HeaderCell.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using Xamarin.Forms;

namespace ListViewDemoVS
{
    // Large black text on white background
    class HeaderCell : ViewCell
    {
        public HeaderCell()
        {
            this.Height = 40;

            var title = new Label
            {
                FontSize = 16,
                TextColor = Color.Black,
                VerticalOptions = LayoutOptions.Center
            };
            title.SetBinding(Label.TextProperty, "Key");

            View = new StackLayout
            {
                HorizontalOptions = LayoutOptions.FillAndExpand,
                HeightRequest = 40,
                BackgroundColor = Color.White,
                Padding = 5,
                Orientation = StackOrientation.Horizontal,
                Children = { title }
            };
        }
    }
}