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Why do string hash codes change for each execution in .NET?

Consider the following code:
Console.WriteLine("Hello, World!".GetHashCode());
First run:
139068974
Second run:
-263623806
Now consider the same thing written in Kotlin:
println("Hello, World!".hashCode())
First run:
1498789909
Second run:
1498789909
Why do hash codes for string change for every execution in .NET, but not on other runtimes like the JVM?

Why do hash codes for string change for every execution in .NET
In short to prevent hash collision attacks. You can roughly find out the reason from the docs of the <UseRandomizedStringHashAlgorithm> configuration element:
The string lookup in a hash table is typically an O(1) operation. However, when a large number of collisions occur, the lookup can become an O(n²) operation. You can use the configuration element to generate a random hashing algorithm per application domain, which in turn limits the number of potential collisions, particularly when the keys from which the hash codes are calculated are based on data input by users.
but not on other runtimes like the JVM?
Not exactly, for example Python's hash function is random. C# also produces identity hash in .net framework, core 1.0 and core 2.0 when <UseRandomizedStringHashAlgorithm> is not enabled.
For Java maybe it's a historical issue because the arithmetic is public, and it's not good, read this.

Why do hash codes change for every execution in .NET?
Because changing the hash code of strings (and other objects!) on each run is a very strong hint to developers that hash codes do not have any meaning outside of the process that generated the hash.
Specifically, the documentation says:
Furthermore, .NET does not guarantee the default implementation of the GetHashCode method, and the value this method returns may differ between .NET implementations, such as different versions of .NET Framework and .NET Core, and platforms, such as 32-bit and 64-bit platforms. For these reasons, do not use the default implementation of this method as a unique object identifier for hashing purposes. Two consequences follow from this:
You should not assume that equal hash codes imply object equality.
You should never persist or use a hash code outside the application domain in which it was created, because the same object may hash across application domains, processes, and platforms.
By changing the hash code of a given object from one run to the next, the runtime is telling the developer not to use the hash code for anything that crosses a process/app-domain boundary. That will help to insulate developers from bugs stemming from changes to the GetHashCode algorithms used by standard classes.
Having hash codes change from one run to the next also discourages things like persisting the hash code for use as a "did this thing change" short-cut. This both prevents bugs from changes to the underlying algorithms and bugs from assuming that two objects of the same type with the same hash code are equal, when no such guarantee is made (in fact, no such guarantee can be made for any data structure which requires or allows more than 32 bits, due to the pigeonhole principle).
Why do other languages generate stable hash codes?
Without a thorough language-by-language review, I can only speculate, but the major reasons are likely to be some combination of:
historical inertia (read: "backwards compatibility")
the disadvantages of stable hash codes were insufficiently understood when the language spec was defined
adding instability to hash codes was too computationally expensive when the language spec was defined
hash codes were less visible to developers

EqualityComparer<String> with stable HashCode and option to ignore diacritics?

I'm looking for a IEqualityComparer<String> that supports stable HashCode, ie the same HashCode between executions/processes. I also need it to ignore casing and nonspacing combining characters (such as diacritics).
Are there any "easy" ways of accomplishing this in .NET? I have started on my custom implementation with a stable HashCode that ignores casing but I'm beginning to wish I could use the already existing implementations in .NET somehow.
The built-in string comparer adds some random seed to HashCodes between procesee to not make it stable (I think because they cannot guarantee it will remain stable between .NET runtimes?) but I think I can handle that by just making sure the HashCodes I persist gets wiped/rebuilt when moving to another runtime.
In any case, is there any way to access the inner checksum calculation (without the randomness)? Perhaps with reflection?
Update: I'm not an expert on the why but it's evident that the HashCode is calculated differently between runtime. I need it because I have a disk based lookup index that is using the hashcode for strings as keys and since it is persistent I obviously need them to be the same between runtime. I could calculate my own checksums in any way I like of course but since .NET already do a very good job with this I wish I could take advantage of that. But without the "seed" or what you want to call it, the thing that makes the hashcodes different between runtimes.

Is there a way to consistently sort/order objects by reference in C#?

So, I have a few object instances, and I want to put them in a consistent order. It doesn't matter what the order is, as long as I can repeat it on any pair of instances, and get the same order. (Assume unique instances.)
Is there any way to do this without relying upon the instances' values? (I know that RuntimeHelpers.GetHashCode can get a consistent hash for each instance without worrying about its value, but the hash isn't guaranteed to be unique.)
Basically, I'm asking for an implementation of IComparer<object>.

