等效的string.Format，不会使用额外的标记引发异常

Question

I want end user of my application to edit format string that is used in string.Format . I want to do that to give user ability to edit templates. Problem occurs when user enters too much markers.

This works fine:

var result = string.Format("test {0}, {1}, {2}", 1, 2, 3);

this works too:

var result = string.Format("test {0}, {1}", 1, 2, 3);

but this throws exception:

var result = string.Format("test {0}, {1}, {2}", 1, 2);

System.FormatException: 'Index (zero based) must be greater than or equal to zero and less than the size of the argument list.'

I'm aware that I must add as much parameters as there are markers, but end user don't know that, or he can make mistakes - add extra marker.

My question is:
Can I use similar approach to format string in a way string.Format works but without exception.

I know I could use try\\catch block and return format string, but I don't want to do that.

I've build very simple method that replace markers in string:

public static string SafeFormat(string format, params object[] args)
{
    var result = format;
    for (int i = 0; i < args.Length; i++)
    {
        result = result.Replace($"{{{i}}}", args[i].ToString());
    }
    return result;
}

but this isn't supporting formatting of data that is used in place of markers.

Internally string.Format is using AppendFormatHelper ( http://referencesource.microsoft.com/#mscorlib/system/text/stringbuilder.cs,2c3b4c2e7c43f5a4 ) that is more complex than my simple replace.

Ideally string.Format("test {0}, {1}, {2}", 1, 2) should return test 1, 2, {2}

Answer 1

One solution is to check how many values are expected, and just append others to argument list (null or placeholders). Example string extension:

public static class StringExtensions
{
    public static string SafeFormat(this string value, params object[] args)
    {
        var pattern = @"{(.*?)}";
        var matches = Regex.Matches(value, pattern);
        var matchCount = matches.Count;
        if (matchCount > args.Length)
        {
            var argsExtended = args.ToList();
            for (int i = args.Length; i < matchCount; i++)
            {
                argsExtended.Add($"{{{i}}}"); //Can add null value to erase them
            }
            return string.Format(value, argsExtended.ToArray());
        }
        return string.Format(value, args);
    }
}

You can easily change this to method if you want