Converting to String with StringFormat

Question

I would like to write a ToString() method for a generic class where I can specify a StringFormat.

So far, I have the following code:

public class Foo<T> : IFoo
{
    public T Value { get; set; }
    string IFoo.Value { get { ValueToString(Value); } }

    public string StringFormat { get; set }

    public Foo(T value, string stringFormat)
    {
        Value = value;
        StringFormat = stringFormat;
    }

    private string ValueToString(T value)
    {
        if (!string.IsNullOrEmpty(StringFormat))
        {
            var formattable = value as IFormattable;
            if (formattable != null)
            {
                var format = CultureInfo.CurrentCulture.GetFormat(typeof(T)) as IFormatProvider;
                if (format != null)
                    return formattable.ToString(StringFormat, format);
            }
        }

        return Convert.ToString(value);
    }
}

public interface IFoo
{
    string Value { get; }
}

I am calling the code like this:

public class Bar
{
    public Bar()
    {
        var dateTimeFoo = new Foo<DateTime>(DateTime.Now, "dd.MM.yyyy");
        var iFoo = dateTimeFoo as IFoo;
        if (Convert.ToString(dateTimeFoo.Value) != iFoo.Value)
        {
            //stringformat worked...
        }
    }
}

The problem with my current implementation is this line of code:

var format = CultureInfo.CurrentCulture.GetFormat(typeof(T)) as IFormatProvider;

It returns null and it should return null for objects that don't have a FormatProvider but DateTime certainly has the ability, how can I get the correct FormatProvider of a generic T?

Answer 1

CultureInfo already implements IFormatProvider, all you want to do is

return formattable.ToString(StringFormat, CulturuInfo.CurrentCulture);

If you look at the implementation of GetFormat method

public virtual Object GetFormat(Type formatType)
{
    if (formatType == typeof (NumberFormatInfo))
    {
        return (NumberFormat);
    }
    if (formatType == typeof (DateTimeFormatInfo))
    {
        return (DateTimeFormat);
    }
    return (null);
}

You can see it only supports two types and returns null otherwise as documented here