ASP.Net Button control that renders with Button tag instead of input

ModernButton.cs

using System;
using System.ComponentModel;
using System.Web;
using System.Web.UI;
using System.Linq;

namespace Public.Html.Controls
{
    public enum ButtonType
    {
        Submit,
        Reset,
        Button
    } ;

    [ToolboxData("<{0}:Button runat=server></{0}:Button>")]
    [ParseChildren(false)]
    [PersistChildren(true)]
    public class Button : System.Web.UI.WebControls.WebControl, IPostBackDataHandler
    {
        //Fields
        private static readonly object EventClick = new object();
        private static readonly object EventCommand = new object();
        private static readonly object EventValueChanged = new object();

        //Events        
        public event EventHandler Click
        {
            add { Events.AddHandler(EventClick, value); }
            remove { Events.RemoveHandler(EventClick, value); }
        }

        public event EventHandler ValueChanged
        {
            add { Events.AddHandler(EventValueChanged, value); }
            remove { Events.RemoveHandler(EventValueChanged, value); }
        }

        public event System.Web.UI.WebControls.CommandEventHandler Command
        {
            add { Events.AddHandler(EventCommand, value); }
            remove { Events.RemoveHandler(EventCommand, value); }
        }

        public Button()
            : base(HtmlTextWriterTag.Button)
        {
        }

        #region Properties

        [Category("Behavior")]
        [DefaultValue("")]
        public string CommandArgument
        {
            get
            {
                return ViewState["CommandArgument"] as string ?? string.Empty;
            }
            set
            {
                ViewState["CommandArgument"] = value;
            }
        }

        [Category("Behavior")]
        [DefaultValue("")]
        public string CommandName
        {
            get
            {
                return ViewState["CommandName"] as string ?? string.Empty;
            }
            set
            {
                ViewState["CommandName"] = value;
            }
        }

        [Category("Behavior")]
        [DefaultValue("")]
        public virtual string PostBackUrl
        {
            get
            {
                return ViewState["PostBackUrl"] as string ?? string.Empty;
            }
            set
            {
                ViewState["PostBackUrl"] = value;
            }
        }

        [Category("Validation")]
        [DefaultValue(true)]
        public virtual bool CausesValidation
        {
            get
            {
                var causesValidation = ViewState["CausesValidation"];
                return (causesValidation == null) ? true : (bool)causesValidation;
            }
            set
            {
                ViewState["CausesValidation"] = value;
            }
        }

        [Category("Validation")]
        [DefaultValue("")]
        public virtual string ValidationGroup
        {
            get
            {
                return ViewState["ValidationGroup"] as string ?? string.Empty;
            }
            set
            {
                ViewState["ValidationGroup"] = value;
            }
        }

        [Category("Behavior")]
        [DefaultValue("")]
        public string Value
        {
            get
            {
                return ViewState["Value"] as string ?? string.Empty;
            }
            set { ViewState["Value"] = value; }
        }

        [Category("Behavior")]
        [DefaultValue(ButtonType.Button)]
        [Description("Defines the type of the button.")]
        public ButtonType ButtonType
        {
            get
            {
                var obj = ViewState["ButtonType"];
                return (obj == null) ? ButtonType.Button : (ButtonType) obj;
            }
            set { ViewState["ButtonType"] = value; }            
        }

        #endregion

        #region Methods

        protected override void AddAttributesToRender(HtmlTextWriter writer)
        {
            base.AddAttributesToRender(writer);

            if(!string.IsNullOrEmpty(Value))
                writer.AddAttribute(HtmlTextWriterAttribute.Value, Value);

            writer.AddAttribute(HtmlTextWriterAttribute.Type, ButtonType.ToString().ToLower());

            var postBackOptions = GetPostBackOptions();

            if(postBackOptions == null)
                throw new Exception("postBackOptions must not be null");

            if (postBackOptions.TargetControl == this)
            {
                writer.AddAttribute(HtmlTextWriterAttribute.Name, ClientID);
            }

            if (Page != null)
            {
                Page.ClientScript.RegisterForEventValidation(postBackOptions);
            }
        }

        protected virtual PostBackOptions GetPostBackOptions()
        {
            var options = new PostBackOptions(this, string.Empty) { ClientSubmit = false };

            if (Page != null)
            {
                if (CausesValidation && (Page.GetValidators(ValidationGroup).Count > 0))
                {
                    options.PerformValidation = true;
                    options.ValidationGroup = ValidationGroup;
                }

                if (!String.IsNullOrEmpty(PostBackUrl))
                {
                    options.ActionUrl = HttpUtility.UrlPathEncode(ResolveClientUrl(PostBackUrl));
                }
            }

            return options;
        }

        protected void OnClick()
        {
            var handler = (EventHandler)Events[EventClick];
            if (handler != null) handler(this, new EventArgs());
        }

        protected void OnValueChanged()
        {
            var handler = (EventHandler)Events[EventValueChanged];
            if (handler != null) handler(this, new EventArgs());
        }

        protected void OnCommand(string commandName, string commandArgument)
        {
            var handler = (System.Web.UI.WebControls.CommandEventHandler)Events[EventCommand];
            var args = new System.Web.UI.WebControls.CommandEventArgs(commandName, commandArgument);

            if (handler != null) handler(this, args);

            RaiseBubbleEvent(this, args);
        }

        protected override void OnPreRender(EventArgs e)
        {
            base.OnPreRender(e);

            if (Page != null)
                Page.RegisterRequiresPostBack(this);
        }

        protected override void LoadControlState(object savedState)
        {
            var pair = savedState as Pair;
            var str = savedState as string;

            if(pair != null)
            {
                base.LoadControlState(pair.First);
                Value = (string) pair.Second;
            }

            if(str != null)
            {
                Value = str;
            }
        }

        protected override object SaveControlState()
        {
            var baseState = base.SaveControlState();

            if (baseState == null)
                return string.IsNullOrEmpty(Value) ? null : Value;
            
            return new Pair(baseState, Value);
        }

        #endregion

        #region IPostBackDataHandler

        public bool LoadPostData(string postDataKey, System.Collections.Specialized.NameValueCollection postCollection)
        {
            if (string.IsNullOrEmpty(postDataKey))
                throw new ArgumentOutOfRangeException("postDataKey");

            if (postCollection == null)
                throw new ArgumentNullException("postCollection");

            if (postCollection.AllKeys.Contains(ClientID))
            {
                var existingValue = Value;
                Value = postCollection[ClientID];

                OnClick();

                if(!string.IsNullOrEmpty(CommandName))
                    OnCommand(CommandName, CommandArgument);

                return (existingValue.Equals(Value, StringComparison.Ordinal));
            }

            return false;
        }

        public void RaisePostDataChangedEvent()
        {
            OnValueChanged();
        }

        #endregion
    }
}

How do I use this?

I included this class in my project, but I can't work out how to reference a Public.Html.Controls.Button rather than a System.Web.UI.WebControls.Button inside my .ascx file - asp:button always seems to create a System.Web.UI.WebControls.Button.

Server Error in '/' Application.

Page.IsValid cannot be called before validation has taken place. It should be queried in the event handler for a control that has CausesValidation=True and initiated the postback, or after a call to Page.Validate.