Is there any way to do this without relying upon the instances' values?
No.
Really?
Well, you could make a weak reference table mapping objects to guids, and then your sort could look up the object in the weak reference table, get the guid, and then sort on the guid.
That's expensive, but it's doable.
It has to be a weak reference table because otherwise you're basically destroying the garbage collector's ability to free any object that's ever been sorted by your scheme.
I know that RuntimeHelpers.GetHashCode can get a consistent hash for each instance without worrying about its value, but the hash isn't guaranteed to be unique.
That's correct. If you're using GetHashCode for something other than balancing a hash table, you're almost certainly doing something wrong. It is not a unique identifier, and it is not stable.
I'm asking for an implementation of IComparer<object>.
You're going to have to live with disappointment. A total ordering on reference objects irrespective of their values is not a service that .NET provides.

Why the GetHashCode does not take advantage of SK.exe tool's hashcode algorithm?

MSDN says:
"The default implementation of the GetHashCode method does not guarantee unique return values for different objects. "
But on the other hand, when I use the sn.exe tool it ensures a unique hash value to create a strongly-named assembly. If I did not get the point wrong, all the content of the assembly is converted to a hash value.
So, why GetHashCode()'s default implementation does not use the same algorithm used by sn.exe to create a unique hash values for objects and expects the developer to implent it?

Not enough bits. GetHashCode() returns 32 of them so there can never be more than 4 billion distinct values. The birthday paradox cuts that down considerably. The strong name generated by sn.exe (not sk.exe) uses a SHA1 hash. Which returns 160 bits, allowing for 2^160 distinct values.
Which is a Really Big Number (1.4E48), ensuring uniqueness by the sheer quantity. Somewhat similar to a Guid which uses 128 bits. Not the same, a Guid generator ensures that no duplicates can occur, SHA1 has no such guarantee.
GetHashCode has a limited number of bits because the primary requirement for the method is that it is fast. Short from providing the bucket index for hashed collections, its use is making equality testing fast. GetHashCode needs to be an order of magnitude faster than Equals(), give or take, to make it useful. That requires many corners to be cut, typically, the GetHashCode implementation for a struct that contains reference types for example only returns the GetHashCode value of the first member.

Those are two entirely different things.
The GetHashCode() function by definition returns (only) a 32 bits integer. It is supposed to use a fast algorithm and does not (can not) guarantee uniqueness. A PC can quickly generate enough strings to show a collision.
When you sign an application (document) you will end up with a lot larger hash (like 128 or 256 bits). While in theory you might still have a collision this has no practical implications.

There's no limit to the number of objects a program can create, call GetHashCode() upon, and abandon. There is, however, a limit of 4,294,967,296 different values GetHashCode() can return. If a program happens to call GetHashCode 4,294,967,297 times, at least one of those calls would have to return a value that had already been returned previously.
It would theoretically be possible for the system to keep a pool of hash-code values, and for objects which are abandoned to have their hash codes put back in the pool so that GetHashCode() could guarantee that it will never return the same value as any other live object (assuming there are no more than 4,294,967,296 live objects, at least). On the other hand, keeping such information would be expensive and not really offer much benefit. From a practical perspective, it's just as good to have the system generate an arbitrary number either when an object is constructed or the first time GetHashCode() is called upon it. There will be occasional collisions, but generally not enough to bother well-written code.
BTW, I've sometimes thought it would be useful for each object to have a 64-bit ID which would be guaranteed unique, and which would also rank objects in order of creation. A 64-bit ID would never overflow within the lifetime of any foreseeable program, and being able to assign objects a ranking could be helpful in some caching or interning scenarios. For example, if a program generates some large objects by reading data from files, and frequently scans them to find differences, it may often find objects that contain identical data but are distinct. If two distinct objects are found to be identical and interchangeable, replacing reference to the newer one with the older one may considerably expedite future comparisons among them; if many matching objects are compared among each other, many of the references to newer objects will get replaced with references to the oldest ones, without having to explicitly cache anything. Absent some means of determining "age", however, such an approaches wouldn't really work, since there would be no way to know which reference should be abandoned in favor of the other.

Unrelated. Wonder how you could relate these two!!
Still, to add more argument:
Hashcode for a value 'can not guarantee' uniqueness for different values. But it does 'guarantee' a same hash code for a given value/object!. That means:
var hashOne = "SO".GetHashCode();
var hastTwo = "SO".GetHashCode();
Debug.Assert(hashOne==hashTwo); //The assertion would succeed.
SN just just generates a random unique number, with no logic over an instance.

