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// Dictonaryのままでソート
Dictionary<DateTime, string> keyValuePairs = datetimeFileDictionary.OrderBy( x => x.Key )
.ToDictionary( pair => pair.Key, pair => pair.Value );

// 昇順
var sorted = fileDictionary.OrderBy( pair => pair.Key );

// 降順
sorted =  fileDictionary.OrderByDescending( pair => pair.Key );