Show / Hide TextBox based on value of DropDownList inside GridView in ASP.Net

1. Add OnSelectedIndexChanged event to the DropDownList and set AutoPostBack True.

2. Inside the Event handler, make the TextBox visible.

HTML

<asp:GridView ID="TypeFruit" runat="server">
<Columns>
    <asp:TemplateField HeaderText="Type">
        <ItemTemplate>
            <asp:DropDownList ID="ddlfruit" runat="server" OnSelectedIndexChanged = "Fruit_Changed" AutoPostBack = "true">
                <asp:ListItem Value="0">Select</asp:ListItem>
                <asp:ListItem Value="1">Fruit</asp:ListItem>
                <asp:ListItem Value="2">Other</asp:ListItem>
            </asp:DropDownList>
            <asp:TextBox ID="TxtOther" runat="server" Visible="false"></asp:TextBox>
        </ItemTemplate>
    </asp:TemplateField>
</Columns>
</asp:GridView>

Code

protected void Fruit_Changed(object sender, EventArgs e)
{
    DropDownList ddlFruits = (sender as DropDownList);
    TextBox txtOther = ((ddlFruits.NamingContainer as GridViewRow).FindControl("TxtOther") as TextBox);
    txtOther.Visible = ddlFruits.SelectedItem.Value == "2";
}