The connection was not closed [duplicate]

This question already has an answer here:

• The connection was not closed the connection's current state is open 4 answers

This is doing my head in. I have tried try and catch with finally and other things but this error keeps coming back. Please some one help. My c# code:

 void show()
    {
       string str = "SELECT PID, Pname, Gender, ContactNum, Category, Department, Ward, AdmitDate, DischargeDate, NumOfDays, CostOfTest, NumOfDocsVisited, DocFee, BedCost, TotalCost, GrandTotal FROM Patient";
        cmd = new SqlCommand(str, con);
            con.Open();
            SqlDataAdapter adp = new SqlDataAdapter(cmd);
            DataTable dt = new DataTable();
            adp.Fill(dt);
            GridView1.DataBind();
                con.Close();
    }

error is: The connection was not closed. The connection's current state is open. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.InvalidOperationException: The connection was not closed. The connection's current state is open.

Source Error:

Line 50:            string str = "SELECT PID, Pname, Gender, ContactNum, Category, Department, Ward, AdmitDate, DischargeDate, NumOfDays, CostOfTest, NumOfDocsVisited, DocFee, BedCost, TotalCost, GrandTotal FROM Patient";
Line 51:             cmd = new SqlCommand(str, con);
Line 52:                 con.Open();
Line 53:                 SqlDataAdapter adp = new SqlDataAdapter(cmd);
Line 54:                 DataTable dt = new DataTable();

Stack Trace:

[InvalidOperationException: The connection was not closed. The connection's current state is open.]

System.Data.ProviderBase.DbConnectionInternal.TryOpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource1 retry, DbConnectionOptions userOptions) +14 System.Data.SqlClient.SqlConnection.TryOpenInner(TaskCompletionSource1 retry) +94 System.Data.SqlClient.SqlConnection.TryOpen(TaskCompletionSource`1 retry) +110 System.Data.SqlClient.SqlConnection.Open() +96 SMC.Billing.show() in e:\VS2013 projects\SMC\SMC\Billing.aspx.cs:52 SMC.Billing.Button3_Click(Object sender, EventArgs e) in e:\VS2013 projects\SMC\SMC\Billing.aspx.cs:97 System.Web.UI.WebControls.Button.OnClick(EventArgs e) +9628722 System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +103 System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +10 System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +13 System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +35 System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1724