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

问题

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?

回答1:

Change this:

name="DatabaseDGVexperiments.Properties.Settings.DatabaseDGVexperimentsConnStg"

to:

name="DatabaseDGVexperimentsConnStg"

or change this:

ConfigurationManager.ConnectionStrings["DatabaseDGVexperimentsConnStg"]

to:

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

回答2:

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;
    }
}

回答3:

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

来源：https://stackoverflow.com/questions/10961348/very-simple-c-sharp-program-generates-a-nullreferenceexception-was-unhandled-e

标签

winforms

configuration

datagridview