.Net under the covers

An interesting little oddity I stumbled across the other day, involving StringBuilder.AppendFormat, and a string which it turned out had had a GUID already expanded into it, and an unexpected exception result

	open System
	open System.Text
	let sb = new StringBuilder()
	// This is the case that surprised me
	sb.AppendFormat("{4f2616db-6e2a-44ca-b2d8-f71ddf94d9d6}");;
	System.FormatException: Index (zero based) must be greater than or equal to zero and less than the size of the argument
	list.
	at System.Text.StringBuilder.AppendFormat(IFormatProvider provider, String format, Object[] args)
	at .$FSI_0009.main@()
	Stopped due to error
	// This is the result that I had expected
	sb.AppendFormat("{4f2616db-6e2a-44ca-b2d8-f71ddf94d9d6}", 0, 1, 2, 3, 4);;
	System.FormatException: Input string was not in a correct format.
	at System.Text.StringBuilder.AppendFormat(IFormatProvider provider, String format, Object[] args)
	at .$FSI_0012.main@()
	Stopped due to error

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so it looks like the parse for a number is run before checking that the format is even sane; not what one would have expected.