Set DropDownList selected Text when ASP.Net GridView cell is empty on SelectedIndexChanged using C# and VB.Net

Hi akhter.

FindByText is not finding an item in DropDownList and you are trying to use the Selected property on a null object.

So It is always important when using methods like FindByValue() or FindByText() to check and ensure that a particular item exists prior to using it.

Check this example. Now please take its reference and correct your code.

HTML

<table>
    <tr>
        <td>Item Name:</td>
        <td><asp:TextBox ID="txtName" runat="server"></asp:TextBox></td>
    </tr>
    <tr>
        <td>UOM :</td>
        <td>
            <asp:DropDownList ID="ddlUOM" runat="server">
                <asp:ListItem Value="0">Select UOM</asp:ListItem>
                <asp:ListItem Value="kg">Kilogram</asp:ListItem>
                <asp:ListItem Value="m">Meter</asp:ListItem>
                <asp:ListItem Value="s">Second</asp:ListItem>
                <asp:ListItem Value="A">Ampere</asp:ListItem>
                <asp:ListItem Value="K">Kelvin</asp:ListItem>
                <asp:ListItem Value="mol">Mole</asp:ListItem>
                <asp:ListItem Value="cd">Candela</asp:ListItem>
            </asp:DropDownList>
        </td>
    </tr>
</table>
<br />
<asp:GridView ID="GridView1" runat="server" AllowPaging="True" AutoGenerateColumns="False"
    DataKeyNames="CodeItem" OnSelectedIndexChanged="OnSelectedIndexChanged">
    <Columns>
        <asp:CommandField ShowSelectButton="True" />
        <asp:BoundField DataField="CodeItem" HeaderText="Code" ReadOnly="True" />
        <asp:BoundField DataField="Description" HeaderText="Description" />
        <asp:BoundField DataField="NameUOM" HeaderText="UOM" />
    </Columns>
</asp:GridView>

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        System.Data.DataTable dt = new System.Data.DataTable();
        dt.Columns.AddRange(new System.Data.DataColumn[] { 
                        new System.Data.DataColumn("CodeItem", typeof(int)),
                        new System.Data.DataColumn("Description", typeof(string)),
                        new System.Data.DataColumn("NameUOM",typeof(string)) });
        dt.Rows.Add(501, "Big Test 501", "");
        dt.Rows.Add(503, "Big Test 503", "Second");
        dt.Rows.Add(502, "Big Test 502", null);
        GridView1.DataSource = dt;
        GridView1.DataBind();
    }
}
protected void OnSelectedIndexChanged(object sender, EventArgs e)
{
    txtName.Text = GridView1.SelectedRow.Cells[2].Text.Trim();
    string umo = GridView1.SelectedRow.Cells[3].Text.Trim();
    ddlUOM.ClearSelection();
    // Check item exists in DropDownList or not.
    if (ddlUOM.Items.FindByText(umo) != null)
    {
        ddlUOM.Items.FindByText(umo).Selected = true;
    }
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        Dim dt As System.Data.DataTable = New System.Data.DataTable()
        dt.Columns.AddRange(New System.Data.DataColumn() {
                            New System.Data.DataColumn("CodeItem", GetType(Integer)),
                            New System.Data.DataColumn("Description", GetType(String)),
                            New System.Data.DataColumn("NameUOM", GetType(String))})
        dt.Rows.Add(501, "Big Test 501", "")
        dt.Rows.Add(503, "Big Test 503", "Second")
        dt.Rows.Add(502, "Big Test 502", Nothing)
        GridView1.DataSource = dt
        GridView1.DataBind()
    End If
End Sub

Protected Sub OnSelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
    txtName.Text = GridView1.SelectedRow.Cells(2).Text.Trim()
    Dim umo As String = GridView1.SelectedRow.Cells(3).Text.Trim()
    ddlUOM.ClearSelection()
    ' Check item exists in DropDownList or not.
    If ddlUOM.Items.FindByText(umo) IsNot Nothing Then
        ddlUOM.Items.FindByText(umo).Selected = True
    End If
End Sub

Screenshot