what is the meaning of exception System.FormatException

Question

in this code it gives the System.FormatException . So what is the solution for it?

 

private DataSet getData()

{

using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["DiabeticCareCenterDbConnectionString"].ConnectionString))

{

if (con.State != ConnectionState.Closed)

{

con.Open();

}

SqlCommand cmd = new SqlCommand("select DId,FName+' '+LName as FullName,Qualification,InTime+' - '+OutTime as Timing,Photo from Doctor", con);

SqlDataAdapter da = new SqlDataAdapter(cmd);

DataSet ds = new DataSet();

da.Fill(ds);

return ds;

}

}

protected void FixAppointment_Click(object sender, EventArgs e)

{

int dr = Convert.ToInt32(((Button)sender).CommandArgument);//error occours here.

Response.Redirect("Home.aspx");

}