how to create registry in c#

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C#3.5

hello sir i want to make registry , here is my code..

RegistryKey key = Microsoft.Win32.Registry.CurrentUser.OpenSubKey("Software\\NERP");
if (key == null)
{
    locServername  = txt_locServer .Text; //.
    locDatabaseName  = txt_locDb .Text;//employee
    locUsername  = txt_locName .Text;//sa
    locPwd  = txt_locPwd .Text;//.
    MainServername = txt_ServerName.Text;
    MainUsername = txt_ServerLoginName.Text ;
    MainPwd = txt_Severpwd.Text;
    
    
    key.SetValue("LocServer", locServername);
    key.SetValue("LocDB", locDatabaseName);
    key.SetValue("LocUsername", locUsername);
    key.SetValue("LocPassword", locPwd);
    key.SetValue("Mainserver", MainServername);
    key.SetValue("MainUsername", MainUsername);
    key.SetValue("MainPwd", MainPwd);
    
    MessageBox.Show("Registry created");
    this.Close();

when i run this program there is error like
object reference not set to an instance of an object" at this line
key.SetValue("LocServer", locServername);

in the "locServername" I pass value ".";
sir tell me how to do this.
i want to make connection string look like this
datasource=.;database=employee;user id=sa;password='';