Let's say I have a class with Name and Type properties, If I store the hashcode as below in a database, can I compare this hashcode in another process which has same name and type?
will the hashcode be same?
Name.ToLower().GetHashCode() + Type.ToLower().GetHashCode();
No, they will not, but look towards the end here for a more durable solution that might fit your needs.
First, there's the warning in the documentation:
Warning
A hash code is intended for efficient insertion and lookup in collections that are based on a hash table. A hash code is not a permanent value. For this reason:
- Do not serialize hash code values or store them in databases.
- Do not use the hash code as the key to retrieve an object from a keyed collection.
- Do not send hash codes across application domains or processes. In some cases, hash codes may be computed on a per-process or per-application domain basis.
...
There's more in the warning but the last one illustrates a rather good point.
Simply create a small program with this code:
Console.WriteLine("Test".GetHashCode());
Then run this for .NET Core 3.1, .NET 5 or .NET 6, repeatedly.
You'll get different values
So to answer your question, no, they will not stay the same.
However, you can roll your own implementation. Here's a rather simple implementation of a hashcode function for strings that would only change if you change it yourself:
public static int SemiStableHashCode(string input)
{
unchecked
{
byte[] bytes = Encoding.UTF8.GetBytes(input.Normalize(NormalizationForm.FormKD));
int result = 17;
foreach (byte b in bytes)
result = result * 23 + b;
return result;
}
}
Having said that, it just occurred to me that .NET 5 and 6 changed their Unicode support to use ICU instead of Win32, so who knows how "stable" my method is.
Do not serialize hash code values or store them in databases.GetHashCode. The underlying implementation may change over time. During the lifetime of a process, it must return the same hash code for the same value or else dictionaries would break (for example). But between process lifetimes, the upgrade of the runtime (or a library) could change the algorithm and result in different hash codes and what's in your database wouldn't match.design was demanding this← What does that mean? Is that a person? What was the exact requirement? Maybe it was to use hash codes but not necessarilyGetHashCode()? In that case there are many different hash implementations that you could use.Console.WriteLine("Test".GetHashCode());will produce different outputs.