Try just grabbing the row from the SelectedRow property:
protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
{
GridViewRow row = GridView1.SelectedRow;
string b = row.Cells[6].Text;
}
It's my understanding that the Rows collection doesn't get repopulated on PostBacks when you're using those data source controls (like SqlDataSource).
You could probably use your existing code if you called .DataBind()
on your GridView prior to trying to iterate through the Rows:
GridView1.DataSourceID="SqlDataSource1";
GridView1.DataBind();
But that seems a little hacky.
After seeing your Page_Load, I see that you need to wrap your databinding code in an if(!Page.IsPostBack)
block. Databinding on every postback is interrupting the process of ASP.NET maintaining the state of your controls via the ViewState.