Catel - Part 4 of n: Unit testing with Catel

// --------------------------------------------------------------------------------------------------------------------
// <copyright file="GetFirstValidationErrorConverter.cs" company="Catel development team">
//   Copyright (c) 2008 - 2011 Catel development team. All rights reserved.
// </copyright>
// <summary>
//   Converts a collection containing <see cref="ValidationError" /> objects to return the first error
//   or an empty string in case there are no errors.
// </summary>
// --------------------------------------------------------------------------------------------------------------------

using System;
using System.Collections.Generic;
using System.Linq;
using System.Windows.Controls;
using System.Windows.Data;

namespace Catel.Windows.Data.Converters
{
	/// <summary>
	/// Converts a collection containing <see cref="ValidationError"/> objects to return the first error
	/// or an empty string in case there are no errors.
	/// </summary>
#if !SILVERLIGHT
	[ValueConversion(typeof(ICollection<ValidationError>), typeof(string))]
#endif
	public class GetFirstValidationErrorConverter : IValueConverter
	{
		/// <summary>
		/// Converts a collection containing <see cref="ValidationError"/> objects to return the first error
		/// or an empty string in case there are no errors.
		/// </summary>
		/// <param name="value">Values to check for. Both collections and arrays are supported.</param>
		/// <param name="targetType">Not used.</param>
		/// <param name="parameter">Not used.</param>
		/// <param name="culture">Not used.</param>
		/// <returns>String containing the first error or <see cref="string.Empty"/> in case there are not errors.</returns>
		public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
		{
			// Check if we have a valid value
			if (value == null)
			{
			    return string.Empty;
			}

			// Convert value
			ICollection<ValidationError> errorCollection = value as ICollection<ValidationError>;
			if (errorCollection == null)
			{
			    return string.Empty;
			}

			// Return error
			return (errorCollection.Count > 0) ? errorCollection.First().ErrorContent.ToString() : string.Empty;
		}

		/// <summary>
		/// This method isn't supported.
		/// </summary>
		/// <param name="value">Not used.</param>
		/// <param name="targetType">Not used.</param>
		/// <param name="parameter">Not used.</param>
		/// <param name="culture">Not used.</param>
		/// <returns>Always null.</returns>
		public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
		{
            return ConverterHelper.DoNothingBindingValue;
		}
	}
}