页面大小导致gridview DataBind中的错误

Question

I am facing a strange issue or maybe i am doing a silly mistake. I have a datatable which contains five rows. I am creating a gridview from codebehind. for pagination if i specify Page size =5 (the size of datatable) everything is fine but when i specify the page size less than the size of the datatable eg Page size=2 , I am getting :

Object reference not set to an instance of an object.

in gvEmployee.DataBind();

using System;
using System.Collections.Generic;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace demogrid
{
    public partial class grid : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                DataTable table = new DataTable();
                table.Columns.Add("Dosage", typeof(int));
                table.Columns.Add("Drug", typeof(string));
                table.Columns.Add("Patient", typeof(string));
                table.Columns.Add("Date", typeof(DateTime));

                // Here we add five DataRows.
                table.Rows.Add(25, "Indocin", "David", DateTime.Now);
                table.Rows.Add(50, "Enebrel", "Sam", DateTime.Now);
                table.Rows.Add(10, "Hydralazine", "Christoff", DateTime.Now);
                table.Rows.Add(21, "Combivent", "Janet", DateTime.Now);
                table.Rows.Add(100, "Dilantin", "Melanie", DateTime.Now);
                BindData(table);
            }
        }
        protected void BindData(DataTable dt)
        {
            GridView gvEmployee = new GridView();
            gvEmployee.AllowPaging = true;
            gvEmployee.PageSize = 5;//getting error if the page size is less than 5
            gvEmployee.AutoGenerateColumns = false;

            if (dt != null)
            {
                for (int i = 0; i < dt.Columns.Count; i++)
                {
                    BoundField boundfield = new BoundField();
                    boundfield.DataField = dt.Columns[i].ColumnName.ToString();
                    boundfield.HeaderText = dt.Columns[i].ColumnName.ToString();
                    gvEmployee.Columns.Add(boundfield);
                }
                gvEmployee.DataSource = dt;
                gvEmployee.DataBind();
                gvEmployee.Width = 600;
                gvEmployee.HeaderStyle.CssClass = "header";
                gvEmployee.RowStyle.CssClass = "rowstyle";

                Panel1.Controls.Add(gvEmployee);
            }
        }
    }
}

StackTrace:

at System.Web.UI.WebControls.GridView.get_StateFormatter()
       at System.Web.UI.WebControls.GridView.BuildCallbackArgument(Int32 pageIndex)
       at System.Web.UI.WebControls.GridView.CreateNumericPager(TableRow row, PagedDataSource pagedDataSource, Boolean addFirstLastPageButtons)
       at System.Web.UI.WebControls.GridView.InitializePager(GridViewRow row, Int32 columnSpan, PagedDataSource pagedDataSource)
       at System.Web.UI.WebControls.GridView.CreateRow(Int32 rowIndex, Int32 dataSourceIndex, DataControlRowType rowType, DataControlRowState rowState, Boolean dataBind, Object dataItem, DataControlField[] fields, TableRowCollection rows, PagedDataSource pagedDataSource)
       at System.Web.UI.WebControls.GridView.CreateChildControls(IEnumerable dataSource, Boolean dataBinding)
       at System.Web.UI.WebControls.CompositeDataBoundControl.PerformDataBinding(IEnumerable data)
       at System.Web.UI.WebControls.GridView.PerformDataBinding(IEnumerable data)
       at System.Web.UI.WebControls.DataBoundControl.OnDataSourceViewSelectCallback(IEnumerable data)
       at System.Web.UI.DataSourceView.Select(DataSourceSelectArguments arguments, DataSourceViewSelectCallback callback)
       at System.Web.UI.WebControls.DataBoundControl.PerformSelect()
       at System.Web.UI.WebControls.BaseDataBoundControl.DataBind()
       at System.Web.UI.WebControls.GridView.DataBind()
       at demogrid.grid.BindData(DataTable dt) in c:\Users\sample\Documents\Visual Studio 2012\Projects\demogrid\demogrid\grid.aspx.cs:line 59
       at demogrid.grid.Page_Load(Object sender, EventArgs e) in c:\Users\sample\Documents\Visual Studio 2012\Projects\demogrid\demogrid\grid.aspx.cs:line 32

Any help would be appreciated.

Thanks in advance.

