Question

Before using String.Format to format a string in C#, I would like to know how many parameters does that string accept?

For eg. if the string was “{0} is not the same as {1}”, I would like to know that this string accepts two parameters
For eg. if the string was “{0} is not the same as {1} and {2}”, the string accepts 3 parameters

How can I find this efficiently?

Answer 1

String.Format receives a string argument with format value, and an params object[] array, which can deal with an arbitrary large value items.

For every object value, it’s .ToString() method will be called to resolve that format pattern

EDIT: Seems I misread your question. If you want to know how many arguments are required to your format, you can discover that by using a regular expression:

string pattern = "{0} {1:00} {{2}}, Failure: {0}{{{1}}}, Failure: {0} ({0})";
int count = Regex.Matches(Regex.Replace(pattern, 
    @"(\{{2}|\}{2})", ""), // removes escaped curly brackets
    @"\{\d+(?:\:?[^}]*)\}").Count; // returns 6

As Benjamin noted in comments, maybe you do need to know number of different references. If you don’t using Linq, here you go:

int count = Regex.Matches(Regex.Replace(pattern, 
    @"(\{{2}|\}{2})", ""), // removes escaped curly brackets
    @"\{(\d+)(?:\:?[^}]*)\}").OfType<Match>()
    .SelectMany(match => match.Groups.OfType<Group>().Skip(1))
    .Select(index => Int32.Parse(index.Value))
    .Max() + 1; // returns 2

This also address @280Z28 last problem spotted.

Edit by 280Z28: This will not validate the input, but for any valid input will give the correct answer:

int count2 =
    Regex.Matches(
        pattern.Replace("{{", string.Empty),
        @"\{(\d+)")
    .OfType<Match>()
    .Select(match => int.Parse(match.Groups[1].Value))
    .Union(Enumerable.Repeat(-1, 1))
    .Max() + 1;