SharePoint Field Control

BaseFieldControl.cs

namespace MyProject.Web.UI.FieldControls
{
    using System;
    using System.Web.UI;
    using Microsoft.SharePoint.WebControls;

    public abstract class UserControlFieldBase : BaseFieldControl
    {
        private SPFieldUserControlBase _usercontrol;

        public virtual string UserControlPath { get; set; }

        public override object Value
        {
            get
            {
                return _usercontrol == null ? base.Value : _usercontrol.Value;
            }
            set
            {
                base.ItemFieldValue = value;
            }
        }

        public UserControlFieldBase()
            : base()
        {
        }

        protected override void OnInit(EventArgs e)
        {
            base.OnInit(e);
            UserControlPath = null;
        }

        protected override void CreateChildControls()
        {
            Controls.Clear();
            base.CreateChildControls();

            if (!string.IsNullOrEmpty(UserControlPath))
            {
                _usercontrol = (SPFieldUserControlBase)Page.LoadControl(UserControlPath);
                _usercontrol.FieldControl = this;

                Controls.Add(_usercontrol);
            }
        }
    }
}

SPFieldUserControlBase.cs

namespace MyProject.Web.UI.FieldControls
{
    using System.Web.UI;

    public abstract class SPFieldUserControlBase : UserControl
    {
        protected SPFieldUserControlBase();

        public BaseFieldControl FieldControl { get; set; }
        public abstract object Value { get; }
    }
}

UserControlFieldBase.cs

namespace MyProject.Web.UI.FieldControls
{
    using System.Web.UI;
    
    public class MyFieldContol : UserControlFieldBase
    {
        public override string UserControlPath
        {
            get
            {
                return "~/_controltemplates/MyFieldControl.ascx";
            }
        }
    }
}