Very simple C# program generates a `NullReferenceException was unhandled` error

Question

I have a simple WinForm that I'm using to try to help me with ADO.NET. It has a datagridview

I have added a compact sql server database to the project called experiment.sdf

In the App.config file I have added the following. I used a previous project app file to base this on so maybe there's an error in here?:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<configSections>
</configSections>
<connectionStrings>
  <add name="DatabaseDGVexperiments.Properties.Settings.DatabaseDGVexperimentsConnStg"
      connectionString="Data Source=|DataDirectory|\experiment.sdf"
      providerName="Microsoft.SqlServerCe.Client.3.5" />
</connectionStrings>
</configuration>

I've added a reference to the configuration library.

Behind the form is the following code:

  SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["DatabaseDGVexperimentsConnStg"].ConnectionString);

When it hits the line SqlConnection conn = ... I get an error NullReferenceException was unhandled. How do I fix this?

Make sure `ConfigurationManager.ConnectionStrings["DatabaseDGVexperimentsConnStg"].ConnectionString` is not null. — Nikhil Agrawal, Jun 09 '12 at 13:18
You calling `.ConnectionString` of a null reference. `ConfigurationManager.ConnectionStrings["DatabaseDGVexperimentsConnStg"]` does not return an object. The string specified does not match a string in the web.config — Nope, Jun 09 '12 at 13:24

score 7 · Accepted Answer · answered Jun 09 '12 at 13:18

7

Change this:

name="DatabaseDGVexperiments.Properties.Settings.DatabaseDGVexperimentsConnStg"

to:

name="DatabaseDGVexperimentsConnStg"

or change this:

ConfigurationManager.ConnectionStrings["DatabaseDGVexperimentsConnStg"]

to:

ConfigurationManager.ConnectionStrings["DatabaseDGVexperiments.Properties.Settings.DatabaseDGVexperimentsConnStg"]

answered Jun 09 '12 at 13:18

Guffa

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I changed the config file. Now it errors on the line `myAdapt.Fill...` with an `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. – whytheq Jun 09 '12 at 13:37
the sdf file is in the same project! why should it have difficulty connecting? – whytheq Jun 09 '12 at 13:38
@whytheq: You need the SQL CE classes to use the SqlServerCe provider. Change SqlConnection to SqlCeConnection, and so on. – Guffa Jun 09 '12 at 15:42
that worked a treat; I also had to change `SELECT * FROM experiment.dbo.helloworld` to `SELECT * FROM helloworld` for the code to work - why? – whytheq Jun 09 '12 at 17:22
@whytheq: Becauase there are no databases and users (the first two parts of the xx.xx.xx notation), the SQL CE file is a single database, and you don't specify a user when you connect to it. – Guffa Jun 09 '12 at 18:09

Damith · Answer 2 · 2012-06-09T14:41:05.047

1

ConfigurationManager.ConnectionStrings["DatabaseDGVexperiments.Properties.Settings.DatabaseDGVexperimentsConnStg"].ConnectionString

note that you need to give name as

DatabaseDGVexperiments.Properties.Settings.DatabaseDGVexperimentsConnStg

not as

DatabaseDGVexperimentsConnStg

EDIT:

You need to use SqlCeConnection and SqlCeDataAdapter too work with SqlServerCe database, add reference to System.Data.SqlServerCe and then code should like below.

using (var conn = new SqlCeConnection(ConfigurationManager.ConnectionStrings["DatabaseDGVexperiments.Properties.Settings.DatabaseDGVexperimentsConnStg"].ConnectionString))
{
    conn.Open();
    using (var myAdapt = new SqlCeDataAdapter("SELECT * FROM experiment.dbo.helloworld", conn))
    {
        DataSet mySet = new DataSet();
        myAdapt.Fill(mySet, "AvailableValues");
        DataTable myTable = mySet.Tables["AvailableValues"];
        this.uxExperimentDGV.DataSource = myTable;
    }
}

edited Jun 09 '12 at 14:41

answered Jun 09 '12 at 13:19

Damith

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Thanks. I changed the config file. Now it errors on the line `myAdapt.Fill...` with an `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.` – whytheq Jun 09 '12 at 13:41
thanks Damith - `using` is a way to avoid persistent database activity? – whytheq Jun 09 '12 at 17:20
Using statement handles the disposing object. you don't need to do it from code. – Damith Jun 09 '12 at 17:27

score 0 · Answer 3 · answered Jun 09 '12 at 13:19

0

You've used a different key value in your code then in web.config. the strings need to match exactly.

answered Jun 09 '12 at 13:19

D Stanley

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Very simple C# program generates a `NullReferenceException was unhandled` error

3 Answers3

EDIT:

Linked