Упрощаем конвертеры для WPF

public abstract class ConverterBase<T> : MarkupExtension, IValueConverter where T: class, new()
{
	private static T instance;
	
	static ConverterBase()
	{
		ConverterBase<T>.instance = Activator.CreateInstance<T>();
	}
	
	protected ConverterBase()
	{
	}
	
	public abstract object Convert(object value, Type targetType, object parameter, CultureInfo culture);
	public virtual object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
	{
		throw new NotImplementedException();
	}
	
	public override object ProvideValue(IServiceProvider serviceProvider)
	{
		if (ConverterBase<T>.instance == null)
		{
			ConverterBase<T>.instance = Activator.CreateInstance<T>();
		}
		return ConverterBase<T>.instance;
	}
	
	public static T Instance
	{
		get
		{
			return ConverterBase<T>.instance;
		}
	}
}

internal class DelayToTimeSpanConverter : ConverterBase<DelayToTimeSpanConverter>
{
	public override object Convert(
		object value, Type targetType, object parameter, CultureInfo culture)
	{
		if (ReferenceEquals(value, null))
		{
			return DependencyProperty.UnsetValue;
		}

		return TimeSpan.FromMilliseconds(System.Convert.ToDouble(value));
	}
}

<TextBlock Text="{Binding Path=Span, Converter={conv:DelayToTimeSpanConverter}, StringFormat=hh\\:mm\\:ss}" />

namespace Converters
{
    public abstract class BaseConverter : IValueConverter
    {
       
        protected abstract object Convert(object value);
        protected virtual object ConvertBack(object value)
        {
            return value;
        }

        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return Convert(value);
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return ConvertBack(value);
        }
    }

    public abstract class BaseConverter<T> : BaseConverter
    {
        protected abstract object Convert(T value);

        protected override object Convert(object value)
        {
            return Convert(value is T ? (T)value : default(T));
        }
    }

    public abstract class BaseConverter<T1, T2> : BaseConverter
    {
        protected abstract T2 Convert(T1 value);

        protected override object Convert(object value)
        {
            return Convert(value is T1 ? (T1)value : default(T1));
        }
    }
}

namespace Converters
{
    public delegate object ConvertHandler(object value, Type targetType, object parameter, CultureInfo culture);

    public class ValueConverter : IValueConverter
    {
        public ConvertHandler ConvertHandler { get; set; }
        public ConvertHandler ConvertBackHandler { get; set; }

        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return ConvertHandler?.Invoke(value, targetType, parameter, culture) ?? value;
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return ConvertBackHandler?.Invoke(value, targetType, parameter, culture) ?? value;
        }
    }

    public class ConvertersFactory
    {
        public static string NullableValue { get; set; }

        public static ConvertHandler IntToString => (value, type, parameter, culture) => value.ToString();
        public static ConvertHandler NullableIntToString => (value, type, parameter, culture) => value is int ? value.ToString() : NullableValue;
        
    }
}

<UserControl x:Class="ConverterControl"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:local="clr-namespace:Converters">

    <UserControl.Resources>
       <local:ValueConverter x:Key="IntToStringConverter"
                              ConvertHandler="{x:Static local:ConvertersFactory.NullableIntToString}" />
    </UserControl.Resources>

    <Grid>
        <TextBlock Text="{Binding Converter={StaticResource IntToStringConverter}}"/>
    </Grid>
</UserControl>