Hide (Remove) selected value of one DropDownList in other DropDownLists in ASP.Net

Hi Karthick34,

I have created one sample that full-fill your requirement.

HTML

<table>
    <tr>
        <td>
            DropDownList1
        </td>
        <td>
            <asp:DropDownList runat="server" ID="ddl1" AutoPostBack="true" OnSelectedIndexChanged="ddl1SelectionChanged">
            </asp:DropDownList>
        </td>
    </tr>
    <tr>
        <td>
            DropDownList2
        </td>
        <td>
            <asp:DropDownList runat="server" ID="ddl2" AutoPostBack="true" OnSelectedIndexChanged="ddl2SelectionChanged">
            </asp:DropDownList>
        </td>
    </tr>
    <tr>
        <td>
            DropDownList3
        </td>
        <td>
            <asp:DropDownList runat="server" ID="ddl3" AutoPostBack="true" OnSelectedIndexChanged="ddl3SelectionChanged">
            </asp:DropDownList>
        </td>
    </tr>
    <tr>
        <td>
            DropDownList4
        </td>
        <td>
            <asp:DropDownList runat="server" ID="ddl4">
            </asp:DropDownList>
        </td>
    </tr>
</table>

Code

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        DataTable dt = GetCountries();
        ddl1.DataSource = dt;
        ddl1.DataTextField = "Name";
        ddl1.DataValueField = "Id";
        ddl1.DataBind();
    }
}
private DataTable GetCountries()
{
    DataTable dt = new DataTable();
    dt.Columns.AddRange(new DataColumn[2] { 
                        new DataColumn("Id", typeof(int)),
                        new DataColumn("Name", typeof(string)) });
    dt.Rows.Add(1, "India");
    dt.Rows.Add(2, "Pakistan");
    dt.Rows.Add(3, "Srilanka");
    dt.Rows.Add(4, "Bangladesh");
    dt.Rows.Add(5, "Australia");
    dt.Rows.Add(6, "England");
    dt.Rows.Add(7, "South Africa");
    return dt;
}
protected void ddl1SelectionChanged(object sender, EventArgs e)
{
    DataTable dt = GetCountries();
    var query = from dd1 in dt.AsEnumerable()
                where dd1.Field<string>("Name") != ddl1.SelectedItem.Text
                select new { ID = dd1.Field<int>("Id"), Name = dd1.Field<string>("Name") };
    ddl2.DataSource = query.ToList();
    ddl2.DataTextField = "Name";
    ddl2.DataValueField = "ID";
    ddl2.DataBind();
}
protected void ddl2SelectionChanged(object sender, EventArgs e)
{
    DataTable dt = GetCountries();
    var query = from dd2 in dt.AsEnumerable()
                where dd2.Field<string>("Name") != ddl1.SelectedItem.Text
                && dd2.Field<string>("Name") != ddl2.SelectedItem.Text
                select new { ID = dd2.Field<int>("Id"), Name = dd2.Field<string>("Name") };
    ddl3.DataSource = query.ToList();
    ddl3.DataTextField = "Name";
    ddl3.DataValueField = "ID";
    ddl3.DataBind();
}
protected void ddl3SelectionChanged(object sender, EventArgs e)
{
    DataTable dt = GetCountries();
    var query = from dd3 in dt.AsEnumerable()
                where dd3.Field<string>("Name") != ddl1.SelectedItem.Text
                && dd3.Field<string>("Name") != ddl2.SelectedItem.Text
                && dd3.Field<string>("Name") != ddl3.SelectedItem.Text
                select new { ID = dd3.Field<int>("Id"), Name = dd3.Field<string>("Name") };
    ddl4.DataSource = query.ToList();
    ddl4.DataTextField = "Name";
    ddl4.DataValueField = "ID";
    ddl4.DataBind();
}

Screenshot