How to center align arguments in a format string

Question

I have a string (like below), and I want each of the argument to be center aligned.

There is an option for left align that is ("+") and right align that is ("-") but I want to center align.

basketItemPrice = string.Format("\n\n{0, -5}{1, -14:0.00}{2, -18:0.00}{3,-14:0.00}{4,6}{5,-12:0.00}", item.Quantity, item.OrderItemPrice, item.MiscellaniousCharges, item.DiscountAmountTotal, "=", item.UpdateItemAmount(Program.currOrder.OrderType));

Answer 1

Unfortunately, this is not supported natively by String.Format. You will have to pad your string yourself:

static string centeredString(string s, int width)
{
    if (s.Length >= width)
    {
        return s;
    }

    int leftPadding = (width - s.Length) / 2;
    int rightPadding = width - s.Length - leftPadding;

    return new string(' ', leftPadding) + s + new string(' ', rightPadding);
}

Usage example:

Console.WriteLine(string.Format("|{0}|", centeredString("Hello", 10)));
Console.WriteLine(string.Format("|{0}|", centeredString("World!", 10)));

Answer 2

I tried to make an extension method which still preserves the IFormattable support. It uses a nested class which remembers the raw value and the desired width. Then when format string is provided, it is used, if possible.

It looks like this:

public static class MyExtensions
{
    public static IFormattable Center<T>(this T self, int width)
    {
        return new CenterHelper<T>(self, width);
    }

    class CenterHelper<T> : IFormattable
    {
        readonly T value;
        readonly int width;

        internal CenterHelper(T value, int width)
        {
            this.value = value;
            this.width = width;
        }

        public string ToString(string format, IFormatProvider formatProvider)
        {
            string basicString;
            var formattable = value as IFormattable;
            if (formattable != null)
                basicString = formattable.ToString(format, formatProvider) ?? "";
            else if (value != null)
                basicString = value.ToString() ?? "";
            else
                basicString = "";

            int numberOfMissingSpaces = width - basicString.Length;
            if (numberOfMissingSpaces <= 0)
                return basicString;

            return basicString.PadLeft(width - numberOfMissingSpaces / 2).PadRight(width);
        }
        public override string ToString()
        {
            return ToString(null, null);
        }
    }
}

Note: You have not indicated if you want the one "extra" space character put to the left or to the right in cases where an odd number of space characters needs to be appended.

This test seems to indicate it works:

double theObject = Math.PI;
string test = string.Format("Now '{0:F4}' is used.", theObject.Center(10));

Of course, format string F4 with a double means "round to 4 decimal places after the decimal point".