Need help with my OracleconnectionStringBuilder

i'm wanting to connect to an Oracle database using the system.data.oracleClient provider and also allows users to be able to authenticate agaisnt the database by passing through textboxes on a login form their username and password..

i've created a AppConfig.config which looks like this
< xml version="1.0" encoding="utf-8" >
<configuration>
<connectionStrings>

<add name="partialConnectString"
providerName="System.Data.OracleClient"
connectionString="Data Source=(DESCRIPTION=(ADDRESS_LIST=(ADDRESS=(PROTOCOL=TCP)(HOST=Cleopatre)(PORT=1521)))(CONNECT_DATA=(SERVER=DEDICATED)(SERVICE_NAME=Jasp)));"
/>

</connectionStrings>
</configuration>

and i'm trying to retrieve this connectionstring using the following code but i'm getting a run-time error of type NullReferenceException unhandled [object reference not set an instance of an object] on the variable settings (which is of type ConnectionStringSettings )
here is the code

private static void BuildConnectionString( string username, string password)
{
// Retrieve the partial connection string named databaseConnection
// from the application's app.config or web.config file.
ConnectionStringSettings settings =
ConfigurationManager.ConnectionStrings["partialConnectString"];

// Retrieve the partial connection string.
string connectString = settings.ConnectionString;
MessageBox.Show("Original: {0}", connectString);

// Create a new oracleConnectionStringBuilder based on the
// partial connection string retrieved from the config file.
OracleConnectionStringBuilder builder =
new OracleConnectionStringBuilder(connectString);

// Supply the additional values.

builder.UserID = userName;
builder.Password = userPassword;
MessageBox.Show("Modified: {0}", builder.ConnectionString);

}

and of course the values passed in textboxes are going to map the values passed in the buildConnectionString

i just do not understand what to do!!!!

ok thanks in advance hope to hearing from you