Follow up to gbogumil s answer, in the first case you get:

"Input string was not in a correct format."

and in the second, you get:

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

If you need to sense which (for user messaging or logging), then you could use a try catch like qor72 suggested, and check for what the error message starts with. In addition, if you need to capture what the format string was, and what the args were, you will need to do something like this:

        string myStr = "{0}{1}{2}";
        string[] strArgs = new string[]{"this", "that"};
        string result = null;

        try { result = string.Format(myStr, strArgs); }

        catch (FormatException fex)
        {
            if (fex.Message.StartsWith("Input"))
                Console.WriteLine
                  ("Trouble with format string: "" + myStr + """);
            else
                Console.WriteLine
                  ("Trouble with format args: " + string.Join(";", strArgs));
            string regex = @"{d+}";
            Regex reg = new Regex(regex, RegexOptions.Multiline);
            MatchCollection matches = reg.Matches(myStr);
            Console.WriteLine
                ("Your format has {0} tokens and {1} arguments", 
                 matches.Count, strArgs.Length );

        }

EDIT: Added the simple regex to count format tokens. Might help...

Hope this helps. Good luck!

The FormatException message property is set to a distinct message in each of those cases.

And you don t want to do...?

works = true;
try {
  String.Parse(Format, ObjectArray);
} catch FormatException {
works = false; }

I recently used the following regular expression below to validate the composite format strings in all our resources files

    /// <summary>
    /// The regular expression to get argument indexes from a composed format string
    /// </summary>
    /// <remarks> 
    /// example         index   alignment   formatString
    /// {0}             0       
    /// {1:d}           1                   d
    /// {2,12}          2       12
    /// {3,12:#}        3       12          #
    /// {{5}}           
    /// {{{6}}}         6
    /// </remarks>
    private static readonly Regex ComposedFormatArgsRegex =
        new Regex(@"(?<!(?<!{){){(?<index>d+)(,(?<alignment>d+))?(:(?<formatString>[^}]+))?}(?!}(?!}))",
            RegexOptions.Compiled | RegexOptions.ExplicitCapture);

For more information about composite formatted strings, see http://msdn.microsoft.com/en-us/library/txafckwd(v=vs.110).aspx