edit

using System;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;
using System.Web.UI.WebControls;

public partial class Pregled : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            ispolniSuppliers();
        }
    }

    protected void ispolniSuppliers()
    {
        SqlConnection conn = new SqlConnection();
        conn.ConnectionString = ConfigurationManager.ConnectionStrings["myConn"].ConnectionString;
        string sqlString = "SELECT * FROM Suppliers";
        SqlCommand command = new SqlCommand(sqlString,conn);
        SqlDataAdapter adapter = new SqlDataAdapter(command);
        DataSet ds = new DataSet();

        try
        {
            conn.Open();
            adapter.Fill(ds);
            gvSuppliers.DataSource = ds;
            gvSuppliers.DataBind();
            ViewState["datasetSuppliers"] = ds;
        }
        catch(Exception err)
        {

        }
        finally
        {
            conn.Close();
        }


    }

    protected void gvSuppliers_PageIndexChanging(object sender, GridViewPageEventArgs e)
    {
        gvSuppliers.PageIndex = e.NewPageIndex;
        gvSuppliers.SelectedIndex = -1;
        DataSet ds = (DataSet)ViewState["datasetSuppliers"];
        gvSuppliers.DataSource = ds;
        gvSuppliers.DataBind();
        gvProducts.Visible = false;
    }

    protected void ispolniProducts()
    {
        SqlConnection conn = new SqlConnection();
        conn.ConnectionString = ConfigurationManager.ConnectionStrings["myConn"].ConnectionString;
        string sqlString = "SELECT Products.ProductID, Suppliers.SupplierID, Products.ProductName, Products.UnitsInStock, Products.UnitPrice FROM Suppliers INNER JOIN Products ON Suppliers.SupplierID = Products.SupplierID where Products.SupplierID = @SupplierID";
        SqlCommand command = new SqlCommand(sqlString, conn);
        command.Parameters.AddWithValue("@SupplierID", gvSuppliers.DataKeys[gvSuppliers.SelectedIndex].Value);
        SqlDataAdapter adapter = new SqlDataAdapter(command);
        DataSet ds = new DataSet();

        try
        {
            conn.Open();
            adapter.Fill(ds);
            gvProducts.DataSource = ds;
            gvProducts.DataBind();
            gvProducts.Visible = true;
            ViewState["datasetProducts"] = ds;
        }
        catch (Exception err)
        {

        }
        finally
        {
            conn.Close();
        }

    }


    protected void gvSuppliers_SelectedIndexChanged(object sender, EventArgs e)
    {
        ispolniProducts();
    }

    protected void gvProducts_RowUpdating(object sender, GridViewUpdateEventArgs e)
    {
        SqlConnection conn = new SqlConnection();
        conn.ConnectionString = ConfigurationManager.ConnectionStrings["myConn"].ConnectionString;
        string sqlString = "UPDATE Products SET ProductName = @ProductName WHERE ProductID = @ProductID";
        SqlCommand command = new SqlCommand(sqlString, conn);

        TextBox tb = (TextBox)gvProducts.Rows[e.RowIndex].Cells[1].Controls[0];
        //lError.Text = tb.Text;
        command.Parameters.AddWithValue("@ProductName",tb.Text);


        //lError.Text += " " + gvProducts.DataKeys[e.RowIndex].Value;
        command.Parameters.AddWithValue("@ProductID", (int)gvProducts.DataKeys[e.RowIndex].Value);

        int effect = 0;

        try
        {
            conn.Open();
            effect = command.ExecuteNonQuery();

        }
        catch (Exception err)
        {
            //r.Text += err.Message;
        }
        finally
        {
            conn.Close();
            gvProducts.EditIndex = -1;

        }


        //lError.Text += " " + effect.ToString();

        if (effect != 0)
        {
            ispolniProducts();
        }


    }

    protected void gvProducts_RowEditing(object sender, GridViewEditEventArgs e)
    {
        DataSet ds = (DataSet)ViewState["datasetProducts"];
        gvProducts.EditIndex = e.NewEditIndex;
        gvProducts.DataSource = ds;
        gvProducts.DataBind();
    }

    protected void gvProducts_RowCancelingEdit(object sender, GridViewCancelEditEventArgs e)
    {
        DataSet ds = (DataSet)ViewState["datasetProducts"];
        gvProducts.EditIndex = -1;
        gvProducts.DataSource = ds;
        gvProducts.DataBind();
    }

    protected void gvProducts_SelectedIndexChanged(object sender, EventArgs e)
    {

    }
}