Generating a unique Hash based on DateTime and String parameter

Question

I would like to create a function which would enable me to generate unique values always based on these two entry parameters:

public string ReturnUniqueValue(DateTime date, string ID)
{
 // Logic here
}

The date parameter is in the following format:

2017-08-14 11:55:32.00

While the ID is in the following format:

112452573848

I would like to generate a unique hash which is 40 characthers in length and that it never repeats.

Is this doable with datetime parameter + unique id string ?

I figured that datetime is never the same (it's nearly impossible), thus this should be able to always generate unique value?

It is possible that you get duplicates from combining these 2 (unless ID is always unique). If you want a somewhat unique string, look into Guid.NewGuid() — Shyju
– Shyju, Commented Aug 21, 2017 at 12:50
@Shyju yes the ID is always unique, what would be the best way to hash this ? :) — User987
– User987, Commented Aug 21, 2017 at 12:51
Use Guid instead, it was created exactly for this. With your method I could pass the same DateTime instance and ID field as many times as I wanted — Camilo Terevinto
– Camilo Terevinto, Commented Aug 21, 2017 at 12:53
@CamiloTerevinto I can't use GUID because the data comes from API source, and I need to somehow hash it in order to compare the values that I have in my DB and from API ^^ — User987
– User987, Commented Aug 21, 2017 at 12:53

Axel Heer · Accepted Answer · 2017-08-21 15:34:14Z

You can

write all your parameters (whichever) into a stream
compute a hash using your favorite algorithm (e.g. SHA265)
and return its first 20 values (= 40 characters) as a final result

In code:

public string ReturnUniqueValue(DateTime date, string ID)
{
    var result = default(byte[]);

    using (var stream = new MemoryStream())
    {
        using (var writer = new BinaryWriter(stream, Encoding.UTF8, true))
        {
            writer.Write(date.Ticks);
            writer.Write(ID);
        }

        stream.Position = 0;

        using (var hash = SHA256.Create())
        {
            result = hash.ComputeHash(stream);
        }
    }

    var text = new string[20];

    for (var i = 0; i < text.Length; i++)
    {
        text[i] = result[i].ToString("x2");
    }

    return string.Concat(text);
}

Note: if you just want a single unique value for multiple parameters, a simple concatenation should already do it. Since you explicitly asked for a "hash" with 40 characters, this more elaborate solution may fit somehow better.

Attention: extending this to more parameters of the same type (e.g. two strings) should include the parameters position within the stream to avoid collisions ((a, b) != (b, a)).

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