How to display data in gridview when click Search button in asp.net using vb.net

Here I have created sample that will help you out.

HTML

<div>
    Country:
    <asp:DropDownList ID="ddlCountry" runat="server">
        <asp:ListItem Text="All" Value="" />
        <asp:ListItem Text="Argentina" Value="Argentina" />
        <asp:ListItem Text="Belgium" Value="Belgium" />
        <asp:ListItem Text="Brazil" Value="Brazil" />
        <asp:ListItem Text="Canada" Value="Canada" />
    </asp:DropDownList>
    <asp:Button Text="Search" runat="server" OnClick="Search" />
    <br />
    <asp:GridView ID="gvCustomers" runat="server" AutoGenerateColumns="false">
        <Columns>
            <asp:BoundField HeaderStyle-Width="150px" DataField="ContactName" HeaderText="Contact Name"
                ItemStyle-CssClass="ContactName" />
            <asp:BoundField HeaderStyle-Width="150px" DataField="CustomerID" HeaderText="CustomerID" />
            <asp:BoundField HeaderStyle-Width="150px" DataField="City" HeaderText="City" />
            <asp:BoundField HeaderStyle-Width="150px" DataField="Country" HeaderText="Country" />
        </Columns>
    </asp:GridView>
</div>

C#

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

protected void Search(object sender, EventArgs e)
{
    BindGrid();
}

private void BindGrid()
{
    string conString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    string query = "SELECT top 10 * FROM Customers where Country=@Country or @Country=''";
    SqlCommand cmd = new SqlCommand(query);
    cmd.Parameters.AddWithValue("@Country", ddlCountry.SelectedItem.Value);
    using (SqlConnection con = new SqlConnection(conString))
    {
        using (SqlDataAdapter sda = new SqlDataAdapter())
        {
            cmd.Connection = con;
            sda.SelectCommand = cmd;
            using (DataSet ds = new DataSet())
            {
                sda.Fill(ds);
                gvCustomers.DataSource = ds;
                gvCustomers.DataBind();
            }
        }
    }
}

VB

Protected Sub Page_Load(sender As Object, e As EventArgs)
	If Not Me.IsPostBack Then
		BindGrid()
	End If
End Sub

Protected Sub Search(sender As Object, e As EventArgs)
	BindGrid()
End Sub

Private Sub BindGrid()
	Dim conString As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
	Dim query As String = "SELECT top 10 * FROM Customers where Country=@Country or @Country=''"
	Dim cmd As New SqlCommand(query)
	cmd.Parameters.AddWithValue("@Country", ddlCountry.SelectedItem.Value)
	Using con As New SqlConnection(conString)
		Using sda As New SqlDataAdapter()
			cmd.Connection = con
			sda.SelectCommand = cmd
			Using ds As New DataSet()
				sda.Fill(ds)
				gvCustomers.DataSource = ds
				gvCustomers.DataBind()
			End Using
		End Using
	End Using
End Sub

Screenshot