{"Format of the initialization string does not conform to specification starting at index 0."}

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C#

SQL-Server

WinForms

when I passing connection string to Connection constructor it giving above exception in windows form application

{"Format of the initialization string does not conform to specification starting at index 0."}

What I have tried:

Connection string in app.config is as follows

<add name="DBConnectionString" connectionString="Data Source=SONALI\HPPAVSQLEXPRESS;Initial Catalog=testdb;Persist Security Info=True;User ID=sa;Passwordadmin123" providerName="System.Data.SqlClient" />

then I am reading Connection string and stored in string Variable
as

public Form1()
       {
           InitializeComponent();
       }

       string connectionstring = ConfigurationManager.ConnectionStrings["DBConnectionString"].ConnectionString;

then passing this string variable to connection class constructor as

SqlConnection connection = new SqlConnection(connectionstring);

but here it is giving above mentioned exception