Hi mahesh,

Please refer below sample.

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    string number = "1234";
    string[] strArr = number.ToString().Split('.').ToArray();
    string finalString = string.Empty;
    if (strArr.Length > 1)
    {
        finalString = string.Format("{0:#,0.00}", Convert.ToDouble(number));
    }
    else
    {
        finalString = string.Format("{0:#,0}", Convert.ToDouble(number));
    }
    ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('" + finalString + "');", true);
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    Dim number As String = "1234"
    Dim strArr As String() = number.ToString().Split("."c).ToArray()
    Dim finalString As String = String.Empty

    If strArr.Length > 1 Then
        finalString = String.Format("{0:#,0.00}", Convert.ToDouble(number))
    Else
        finalString = String.Format("{0:#,0}", Convert.ToDouble(number))
    End If

    ClientScript.RegisterStartupScript(Me.GetType(), "alert", "alert('" & finalString & "');", True)
End Sub

Screenshot