Connecting to sql server express instance using web config connection string and entity framework

This connection string does NOT work:

  <connectionStrings>
    <!--<add name="TodosConnection" providerName="System.Data.SqlClient"
          connectionString="Server=.\SqlExprss;Initial Catalog=Todos;Integrated Security=True;"/>-->

This connection string works:

    <add name="TodosConnection"
     connectionString="data source=.\SQLEXPRESS;Integrated Security=True; database=Todos;"
     providerName="System.Data.SqlClient" />
    
  </connectionStrings>

And in the code this how to use it:

namespace Todo.Api.Data

{

    public class TodosContext : DbContext

    {

        public TodosContext()

            : base("name=TodosConnection")

        {

            Debug.Write(Database.Connection.ConnectionString);

        }

        public DbSet<Todo> Todos { get; set; }

    }

}