In order to retrieve SharePoint list items,  SPQuery is object of choice most of the times. Also for better performance, we may need to display limited items (PageSize) and  use paging to fetch next set of items.  Paging is not that straightforward task when you want to use it with SPQuery.

SPListItemCollectionPosition is the class which helps to achieve paging and sorting the results fetched by SPQuery.

SPQuery.ListItemCollectionPosition is a property of SPQuery that getssets an object of type SPListItemCollectionPosition that is used to obtain the next set of list item rows.

The ListItemCollectionPosition property is used together with the SPQuery.RowLimit property to define paging in a CAML query. The SPListItemCollectionPosition object is used to traverse through all the items in a collection n items at a time, where n is  row limit.

SPListItemCollectionPosition object needs to be initialized with correct PagingInfo string prior to use it with SPQuery. Below is the constructor signature

public SPListItemCollectionPosition(
         string PagingInfo
)

PagingInfo is the string containing paging information used to fetch the next page of list items. The PagingInfo string can have below formats depending upon how you need to traverse, forward or backward.

For traversing forward , PagingInfo string becomes :

Paged=TRUE&p_ID={l}

In case of paging forward p_ID is list item Id of last item (=l) on current page.

For traversing  backward , PagingInfo  string becomes

Paged=TRUE &PagedPrev=TRUE&p_ID={f}

In case of paging backward(previous) , p_ID is item Id of first item (=f) on current page.

Now, Let’s assume that we want to set page size of 5. For that, we first need to set SPQuery.RowLimit to 5 to return 5 list items at one time. When the page loads first time, you need not use SPListItemCollectionPosition .For the next page,The blow code retrieves next 5 items assuming last item in current page have Id=5 .

[Note: If items are deleted from lists, the 5^thitem Id may not necessarily be 5.You will need correct list item id to go to next page. In below example ,itemCollec[itemCollec.Count - 1].ID gives correct item Id]

            SPQuery query = new SPQuery();
            query.RowLimit = 5;
            query.Query = "<IsNotNull><FieldRef Name='Title' /></IsNotNull>";
            query.ViewFields = "<FieldRef Name='Title' /><FieldRef Name='Id' />";
            SPListItemCollectionPosition position = new SPListItemCollectionPosition(“Paged=TRUE&p_ID=5”);
            query.ListItemCollectionPosition = position;
            itemCollec = oWeb.Lists["Tasks"].GetItems(query);
            return itemCollec;

If you are on Page 2 and from here if you want to go previous,the pagingInfo becomes Paged=TRUE &PagedPrev=TRUE&p_ID=6

            SPQuery query = new SPQuery();
            query.RowLimit = 5;
            query.Query = "<IsNotNull><FieldRef Name='Title' /></IsNotNull>";
            query.ViewFields = "<FieldRef Name='Title' /><FieldRef Name='Id' />";
            SPListItemCollectionPosition position = new SPListItemCollectionPosition(“Paged=TRUE &PagedPrev=TRUE&p_ID=6”);
            query.ListItemCollectionPosition = position;
            itemCollec = oWeb.Lists["Tasks"].GetItems(query);
            return itemCollec;

[Note: If items are deleted from lists, the 1st item Id on second page may not necessarily be 6 .You will need correct list item id to go to previous page. In below example ,itemCollec[0].ID gives correct item Id]

Using above logic you can create a webpart that uses  querystring to pass Page No and Item Id to construct PagingInfo string on fly. You can also use ViewState instead of Query String to persist the necessary information to construct the PagingInfo.

Check the below working code sample.

public class SPQueryPaging : WebPart
    {
        string prevPageUrl;
        string nextPageUrl;
        GridView GridView1=null;
        Label lblcurrPageItems;
        protected override void CreateChildControls()
        {
            GridView1 = new GridView();
            this.Controls.Add(GridView1);
            lblcurrPageItems = new Label();
            this.Controls.Add(lblcurrPageItems);
            //Fetch ‘page no’ and ‘item id’ from querystring to get items for current page.
            GetListItems(GetcurrPage("page"), GetCurrentItemId("itemid"));
        }
        private string GetQueryStringValue(string param)
        {
            if (!string.IsNullOrEmpty(Page.Request.QueryString[param]))
                return Page.Request.QueryString[param];
            return string.Empty;
        }
        private int GetcurrPage(string currPage)
        {
            int page = 1;
            try
            {
                page = Int32.Parse(GetQueryStringValue(currPage));
            }
            catch
            {
                page = 1;
            }
            return page;
        }
        private int GetCurrentItemId(string currItemId)
        {
            int currItem = -1;
            try
            {
                currItem = Int32.Parse(GetQueryStringValue(currItemId));
            }
            catch
            {
                currItem = -1;
            }
            return currItem;
        }
        protected override void Render(HtmlTextWriter writer)
        {
            lblcurrPageItems.RenderControl(writer);
            GridView1.RenderControl(writer);
            if (!string.IsNullOrEmpty(prevPageUrl))
                writer.Write("<a href='" + prevPageUrl + "'><< Prev</a>");
            writer.Write("&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;");
            if (!string.IsNullOrEmpty(nextPageUrl))
                writer.Write("<a href='" + nextPageUrl + "'>Next >></a>");
        }
        private void GetListItems(int currPage, int itemId)
        {
            uint rowCount = 5;
            string pagingInfo = string.Empty;
            if (itemId != -1)
            {
                //Generate PagingInfo from current Page No and Item Id
                if (GetQueryStringValue("prev") == "1")
                    pagingInfo = string.Format("Paged=TRUE&PagedPrev=TRUE&p_ID={0}", itemId);
                else
                    pagingInfo = string.Format("Paged=TRUE&p_ID={0}", itemId);
            }
            SPListItemCollection itemCollec;
            itemCollec = GetListItems(pagingInfo, rowCount);
            DataTable objDataTable = itemCollec.GetDataTable();
            GridView1.DataSource = objDataTable;
            GridView1.DataBind();

            if (itemCollec.ListItemCollectionPosition != null)
            //Generate previous PageUrl with correct item id and Page No    
             nextPageUrl = Page.AppRelativeVirtualPath.Replace("~", string.Empty) + "?itemid=" + itemCollec[itemCollec.Count - 1].ID + "&page=" + (currPage + 1);
            else
                nextPageUrl = string.Empty;
            if (currPage > 1)

 //Generate next PageUrl with correct item id and Page No   
        prevPageUrl = Page.AppRelativeVirtualPath.Replace("~", string.Empty) + "?itemid=" + itemCollec[0].ID + "&page=" + (currPage - 1) + "&prev=1";
            else
                prevPageUrl = string.Empty;
            lblcurrPageItems.Text = ((currPage - 1) * rowCount) + 1 + " - " + currPage * rowCount;
        }
        public SPListItemCollection GetListItems(string pagingInfo, uint rowLimit)
        {
            SPWeb oWeb = SPContext.Current.Web;
            SPListItemCollection itemCollec;
            SPQuery query = new SPQuery();
            query.RowLimit = rowLimit;
            query.Query = "<IsNotNull><FieldRef Name='Title' /></IsNotNull>";
            query.ViewFields = "<FieldRef Name='Title' /><FieldRef Name='Id' />";
            if (!string.IsNullOrEmpty(pagingInfo))
            {
                SPListItemCollectionPosition position = new SPListItemCollectionPosition(pagingInfo);
                query.ListItemCollectionPosition = position;
            }
            itemCollec = oWeb.Lists["Tasks"].GetItems(query);
            return itemCollec;
        }
    }