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

Here I have Created Sample Based on your requirment.

HTML

<div>
<script type="text/javascript">
    window.onload = function () {
        debugger
        var table = document.getElementById("<%=Gridview1.ClientID %>");
        var dropdownlist = table.getElementsByTagName('select');
        for (var i = 0; i < dropdownlist.length; i++) {
            if (dropdownlist[i].value == "Yes") {
                var parent = dropdownlist[i].parentNode.parentNode;
                var control = parent.getElementsByTagName('input');
                for (var j = 0; j < control.length; j++) {
                    if (control[j].type == "text") {
                        control[j].style.display = "none";
                    }
                }
            }
        }

    }
</script>
<script type="text/javascript">
    function EnableDisableTextBox(ddl) {
        debugger;
        var parent = ddl.parentNode.parentNode;
        var control = parent.getElementsByTagName('input');
        for (var i = 0; i < control.length; i++) {
            if (control[i].type == "text" && ddl.value == "Yes") {
                control[i].style.display = "none";
            }
            if (control[i].type == "text" && ddl.value != "Yes") {
                control[i].style.display = "block";
            }
        }
    }
</script>
<asp:GridView ID="Gridview1" runat="server" ShowFooter="true" AutoGenerateColumns="false"
    OnRowCreated="Gridview1_RowCreated">
    <Columns>
        <asp:TemplateField HeaderText="RowNumber">
            <ItemTemplate>
                <%# Container.DataItemIndex + 1 %>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Header 1">
            <ItemTemplate>
                <%--<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>--%>
                <asp:DropDownList ID="DropDownList1" onchange="EnableDisableTextBox(this);" runat="server">
                    <asp:ListItem Text="Please Select" Value="Please Select" />
                    <asp:ListItem Text="Yes" Value="Yes" />
                    <asp:ListItem Text="No" Value="No" />
                </asp:DropDownList>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Header 2">
            <ItemTemplate>
                <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Header 3">
            <ItemTemplate>
                <asp:TextBox ID="TextBox3" runat="server"></asp:TextBox>
            </ItemTemplate>
            <FooterStyle HorizontalAlign="Right" />
            <FooterTemplate>
                <asp:Button ID="ButtonAdd" runat="server" Text="Add New Row" OnClick="ButtonAdd_Click" />
            </FooterTemplate>
        </asp:TemplateField>
        <asp:TemplateField>
            <ItemTemplate>
                <asp:LinkButton ID="LinkButton1" runat="server" OnClick="LinkButton1_Click">Remove</asp:LinkButton>
            </ItemTemplate>
        </asp:TemplateField>
    </Columns>
</asp:GridView>
</div>

Code

private void SetInitialRow()
{
    DataTable dt = new DataTable();
    DataRow dr = null;

    dt.Columns.Add(new DataColumn("Column1", typeof(string)));
    dt.Columns.Add(new DataColumn("Column2", typeof(string)));
    dt.Columns.Add(new DataColumn("Column3", typeof(string)));
    dr = dt.NewRow();

    dr["Column1"] = string.Empty;
    dr["Column2"] = string.Empty;
    dr["Column3"] = string.Empty;
    dt.Rows.Add(dr);

    //Store the DataTable in ViewState
    ViewState["CurrentTable"] = dt;

    Gridview1.DataSource = dt;
    Gridview1.DataBind();
}

protected void Page_Load(object sender, EventArgs e)
{
    if (!Page.IsPostBack)
    {
        SetInitialRow();
    }
}

protected void ButtonAdd_Click(object sender, EventArgs e)
{
    DataTable dtCurrentTable = (DataTable)ViewState["CurrentTable"];
    dtCurrentTable.Rows.Clear();
    foreach (GridViewRow row in Gridview1.Rows)
    {
        if (row.RowType == DataControlRowType.DataRow)
        {
            DropDownList box1 = (DropDownList)row.FindControl("DropDownList1");
            TextBox box2 = (TextBox)row.FindControl("TextBox2");
            TextBox box3 = (TextBox)row.FindControl("TextBox3");
            dtCurrentTable.Rows.Add(box1.SelectedItem.Value, box2.Text, box3.Text);
        }
    }


    dtCurrentTable.Rows.Add("", "", "");
    ViewState["CurrentTable"] = dtCurrentTable;
    this.Gridview1.DataSource = dtCurrentTable;
    this.Gridview1.DataBind();
    for (int i = 0; i < dtCurrentTable.Rows.Count; i++)
    {
        if (this.Gridview1.Rows[i].RowType == DataControlRowType.DataRow)
        {
            DropDownList box1 = (DropDownList)this.Gridview1.Rows[i].FindControl("DropDownList1");
            TextBox box2 = (TextBox)this.Gridview1.Rows[i].FindControl("TextBox2");
            TextBox box3 = (TextBox)this.Gridview1.Rows[i].FindControl("TextBox3");
            box1.Text = dtCurrentTable.Rows[i]["Column1"].ToString();
            box2.Text = dtCurrentTable.Rows[i]["Column2"].ToString();
            box3.Text = dtCurrentTable.Rows[i]["Column3"].ToString();
        }
    }
}

protected void Gridview1_RowCreated(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow)
    {
        DataTable dt = (DataTable)ViewState["CurrentTable"];
        LinkButton lb = (LinkButton)e.Row.FindControl("LinkButton1");
        if (lb != null)
        {
            if (dt.Rows.Count > 1)
            {
                if (e.Row.RowIndex == dt.Rows.Count - 1)
                {
                    lb.Visible = false;
                }
            }
            else
            {
                lb.Visible = false;
            }
        }
    }
}
//Deleting the Row
protected void LinkButton1_Click(object sender, EventArgs e)
{
    LinkButton lb = (LinkButton)sender;
    GridViewRow gvRow = (GridViewRow)lb.NamingContainer;
    DataTable dt = (DataTable)ViewState["CurrentTable"];
    dt.Rows.RemoveAt(gvRow.RowIndex);


    this.Gridview1.DataSource = dt;
    this.Gridview1.DataBind();
    ViewState["CurrentTable"] = dt;
    for (int i = 0; i < dt.Rows.Count; i++)
    {
        if (this.Gridview1.Rows[i].RowType == DataControlRowType.DataRow)
        {
            DropDownList box1 = (DropDownList)this.Gridview1.Rows[i].FindControl("DropDownList1");
            TextBox box2 = (TextBox)this.Gridview1.Rows[i].FindControl("TextBox2");
            TextBox box3 = (TextBox)this.Gridview1.Rows[i].FindControl("TextBox3");
            box1.Text = dt.Rows[i]["Column1"].ToString();
            box2.Text = dt.Rows[i]["Column2"].ToString();
            box3.Text = dt.Rows[i]["Column3"].ToString();
        }
    }
}