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Command behind a GridView ButtonField
<asp:ButtonField ButtonType="Image" ImageUrl="~/Images/pdf.png"  CommandName="pdf_click" />

protected void pdf_click(object sender, EventArgs e)
{
    lblWelkom.Text = "Succes!";
}

void GridView1_RowCommand(Object sender, GridViewCommandEventArgs e)
 {
 // If multiple buttons are used in a GridView control, use the
 // CommandName property to determine which button was clicked.
 if(e.CommandName=="pdf_click")
  {
   // Convert the row index stored in the CommandArgument
   // property to an Integer.
   int index = Convert.ToInt32(e.CommandArgument);

   // Retrieve the row that contains the button clicked
   // by the user from the Rows collection.
   GridViewRow row = ContactsGridView.Rows[index];
   // Now you have access to the gridviewrow.
  }
}

int index = Convert.ToInt32(e.CommandArgument);
    int ServerID = Convert.ToInt32(GridView1.DataKeys[index].Value);