PostgreSQL bugs

Hi

I'm using PostgreSQL in my project, so referenced Npgsql.dll, A .net wrapper for pgsql. What's more, I wrote a helper class, and it works fine under Visual Studio.

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Then, in my code(Unity)

 PostgresHelper pg = new PostgresHelper();
 pg.conn.Open();
 pg.conn.Close();

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Verything is happy, Until, In my Helper Class

 public void InsideCall() {
             this.conn.Open();
             this.conn.Close();
         }   

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And Call it in Unity c# script

 PostgresHelper pg = new PostgresHelper();
 pg.InsideCall();

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Then, Unity Crashed... I have no idea what's the difference and why it crashed(really weird). Sometimes Unity may not crash, showing me a error message like "Npgsql timeout" instead Can anyone help? Thanks!

p.s Unity 3.5.5f3

     //Creator
     public PostgresHelper()
     {
         string connectString = String.Format("Server={0};Port={1};" +
                 "User Id={2};Password={3};Database={4};",
                 D_SERVER, D_PORT, D_USER,
                 D_PASS, D_DATABASE);
         conn = new NpgsqlConnection(connectString);
     }

     //This Call will cause Unity CRASH,but Conn.open()/Close() is fine
     public void InsideCall() {
         this.conn.Open();
         this.conn.Close();
     }       
 }