using System;

 using System.Collections.Generic;

 using System.Linq;

 using System.Text;

 using System.Threading.Tasks;

 using System.Windows.Forms.Design;

 using System.Drawing.Design;

 using System.Windows.Forms;

 namespace PropertyGridDemo

 {

     public class PropertyGridRichText:UITypeEditor

     {

         public override UITypeEditorEditStyle GetEditStyle(System.ComponentModel.ITypeDescriptorContext context)

         {

             return UITypeEditorEditStyle.DropDown;

         }

         public override object EditValue(System.ComponentModel.ITypeDescriptorContext context,System.IServiceProvider provider,object value)

         {

             try

             {

                 IWindowsFormsEditorService edSvc = (IWindowsFormsEditorService)provider.GetService(typeof(IWindowsFormsEditorService));

                 if (edSvc != null)

                 {

                     if (value is string)

                     {

                         RichTextBox box = new RichTextBox();

                         box.Text = value as string;

                         edSvc.DropDownControl(box);

                         return box.Text;

                     }

                 }

             }

             catch (Exception ex)

             {

                 System.Console.WriteLine("PropertyGridRichText Error : " + ex.Message);

                 return value;

             }

             return value;

         }

     }

 }

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

namespace PropertyGridDemo

{

    public class Person

    {

        public int Id { get; set; }

        public string Name { get; set; }

        public string Sex { get; set; }

        [EditorAttribute(typeof(PropertyGridRichText), typeof(System.Drawing.Design.UITypeEditor)),Description("The person content!")]

        public string Content { get; set; }

    }

}

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

using System.Windows.Forms;

namespace PropertyGridDemo

{

    public partial class Form1 : Form

    {

        public Form1()

        {

            InitializeComponent();

            propertyGrid1.SelectedObject = new Person();

        }

    }

}