this should work:
SqlCeConnection sqlConnection = new SqlCeConnection(#"Data Source=C:\Users\admin\documents\visual studio 2010\Projects\WFA1\WFA1\database.sdf");

Create you connection string as follow :
add in the namespace using System.Data.SqlServerCe;
SqlCeConnection conn = new SqlCeConnection(#"Data Source=C:\Users\admin\documents\visual
studio 2010\Projects\WFA1\WFA1\database.sdf; Persist Security Info=False;");
if .sdf file inside you project then you can use :
string strConnectionString ="Data Source=" + (System.IO.Path.GetDirectoryName(System.Reflection.Assembly.
GetExecutingAssembly().GetName().CodeBase) + "\\database.sdf;Persist Security Info=False;";
SqlCeConnectionconn = new SqlCeConnection(strConnectionString );

SQL Server 2008 connection with C#

No matter what I try I keep getting the following error when I try to run a simple select query to the database I created (I've run the exact query in the Management studio and it works fine and the login used is in the sysadmin group):
{"Cannot open database \"Test\" requested by the login. The login
failed.\r\nLogin failed for user
'JackLarson'."} System.SystemException
{System.Data.SqlClient.SqlException}
My connection string is:
Data Source=.\MPIT_TEST;Initial Catalog=Test;Integrated
Security=True;Connect Timeout=30;User Instance=True

you have to set your server name in your connection string
if your servername is SERVERSQLCOMPUTER your connection string will be:
Data Source=SERVERSQLCOMPUTER \MPIT_TEST;Initial Catalog=[Test_MPITRACKER];Integrated Security=True;Connect Timeout=30;User Instance=True
Moreover, always specify server name to be able to use your application on every computer in network entreprise (servername\serverinstance is an absolute path)
.\serverinstance means that your application is on the same computer than your Sql server

Take a look at the error message. It says it cannot find database named Test although you specified Test_MPITRACKER in the connection string. It looks like there is an issue when you have a database with undescore _ character in its name.
The easiest way would be to remove the undescore from the name of the database.
If you want to use database name with the underscore character, try to wrap it with square brackets, so it looks like this:
Data Source=.\MPIT_TEST;Initial Catalog=[Test_MPITRACKER];Integrated Security=True;Connect Timeout=30;User Instance=True
However, I haven't tested this solution, it's based on information found here: Database created in SQL Server Management Studios cannot be found by Visual Studio 2010.
EDIT
According to the conversation in the comments this is not the cause of the issue in this case.

How do I connect to an .mdf (Microsoft SQL Server Database File) in a simple web project?

Specifically, in VS 2008, I want to connect to a data source that you can have by right-clicking on the automatically-generated App_Data folder (an .mdf "database"). Seems easy, and it is once you know how.

A great resource I always keep around is connectionstrings.com.
It's really handy for finding these connection strings when you can't find an example.
Particularly this page applied to your problem
Attach a database file on connect to a local SQL Server Express instance
Driver={SQL Native Client};Server=.\SQLExpress;AttachDbFilename=c:\asd\qwe\mydbfile.mdf; Database=dbname;Trusted_Connection=Yes;

So here's the answer from MSDN:
Choos[e] "Add New Data Source" from the
Data menu.[And follow the connection wizard]
Very easy, except that I have no Data menu. If you don't have a Data menu, do the following:
Click on Tools >> Connect to Database...
Select "Microsoft SQL Server Database File", take the default Data provider, and click OK
On the next screen, browse to your Database file, which will be in your VS Solution folder structure somewhere.
Test the connection. It'll be good. If you want to add the string to the web.config, click the Advanced button, and copy the Data Source line (at the bottom of the dialog box), and paste it into a connection string in the appropriate place in the web.config file. You will have to add the "AttachDbFilename" attribute and value. Example:
The raw text from the Advanced panel:
Data Source=.\SQLEXPRESS;Integrated Security=True;Connect Timeout=30;User Instance=True
The actual entry in the web.config:
<add name="SomeDataBase" connectionString="Data Source=.\SQLEXPRESS;
AttachDbFilename=C:\Development\blahBlah\App_Data\SomeDataFile.mdf;
Integrated Security=True; Connect Timeout=30; User Instance=True" />

Just one more -- i've always kept a udl file on my desktop to easily create and test connection strings. If you've never done it before - create a new text file and name it to connection.udl (the ext is the only important part). Open the file, start on the Provider tab and work your way through. Once you're happy with the connection rename the file giving it a .txt extension. Open the file and copy the string - it's relatively easy and lets you test the connection before using it.

<add name="Your Database" connectionString="metadata=res://*/Model1.csdl|res://*/Model1.ssdl|res://*/Model1.msl;provider=System.Data.SqlClient;provider connection string="Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\Expanse.mdf;Integrated Security=True;User Instance=True;MultipleActiveResultSets=True"" providerName="System.Data.EntityClient"/>

In your Login.aspx.cs (the code behind file for your login page in the submit button click event) add
string constr = #"Data Source=(LocalDB)\v11.0; AttachDbFilename=|DataDirectory|\myData.mdf; Integrated Security=True; Connect Timeout=30;";
using (SqlConnection conn = new SqlConnection(constr))
string constr = ConfigurationManager.ConnectionStrings["myData"].ToString();
using (SqlConnection conn = new SqlConnection(constr))
{
sqlQuery=" Your Query here"
SqlCommand com = new SqlCommand(sqlQuery, conn);
com.Connection.Open();
string strOutput = (string)com.ExecuteScalar();
}