Auto generate invoice number with Prefix using C# and VB.Net in ASP.Net

Refer

HTML

<asp:Label ID="lblID" runat="server" />
<asp:Button ID="btnGenerate" Text="Generate ID" runat="server" OnClick="GenerateID" />

C#

protected void GenerateID(object sender, EventArgs e)
{
    string numbers = "1234567890";

    string characters = numbers;
    int length = 10;
    string id = string.Empty;
    for (int i = 0; i < length; i++)
    {
        string character = string.Empty;
        do
        {
            int index = new Random().Next(0, characters.Length);
            character = characters.ToCharArray()[index].ToString();
        } while (id.IndexOf(character) != -1);
        id += character;
    }
    lblID.Text = "FF" + id;
}

VB.Net

Protected Sub GenerateID(sender As Object, e As EventArgs)
    Dim numbers As String = "1234567890"

    Dim characters As String = numbers
    Dim length As Integer = 10
    Dim id As String = String.Empty
    For i As Integer = 0 To length - 1
        Dim character As String = String.Empty
        Do
            Dim index As Integer = New Random().Next(0, characters.Length)
            character = characters.ToCharArray()(index).ToString()
        Loop While id.IndexOf(character) <> -1
        id += character
    Next
    lblID.Text = "FF" + id
End Sub