Something like this?
var dict = text.Split(new[] {';'}, StringSplitOptions.RemoveEmptyEntries)
.Select(part => part.Split('='))
.ToDictionary(split => split[0], split => split[1]);
Of course, this will fail if the assumptions aren’t met. For example, an IndexOutOfRangeException could be thrown if the text isn’t in the right format and an ArgumentException will be thrown if there are duplicate keys. Each of these scenarios will require different modifications. If redundant white-space could be present, you may need some string.Trim calls as necessary.