How to check whether HTML CheckBox is Checked or Unchecked in ASP.Net

You need to add runat = server and use onserverclick for Input type Button

HTML

<div>
    <input id="checkbox1" runat="server" type="checkbox" name="name" value="" />
    <input type="button" runat="server" onserverclick="ValidateCheckBox" name="name"  value="Validate" />
</div>

 C#

protected void ValidateCheckBox(object sender, EventArgs e)
{
    if (this.checkbox1.Checked != true)
    {
        ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('Please Check the CheckBox')", true);
    }
}

Without runat=server you can get the values by name

string s  = Request.Form["<name>"];