According to the documentation (http://msdn.microsoft.com/en-us/library/ms521832.aspx), System.Windows.Rect.ToString() returns a string in the form "X,Y,Width,Height". This seems like a perfect match for the Parse method (http://msdn.microsoft.com/en-us/library/system.windows.rect.parse.aspx) which accepts a string in that same format.
However, ToString() follows the .NET conventions of returning locale-sensitive results and requires that the ToString(IFormatProvider) overload be called to return culture-invariant output. In most European locales (e.g., "de" or "fr"), new Rect(1.5, 2, 3.5, 4).ToString() will return "1,5;2;3,5;4". If this string is passed to Parse, it will throw a FormatException.
Since it makes sense for Rect.ToString() to follow the overall .NET convention (of producing locale-sensitive output unless specifically overridden), the documentation should be corrected to make it clear that the returned string is locale-sensitive.