public void AddOrOverwrite() { var d = new ConcurrentDictionary <string, int>(StringComparer.OrdinalIgnoreCase); var p1 = d.AddOrOverwrite("potato", 9); var p2 = d.AddOrOverwrite("pOtato", 45); var p3 = d.AddOrOverwrite("orange", 4); Assert.Equal(9, p1); Assert.Equal(45, p2); Assert.Equal(45, d["Potato"]); Assert.Equal(4, d["orange"]); Assert.NotEqual(9, d["potato"]); }