SQL Server CE connection error message

In app.config file:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <configSections>
    </configSections>
    <connectionStrings>
        <add name="WindowsFormsLearning_C.Properties.Settings.NorthwindConnectionString"
            connectionString="Data Source=E:\Documents and Settings\SampleDatabase\Northwind.sdf"
            providerName="Microsoft.SqlServerCe.Client.3.5" />
    </connectionStrings>
</configuration>

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In button code:

// connect to a database and retrieve data:
            try
            {
                SqlConnection sqlc = new SqlConnection();
                sqlc.ConnectionString = Properties.Settings.Default.NorthwindConnectionString;
                

                sqlc.Open();   <<------Fails

                DataSet xds;
                SqlCommand xsqlcmd = new SqlCommand();
                StringBuilder sbsql = new StringBuilder();

                sbsql.Append("Select distinct ");
                sbsql.Append(" 'Company Name' ");
                sbsql.Append(" from Customers ");

                xds = new DataSet();

                SqlDataAdapter sqdap = new SqlDataAdapter(sbsql.ToString(),sqlc);

                sqdap.Fill(xds, "Customers");

                if (xds == null) { 
                
                    //do something
                }
               else {

                   foreach (DataRow xdr in xds.Tables[0].Rows)
                   {

                       //ds.Tables[0].Rows[0]["ColumnName"].ToString() 
                       Console.WriteLine(xdr["Company Name"]);
                   }

                    // do something else
                    
                    }
                sqlc.Close();
            }
            catch(Exception exc){

                MessageBox.Show(exc.Message);
                
              }
            }

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Exception message (some of it, anyway):

System.Data.SqlClient.SqlException was caught
  Message=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)
  Source=.Net SqlClient Data Provider
  ErrorCode=-2146232060
  Class=20
  LineNumber=0
  Number=-1
  Server=""
  State=0

I know the path to the database file is correct, because I can successfully connect to it and test the connection from within the IDE connection dialog.  I copied and pasted that connection string into the config file.

So what am I doing wrong?

Thanks in advance!