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dgvnew.DataSource = dt3;
dgvnew.AutoSizeColumnsMode = DataGridViewAutoSizeColumnsMode.DisplayedCells;
this.Controls.Add(dgvnew);					

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private void dgvnew_CurrentCellChanged(object sender, EventArgs e)
        {
            try
            {
                int currentrow = dgvnew.CurrentRow.Index;  //<--- hier taucht der Fehler auf: NullReferenceException wurde nicht behandelt Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt.
                string myurl = this.dgvnew[23, currentrow].Value.ToString();


                HttpWebRequest myHttpWebRequest = (HttpWebRequest)WebRequest.Create(new Uri(myurl));
                HttpWebResponse myHttpWebResponse = (HttpWebResponse)myHttpWebRequest.GetResponse();
                Stream resStream = myHttpWebResponse.GetResponseStream();
                Image ProductImage = new Bitmap(this.pictureBox1.Size.Width, this.pictureBox1.Size.Height);
                ProductImage = ((System.Drawing.Image)(Image.FromStream(resStream)));
                this.pictureBox1.Image = ProductImage;
                resStream.Close();
                myHttpWebResponse.Close();

            }
            catch (WebException)
            {
                MessageBox.Show("Fehler");
            }
        }					

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            try
            {
                if(dgvnew.CurrentRow != null)
                {
                     int currentrow = dgvnew.CurrentRow.Index; 
                     .
                     .
                     .
                }  

            }
            catch (WebException)
            {
                MessageBox.Show("Fehler");
            }					

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dgvnew.CurrentCellChanged -= new EventHandler(dgvnew_CurrentCellChanged);