Generate a Hashcode for a string that is platform independent

We have an application that
Generates a hash code on a string
Saves that hash code into a DB along with associated data
Later, it queries the DB using the string hash code for retrieving the data
This is obviously a bug because the value returned from string.GetHashCode() varies from .NET versions and architectures (32/64 bit). To complicate matters, we're too close to a release to refactor our application to stop serializing hash codes and just query on the strings instead. What we'd like to do is come up with a quick and dirty fix for now, and refactor the code later to do it the right way.
The quick and dirty fix seems like creating a static GetInvariantHashCode(string s) helper method that is consistent across architectures.
Can suggest an algorithm for generating a hashcode on a string that is equivalent on 32 bit and 64 bit architecture?
A few more notes:
I'm aware that HashCodes are not unique. If a hashcode returns a match on two different strings, we post process the results to find the exact match. It is not used as a primary key.
I believe the architect's intent was to speed up the searches by querying on a long instead of an NVarChar

I'm aware that HashCodes are not unique. If a hashcode returns a match on two different strings, we post process the results to find the exact match. It is not used as a primary key.
I believe the architect's intent was to speed up the searches by querying on a long instead of an NVarChar
Then just let the database index the strings for you!
Look, I have no idea how large your domain is, but you're going to get collisions very rapidly with very high likelihood if it's of any decent size at all. It's the birthday problem with a lot of people relative to the number of birthdays. You're going to have collisions, and lose any gain in speed you might think you're gaining by not just indexing the strings in the first place.
Anyway, you don't need us if you're stuck a few days away from release and you really need an invariant hash code across platform. There are really dumb, really fast implementations of hash code out there that you can use. Hell, you could come up with one yourself in the blink of an eye:
string s = "Hello, world!";
int hash = 17;
foreach(char c in s) {
unchecked { hash = hash * 23 + c.GetHashCode(); }
}
Or you could use the old Bernstein hash. And on and on. Are they going to give you the performance gain you're looking for? I don't know, they weren't meant to be used for this purpose. They were meant to be used for balancing hash tables. You're not balancing a hash table. You're using the wrong concept.
Edit (the below was written before the question was edited with new salient information):
You can't do this, at all, theoretically, without some kind of restriction on your input space. Your problem is far more severe than String.GetHashCode differening from platform to platform.
There are a lot of instances of string. In fact, way more instances than there are instances of Int32. So, because of the piegonhole principle, you will have collisions. You can't avoid this: your strings are pigeons and your Int32 hash codes are piegonholes and there are too many pigeons to go in the pigeonholes without some pigeonhole getting more than one pigeon. Because of collision problems, you can't use hash codes as unique keys for strings. It doesn't work. Period.
The only way you can make your current proposed design work (using Int32 as an identifier for instances of string) is if you restrict your input space of strings to something that has at size less than or equal to the number of Int32s. Even then, you'll have difficulty coming up with an algorithm that maps your input space of strings to Int32 in a unique way.
Even if you try to increase the number of pigeonholes by using SHA-512 or whatever, you still have the possibility of collisions. I doubt you considered that possibility previously in your design; this design path is DOA. And that's not what SHA-512 is for anyway, it's not to be used for unique identification of messages. It's just to reduce the likelihood of message forgery.
To complicate matters, we're too close to a release to refactor our application to stop serializing hash codes and just query on the strings instead.
Well, then you have a tremendous amount of work ahead of you. I'm sorry you discovered this so late in the game.
I note the documentation for String.GetHashCode:
The behavior of GetHashCode is dependent on its implementation, which might change from one version of the common language runtime to another. A reason why this might happen is to improve the performance of GetHashCode.
And from Object.GetHashCode:
The GetHashCode method is suitable for use in hashing algorithms and data structures such as a hash table.
Hash codes are for balancing hash tables. They are not for identifying objects. You could have caught this sooner if you had used the concept for what it is meant to be used for.

You should just use SHA512.
Note that hashes are not (and cannot be) unique.
If you need it to be unique, just use the identity function as your hash.

You can use one of the managed cryptography classes (such as SHA512Managed) to compute a platform independent hash via ComputeHash. This will require converting the string to a byte array (ie: using Encoding.GetBytes or some other method), and be slow, but be consistent.
That being said, a hash is not guaranteed unique, and is really not a proper mechanism for uniqueness in a database. Using a hash to store data is likely to cause data to get lost, as the first hash collision will overwrite old data (or throw away new data).