Download file from a sharepoint site from code behind

Question

I have an Image button in a custom 2010 webpart, on click of that I would like to download a file.

Code Used :

foreach (string filename in item.Attachments)
{
    attachmentURL = SiteListURL + "attachments/" + item.ID + "/" + filename;
}

//OpenFile(attachmentURL);
Response.Redirect(attachmentURL);

Here Openfile throws an error -

The file http://so4:60000/Lists/AbilityTableSales/attachments/1/Dashboard Solutions.pptx does not exist.

Though the line Response.Redirect(attachmentURL); working and it is opening the save dialogue box. If I click the image button again the event is not firing, I guess it is because I used Response.Redirectmethod.

How can I do this?

UPDATE

Entire Code

private string getlist(string Title)
{
    string SiteListURL = "http://so4:60000/Lists/CapabilityTableSales/";
    using (SPSite oSiteCollection = new SPSite(SiteListURL))
    {
        using (SPWeb web = oSiteCollection.OpenWeb())
        {
            SPQuery query = new SPQuery();

            query.Query = @"<Where><Eq><FieldRef Name='Title' /><Value Type='Text'>"+Title+"</Value></Eq></Where>"; 
            //query.RowLimit = 6;
            SPList list = web.Lists["CapabilityTableSales"];
            SPListItemCollection col = list.GetItems(query);
            int Id = -1 ;
            foreach (SPListItem LI in col)
            {
                Id = Convert.ToInt32(LI["ID"]);
            }
            string attachmentURL="";
            if (Id != -1)
            {
                SPListItem item = list.GetItemById(Id);

                foreach (string filename in item.Attachments)
                {
                    attachmentURL = SiteListURL + "attachments/" + item.ID + "/" + filename;
                }
            }
            OpenFile(attachmentURL);

            //System.Net.WebClient _WebClient = new System.Net.WebClient();
            //// Downloads the resource with the specified URI to a local file.
            //_WebClient.OpenWrite(attachmentURL);

            return attachmentURL;

            //Response.Redirect(attachmentURL);
            //Response.Redirect("http://so4:60000/SitePages/Capability%20Presentation.aspx");
        }
    }
}

Code for Button click :

protected void ImageButton11_Click(object sender, ImageClickEventArgs e)
{
    string redirectURL11 = getlist("1.1");

    ImageButton11.PostBackUrl = redirectURL11;
}

First time it will work properly, the save dialogue box will appear. However 2nd time its just not working until I refresh the page. Another problem is that in this page I have got another web part - no relation to this one. Even that button will not fire!

Answer 1

Got it worked !! Used an anchor control with Image instead of Image Button!!

<a href="" runat="server" id="aAttachmentURL11" target="_blank"> <img src="~/_layouts/images/ipw/PowerPoint-64.png" alt="" style="border: 0px" width="63px" /> </a>

Code : And then called the method getlist(string Title) to return the related attachment URL!!

string redirectURL11 = getlist("1.1"); aAttachmentURL11.HRef = redirectURL11;

Answer 2

You can construct the attachment URL by using the SPAttachmentCollection.UrlPrefix property of the attachments collection. For example:

string attachmentUrl = item.Attachments.UrlPrefix + item.Attachments[0];