Why don't you define an abstract class with the following two methods:

• One which can convert an object to string
• Another which can convert an object to double

public abstract class FromObjectConverter
{
    protected double ToDouble(object value)
        => double.TryParse(value as string, out var x) ? x : double.NaN;

    protected string ToString(object value)
        => value is double x ? x.ToString("f3") : string.Empty;
}

With this class in our hand the two concrete converter classes are just simple wrappers

public sealed class StringToDoubleConverter : FromObjectConverter, IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        => ToDouble(value);

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        => ToString(value);
}

public sealed class DoubleToStringConverter : FromObjectConverter, IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        => ToString(value);

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        => ToDouble(value);
}

Why don't you define an abstract class with the following two methods:

• One which can convert an object to string
• Another which can convert an object to double

public abstract class FromObjectConverter
{
    protected double ToDouble(object value)
        => double.TryParse(value as string, out var x) ? x : double.NaN;

    protected string ToString(object value)
        => value is double x ? x.ToString("f3") : string.Empty;
}

With this class in our hand the two concrete converter classes are just simple wrappers

public sealed class StringToDoubleConverter : FromObjectConverter, IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        => ToDouble(value);

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        => ToString(value);
}

public sealed class DoubleToStringConverter : FromObjectConverter, IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        => ToString(value);

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        => ToDouble(value);
}

Or if you want to avoid inheritance then define the FromObjectConverter class and its methods as static.

Why don't you define an abstract class which defines two methods

• One which can convert an object to string
• Another which can convert an object to double

public abstract class FromObjectConverter
{
    protected double ToDouble(object value)
        => double.TryParse(value as string, out var x) ? x : double.NaN;

    protected string ToString(object value)
        => value is double x ? x.ToString("f3") : string.Empty;
}

With this class in our hand the two concrete converter classes are just simple wrappers

public sealed class StringToDoubleConverter : FromObjectConverter, IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        => ToDouble(value);

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        => ToString(value);
}

public sealed class DoubleToStringConverter : FromObjectConverter, IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        => ToString(value);

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        => ToDouble(value);
}

Why don't you define an abstract class with the following two methods:

• One which can convert an object to string
• Another which can convert an object to double

public abstract class FromObjectConverter
{
    protected double ToDouble(object value)
        => double.TryParse(value as string, out var x) ? x : double.NaN;

    protected string ToString(object value)
        => value is double x ? x.ToString("f3") : string.Empty;
}

With this class in our hand the two concrete converter classes are just simple wrappers

public sealed class StringToDoubleConverter : FromObjectConverter, IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        => ToDouble(value);

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        => ToString(value);
}

public sealed class DoubleToStringConverter : FromObjectConverter, IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        => ToString(value);

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        => ToDouble(value);
}