Generate Unique Hash code based on String

string.GetHashCode is indeed inappropriate for real hashing:

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 use the hash code instead of a value returned by a cryptographic hashing function. For cryptographic hashes, use a class derived from the System.Security.Cryptography.HashAlgorithm or System.Security.Cryptography.KeyedHashAlgorithm class.
• Do not test for equality of hash codes to determine whether two objects are equal. (Unequal objects can have identical hash codes.) To test for equality, call the ReferenceEquals or Equals method.

and has high possibility of duplicates.

Consider HashAlgorithm.ComputeHash. The sample is slightly changed to use SHA256 instead of MD5, as @zaph suggested:

static string GetSha256Hash(SHA256 shaHash, string input)
{
    // Convert the input string to a byte array and compute the hash.
    byte[] data = shaHash.ComputeHash(Encoding.UTF8.GetBytes(input));

    // Create a new Stringbuilder to collect the bytes
    // and create a string.
    StringBuilder sBuilder = new StringBuilder();

    // Loop through each byte of the hashed data 
    // and format each one as a hexadecimal string.
    for (int i = 0; i < data.Length; i++)
    {
        sBuilder.Append(data[i].ToString("x2"));
    }

    // Return the hexadecimal string.
    return sBuilder.ToString();
}

using System.Security.Cryptography;
string data="test";
byte[] hash;
using (MD5 md5 = MD5.Create())
{
    md5.Initialize();
    md5.ComputeHash(Encoding.UTF8.GetBytes(data));
    hash = md5.Hash;
}

hash is a 16 byte array, which in turn you could covert to some hex-string or base64 encoded string for storage.

EDIT:

What's the purpose of that hash code?

From hash(x) != hash(y) you can derive x!=y, but

from hash(x) == hash(y) you canNOT derive x==y in general!

I also had a problem similar to yours which I have solved in the following way and with a test software I have tested up to 50 million texts, all of which were in English, and there was no problem in testing them, but surely, considering the statistical population of the input data, it is completely logical that it cannot be For all cases of a string with any string length, it will produce a unique long numeric value, but for strings with special and meaningful conditions, such as the names of tables and columns, this method can be used. If there is a better solution, please guide me

public class ContextIdGenerator 
{
    public long CreateTableId(string schemaName, string tableName) => CreateId($"{schemaName}.{tableName}", false, Encoding.ASCII);
    public long CreateTableColumnId(string schemaName, string tableName, string columnName) => CreateId($"{schemaName}.{tableName}.{columnName}", false, Encoding.ASCII);
    public long CreateId(string context, bool sensitive, Encoding encoding)
    {
        if (string.IsNullOrEmpty(context)) return 0;
        if (!sensitive) context = context.ToUpper();
        using var hasher = MD5.Create();
        var bytes = encoding.GetBytes(context.ToUpper());
        var hBytes = hasher.ComputeHash(bytes);

        return hBytes.Select((q, i) => Convert.ToInt64(q * Math.Pow(10, i + 1))).Sum();
    }
}

The test source is mentioned below (ASCII state)

private static Random random = new Random();
public static string RandomString(int length)
{
    const string chars = ".ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    return new string(Enumerable.Repeat(chars, length)
        .Select(s => s[random.Next(s.Length)]).ToArray());
}

private void TestButton_Click(object sender, EventArgs e)
{
    Enabled = false;
    Dictionary<long, string> db = new();
    var A = new ContextIdGenerator();
    var errorCounter = 0;
    var counter = 0;
    while (counter <= 50000000)
    {
        Application.DoEvents();
        var len = random.Next(20, 30);
        var text = RandomString(len);
        var keyId = A.CreateId(text, false, Encoding.ASCII);
        counter++;
        if (keyId == 0) continue;
        try
        {
            db.Add(keyId, text);
        }
        catch
        {
            if (db.Values.Contains(text, StringComparer.OrdinalIgnoreCase))
                continue;
            errorCounter++;
        }
    }
    Enabled = true;
    MessageBox.Show($"ErrorCount : {errorCounter}");
}