Hi YulIhnio,

Refer below sample.

HTML

<asp:GridView ID="GridView1" runat="server" CssClass="Grid" AutoGenerateColumns="false"
    EmptyDataText="No records has been added.">
    <Columns>
        <asp:BoundField DataField="Name" HeaderText="Name" ItemStyle-Width="120" />
        <asp:BoundField DataField="Country" HeaderText="Country" ItemStyle-Width="120" />
    </Columns>
</asp:GridView>
<br />
<table border="0" cellpadding="0" cellspacing="0" style="border-collapse: collapse">
    <tr>
        <td style="padding-bottom: 10px">
            Name:<br />
            <asp:TextBox ID="txtName" runat="server" />
        </td>
    </tr>
    <tr>
        <td style="padding-bottom: 10px">
            Country:<br />
            <asp:TextBox ID="txtCountry" runat="server" />
        </td>
    </tr>
    <tr>
        <td style="width: 100px">
            <asp:Button ID="btnAdd" runat="server" Text="Add" OnClick="Insert" />
        </td>
        <td style="width: 100px">
            <asp:Button ID="btnClear" runat="server" Text="Clear" OnClick="ClearData" />
        </td>
    </tr>
</table>

Namespaces

C#

using System.Data;

VB.Net

Imports System.Data

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        DataTable dt = new DataTable();
        dt.Columns.AddRange(new DataColumn[2] { new DataColumn("Name"), new DataColumn("Country") });
        ViewState["Customers"] = dt;
        this.BindGrid();
    }
}

protected void BindGrid()
{
    GridView1.DataSource = (DataTable)ViewState["Customers"];
    GridView1.DataBind();
}

protected void Insert(object sender, EventArgs e)
{
    DataTable dt = (DataTable)ViewState["Customers"];
    dt.Rows.Add(txtName.Text.Trim(), txtCountry.Text.Trim());
    ViewState["Customers"] = dt;
    this.BindGrid();
    txtName.Text = string.Empty;
    txtCountry.Text = string.Empty;
}

protected void ClearData(object sender, EventArgs e)
{
    ViewState["Customers"] = null;
    GridView1.DataSource = (DataTable)ViewState["Customers"];
    GridView1.DataBind();
}

VB.Net

Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        Dim dt As New DataTable()
        dt.Columns.AddRange(New DataColumn(1) {New DataColumn("Name"), New DataColumn("Country")})
        ViewState("Customers") = dt
        Me.BindGrid()
    End If
End Sub

Protected Sub BindGrid()
    GridView1.DataSource = DirectCast(ViewState("Customers"), DataTable)
    GridView1.DataBind()
End Sub

Protected Sub Insert(sender As Object, e As EventArgs)
    Dim dt As DataTable = DirectCast(ViewState("Customers"), DataTable)
    dt.Rows.Add(txtName.Text.Trim(), txtCountry.Text.Trim())
    ViewState("Customers") = dt
    Me.BindGrid()
    txtName.Text = String.Empty
    txtCountry.Text = String.Empty
End Sub

Protected Sub ClearData(ByVal sender As Object, ByVal e As EventArgs)
    ViewState("Customers") = Nothing
    GridView1.DataSource = CType(ViewState("Customers"), DataTable)
    GridView1.DataBind()
End Sub

Screenshot