Answer 1

The Exception is occurring because your GridView is not a control on PAGE yet and your are calling DataBind .
You must add the control on the page before you databind .
And Also you must implement PageIndexChanging Event to handle Page Changing Event.

One more thing is that you need to move your datatable code from if (!IsPostBack) block, because it wont be executed when Page POSTBACK and control wont be on page anymore when page is changed or any POSTBACK event is fired. Here is a working code for you.

using System;
using System.Collections.Generic;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace demogrid
{
    public partial class grid : System.Web.UI.Page
    {
    GridView gvEmployee = new GridView();

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {

        }
        DataTable table = new DataTable();
        table.Columns.Add("Dosage", typeof(int));
        table.Columns.Add("Drug", typeof(string));
        table.Columns.Add("Patient", typeof(string));
        table.Columns.Add("Date", typeof(DateTime));

        // Here we add five DataRows.
        table.Rows.Add(25, "Indocin", "David", DateTime.Now);
        table.Rows.Add(50, "Enebrel", "Sam", DateTime.Now);
        table.Rows.Add(10, "Hydralazine", "Christoff", DateTime.Now);
        table.Rows.Add(21, "Combivent", "Janet", DateTime.Now);
        table.Rows.Add(100, "Dilantin", "Melanie", DateTime.Now);
        BindData(table);
    }
    protected void BindData(DataTable dt)
    {
        gvEmployee.AllowPaging = true;
        gvEmployee.PageSize = 2;
        gvEmployee.AutoGenerateColumns = false;
        gvEmployee.PageIndexChanging += gvEmployee_PageIndexChanging;
        if (dt != null)
        {
            for (int i = 0; i < dt.Columns.Count; i++)
            {
                BoundField boundfield = new BoundField();
                boundfield.DataField = dt.Columns[i].ColumnName.ToString();
                boundfield.HeaderText = dt.Columns[i].ColumnName.ToString();
                gvEmployee.Columns.Add(boundfield);
            }
            Panel1.Controls.Add(gvEmployee);
            gvEmployee.DataSource = dt;
            gvEmployee.DataBind();
            gvEmployee.Width = 600;
            gvEmployee.HeaderStyle.CssClass = "header";
            gvEmployee.RowStyle.CssClass = "rowstyle";

        }
    }

    void gvEmployee_PageIndexChanging(object sender, GridViewPageEventArgs e)
    {
        gvEmployee.PageIndex = e.NewPageIndex;
        gvEmployee.DataBind();

    }


}
}

Answer 2

This is caused because no Pager Template is set to your GridView. I have modified your code to get you started. First add RowDataBound handler and set your Pager:

    protected void BindData(DataTable dt)
    {
        GridView gvEmployee = new GridView();
        gvEmployee.AllowPaging = true;
        gvEmployee.PageSize = 4;//this Will work now
        gvEmployee.AutoGenerateColumns = false;

        if (dt != null)
        {
            for (int i = 0; i < dt.Columns.Count; i++)
            {
                BoundField boundfield = new BoundField();
                boundfield.DataField = dt.Columns[i].ColumnName.ToString();
                boundfield.HeaderText = dt.Columns[i].ColumnName.ToString();
                gvEmployee.Columns.Add(boundfield);
            }
            gvEmployee.DataSource = dt;
            gvEmployee.RowDataBound += gvEmployee_RowDataBound;
            gvEmployee.PagerTemplate = new MyTemplate();
            gvEmployee.DataBind();
            gvEmployee.Width = 600;
            gvEmployee.HeaderStyle.CssClass = "header";
            gvEmployee.RowStyle.CssClass = "rowstyle";

            Panel1.Controls.Add(gvEmployee);
        }
    }

    void gvEmployee_RowDataBound(object sender, GridViewRowEventArgs e)
    {
        switch (e.Row.RowType)
        {
            case DataControlRowType.DataRow:
                break;
            case DataControlRowType.EmptyDataRow:
                break;
            case DataControlRowType.Footer:
                break;
            case DataControlRowType.Header:
                break;
            case DataControlRowType.Pager:
                //TODO: handle your pager...
                break;
            case DataControlRowType.Separator:
                break;
            default:
                break;
        }
    }

And here is basic Pager control:

public class MyTemplate : WebControl, ITemplate
{
    public void InstantiateIn(Control container)
    {
        //TODO: create your pager controls...
    }
}

This will do the job.