Shows how to bind a control's BackgroundProperty from a bool

BindingBackgroundColorFromBool.cs

using System;
using System.ComponentModel;
using Eto;
using Eto.Forms;
using Eto.Drawing;

namespace BindingBackgroundColorFromBool
{
	public class Model : INotifyPropertyChanged
	{
		string text;
		bool isValid = true;

		public string Text
		{
			get { return text; }
			set
			{
				text = value;
				OnPropertyChanged("Text");
				IsValid = text == null || text.Length <= 4;
			}
		}

		public bool IsValid
		{
			get { return isValid; }
			set
			{
				if (value != isValid)
				{
					isValid = value;
					OnPropertyChanged("IsValid");
				}
			}
		}

		public event PropertyChangedEventHandler PropertyChanged;

		protected virtual void OnPropertyChanged(string propertyName)
		{
			if (PropertyChanged != null)
				PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
		}
	}

	public class TestForm : Form
	{
		public TestForm()
		{
			Menu = new MenuBar();

			var textBox = new TextBox();
			var defaultColor = textBox.BackgroundColor;
			textBox.TextBinding.BindDataContext<Model>(r => r.Text);
			textBox.BindDataContext(c => c.BackgroundColor, Binding.Property((Model m) => m.IsValid).Convert(val => !val ? Colors.Red : defaultColor));

			Content = new TableLayout
			{
				Padding = new Padding(10),
				Spacing = new Size(5, 5),
				Rows =
				{
					new TableRow(new Label { Text = "Text:" }, textBox),
				}
			};

			// set the data context
			DataContext = new Model();
		}
	}
}