Example of how to validate the input of controls on an Eto.Forms dialog.

MyValidationDialog.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Eto.Forms;
using Eto.Drawing;

namespace MyValidationApp
{
	// base validator that can be extended to do custom validation
	public class ControlValidator
	{
		public Control Target { get; private set; }

		public string Message { get; set; }

		readonly Func<bool> validate;

		public ControlValidator (Control target, Func<bool> validate, string message = null)
		{
			Target = target;
			this.validate = validate;
			Message = message;
		}

		protected virtual void SetValid()
		{
			Target.BackgroundColor = SystemColors.ControlBackground;
		}

		protected virtual void SetInvalid()
		{
			Target.BackgroundColor = Colors.Red;
		}

		public bool Validate()
		{
			if (validate())
			{
				SetValid ();
				return true;
			}
			SetInvalid ();
			return false;
		}
	}

	// collection of validations
	class ValidationHandler : List<ControlValidator>
	{
		public Action<IEnumerable<ControlValidator>> IsInvalid { get; set; }

		public IEnumerable<ControlValidator> Validate()
		{
			var invalidItems = new List<ControlValidator>();
			foreach (var item in this) {
				if (!item.Validate ())
					invalidItems.Add (item);
			}
			if (invalidItems.Count > 0 && IsInvalid != null)
				IsInvalid (invalidItems);

			return invalidItems;
		}
	}

	// dialog that uses control validation
	public class MyValidationDialog : Dialog<bool>
	{
		public MyValidationDialog ()
		{

			var validations = new ValidationHandler();
			validations.IsInvalid = items => {
				// focus the first item with an error
				items.First().Target.Focus();

				// show error
				var message = string.Join("\n", items.Where(r => r.Message != null).Select(r => " > " + r.Message));
				if (!string.IsNullOrEmpty(message))
					MessageBox.Show("Errors in the form:\n" + message, null, MessageBoxButtons.OK, MessageBoxType.Error);
			};

			// input controls
			var myTextBox = new TextBox ();
			var myDropDown = new DropDown  { Items = { "Male", "Female" } };


			// validations
			validations.Add (new ControlValidator (myTextBox, () => !string.IsNullOrEmpty(myTextBox.Text), "Name is required"));
			validations.Add (new ControlValidator (myDropDown, () => myDropDown.SelectedValue != null, "Gender is required"));

			// buttons
			var btnOk = new Button { Text = "Ok" };
			btnOk.Click += (sender, e) => {
				if (!validations.Validate().Any())
					Close (true);
			};

			var btnCancel = new Button { Text = "Cancel" };
			btnCancel.Click += (sender, e) => Close(false);

			// content
			Content = new StackLayout {
				HorizontalContentAlignment = HorizontalAlignment.Stretch,
				Spacing = 10,
				Items = {
					new StackLayoutItem(new TableLayout {
						Padding = 10,
						Spacing = new Size(5, 5),
						Rows = {
							new TableRow("Name", myTextBox),
							new TableRow("Gender", myDropDown)
						}
					}, expand: true),
					new StackLayout {
						Orientation = Orientation.Horizontal,
						Spacing = 5,
						Items = { null, btnOk, btnCancel }
					}
				}
			};

			Title = "My Dialog with Validation";
			DefaultButton = btnOk;
			AbortButton = btnCancel;
		}
	}
}