This Way:
<form id="form1" runat="server">
<div>
<asp:TextBox ID="txtNumber" runat="server" />
<asp:Button ID="btnSubmitNumber" runat="server" OnClick="SubmitNumber" Text="Submit Number" />
</div>
</form>
C#:
protected void SubmitNumber(object sender, EventArgs e)
{
if (string.IsNullOrEmpty(this.txtNumber.Text.Trim()) || Convert.ToInt32(this.txtNumber.Text.Trim()) < 0)
{
ClientScript.RegisterClientScriptBlock(this.GetType(), "alert", "alert('Number is not correct')", true);
}
}
Thank You.