Object reference not set to an instance of an object in drop down list in C# ASP.Net

Hi theunclevince,

Write the DropDownList population code inside the Not IsPostBack condition.

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        DateTime dateTime = DateTime.Now;

        // List of Months to be display.
        List<ListItem> months = new List<ListItem>();
        for (int i = 1; i <= 8; i++)
        {
            months.Add(ddlMonths.Items.Cast<ListItem>()
                                .Where(x => x.Value.Equals(dateTime.AddMonths(-i).Month.ToString()))
                                .FirstOrDefault());
        }

        //Determines first item i.e.Equals Default Item.
        ListItem defaultItem = ddlMonths.Items[0];

        //Clears all items in the DropDownList.
        ddlMonths.Items.Clear();

        //Reverse the List of last two months.
        months.Reverse();

        //Inserts Default Item.
        months.Insert(0, defaultItem);

        //Assigning month to DropDownList.
        ddlMonths.DataSource = months;
        ddlMonths.DataBind();
    }
}

Okay, thanks

Now it doesn't return an error but the name of the month, I would need the number of the month.

For example in drop down list list input OCT output 10

I have tried using

    protected void ddlMonths_SelectedIndexChanged(object sender, EventArgs e)
    {
        Response.Write(ddlMonths.SelectedIndex + "<br /><br />");
    }

But the output is wrong

Hi theunclevince,

Refer complete sample.

HTML

<asp:DropDownList ID="ddlMonths" runat="server" AutoPostBack="true"
    OnSelectedIndexChanged="ddlMonths_SelectedIndexChanged">
    <asp:ListItem Text="Select" Value=""></asp:ListItem>
    <asp:ListItem Text="JAN" Value="1"></asp:ListItem>
    <asp:ListItem Text="FEB" Value="2"></asp:ListItem>
    <asp:ListItem Text="MAR" Value="3"></asp:ListItem>
    <asp:ListItem Text="APR" Value="4"></asp:ListItem>
    <asp:ListItem Text="MAY" Value="5"></asp:ListItem>
    <asp:ListItem Text="JUN" Value="6"></asp:ListItem>
    <asp:ListItem Text="JUL" Value="7"></asp:ListItem>
    <asp:ListItem Text="AUG" Value="8"></asp:ListItem>
    <asp:ListItem Text="SEP" Value="9"></asp:ListItem>
    <asp:ListItem Text="OCT" Value="10"></asp:ListItem>
    <asp:ListItem Text="NOV" Value="11"></asp:ListItem>
    <asp:ListItem Text="DEC" Value="12"></asp:ListItem>
</asp:DropDownList>
<br />
<asp:Label ID="lblMesssage" runat="server" />

Code

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        DateTime dateTime = DateTime.Now;

        // List of Months to be display.
        List<ListItem> months = new List<ListItem>();
        for (int i = 1; i <= 8; i++)
        {
            months.Add(ddlMonths.Items.Cast<ListItem>()
                                .Where(x => x.Value.Equals(dateTime.AddMonths(-i).Month.ToString()))
                                .FirstOrDefault());
        }

        //Determines first item i.e.Equals Default Item.
        ListItem defaultItem = ddlMonths.Items[0];

        //Clears all items in the DropDownList.
        ddlMonths.Items.Clear();

        //Reverse the List of last two months.
        months.Reverse();

        //Inserts Default Item.
        months.Insert(0, defaultItem);

        //Assigning month to DropDownList.
        ddlMonths.DataSource = months;
        ddlMonths.DataTextField = "Text";
        ddlMonths.DataValueField = "Value";
        ddlMonths.DataBind();
    }
}

protected void ddlMonths_SelectedIndexChanged(object sender, EventArgs e)
{
    lblMesssage.Text = "Selected Index: " + ddlMonths.SelectedIndex + "<br />Selected Text: " + ddlMonths.SelectedItem.Text + "<br />Selected Value:" + ddlMonths.SelectedItem.Value;
}

Screenshot