检查值是否已存在

Dictionary<string, book> books

class book
{
    string author { get; set; }

    string title { get; set; }
} 

foreach(KeyValuePair<string, book> b in books) // or foreach(book b in books.Values)
{
    if(b.Value.title.Equals("some title", StringComparison.CurrentCultureIgnoreCase))
        return true
}

books.Any(tr => tr.Value.title.Equals("some title", StringComparison.CurrentCultureIgnoreCase))

books.ContainsKey("some title");