Change value of HTML5 data attribute from server side (code behind) in ASP.Net

You need to do this way

HTML

<div id="div1" runat="server">
</div>

 C#

protected void Page_Load(object sender, EventArgs e)
{
    //Set it from Database, Here i have set the property with hard coded value.
    this.PID = 1;
    div1.Attributes["data-href"] = string.Format("http://www.aspforums.net/mypage?PID={0}", this.PID);
}

Screenshot