Question

I have

Dictionary<Guid, DateTime> d = new Dictionary<Guid, DateTime>();

How I can get an Guid which has MAX value?

Answer 1

Since this was the accepted answer, I’ll try to cover every possible meaning of the question:

var dict = 
    new Dictionary<string, int> 
    { 
        ["b"] = 3, 
        ["a"] = 4 
    };

// greatest key
var maxKey = dict.Keys.Max(); // "b"

// greatest value
var maxValue = dict.Values.Max(); // 4

// key of the greatest value
// 4 is the greatest value, and its key is "a", so "a" is the answer.
var keyOfMaxValue =
    dict.MaxBy(entry => entry.Value).Key; // "a"

// Note: On .NET < 6, MaxBy is not available. You can use:
var keyOfMaxValue = 
    dict.Aggregate((x, y) => x.Value > y.Value ? x : y).Key; // "a"

Note: the question has System.Guid as the key type. It might not make sense to ask "what is the greatest GUID", since they are simply intended to be unique values, rather than represent any orderable concept. Nonetheless, the above code will work with any type that supports the > operator, string and int being chosen here for conciseness.