C# generated __lookup_by_key fails due to CompareTo instead of string.Compare(..., StringComparison.Ordinal)

[timestee]

Description

I ran into an issue with the generated C# code for __lookup_by_key.

The current implementation of the binary search uses:

int comp = obj_.Key.CompareTo(key);

However, this fails to find elements even when the vector is sorted correctly.
For example, when looking up "OpenBox_point", the search fails despite the key being present in the table.

When I change the code to:

int comp = string.Compare(obj_.Key, key, StringComparison.Ordinal);

the lookup works as expected.

---

Steps to reproduce

1. Create a FlatBuffers table with a sorted key vector containing mixed-case strings (e.g., "OpenBox_point").
2. Use the generated C# __lookup_by_key to search for the key.
3. Observe that CompareTo fails to find the entry.
4. Replace CompareTo with string.Compare(..., StringComparison.Ordinal) and see that the search succeeds.

---

Expected behavior

__lookup_by_key should reliably find keys as long as the vector is sorted correctly, regardless of case differences or locale issues.

---

Actual behavior

The lookup fails because CompareTo does not behave the same as string.Compare(..., StringComparison.Ordinal).

---

Minimal Reproduction Code

class Program
{
    [Test]
    public void TestBinarySearch()
    {
        // Simulate a sorted key array. The keys are already sorted when the data is generated, using Go's string sort.
        string[] keys = {
            "30d_card_collection",
            "30d_card_defense",
            "30d_card_discount",
            "30d_card_dispatch",
            "30d_card_queue",
            "OpenBox_point",
            "allianceBoss_buff_upgrade_item",
            "alliance_box_1",
            "alliance_box_2",
            "alliance_box_3",
            "alliance_box_pay1",
            "alliance_box_pay2",
            "alliance_box_pay3",
            "alliance_box_pay4",
            "alliance_box_pay5",
            "alliance_box_pay6",
            "alliance_box_pay7",
            "alliance_points_2500"
        };

        // Generated FlatBuffers __lookup_by_key logic (uses CompareTo)
        string target = "OpenBox_point";
        int idx = BinarySearch(keys, target);
        Console.WriteLine($"Search result with CompareTo: {idx}"); // ❌ Not found

        // Modified version using StringComparison.Ordinal
        idx = BinarySearchOrdinal(keys, target);
        Console.WriteLine($"Search result with Ordinal: {idx}");   // ✅ Found correctly
    }

    static int BinarySearch(string[] arr, string key)
    {
        int start = 0;
        int span = arr.Length;

        while (span != 0)
        {
            int middle = span / 2;
            int comp = arr[start + middle].CompareTo(key);
            if (comp > 0)
            {
                span = middle;
            }
            else if (comp < 0)
            {
                middle++;
                start += middle;
                span -= middle;
            }
            else
            {
                return start + middle;
            }
        }
        return -1;
    }

    static int BinarySearchOrdinal(string[] arr, string key)
    {
        int start = 0;
        int span = arr.Length;

        while (span != 0)
        {
            int middle = span / 2;
            int comp = string.Compare(arr[start + middle], key, StringComparison.Ordinal);
            if (comp > 0)
            {
                span = middle;
            }
            else if (comp < 0)
            {
                middle++;
                start += middle;
                span -= middle;
            }
            else
            {
                return start + middle;
            }
        }
        return -1;
    }
}

---

Search result with CompareTo: -1
Search result with Ordinal: 5

Question

Is this the intended behavior for the C# code generator, or is this a bug?
It seems that using string.Compare with StringComparison.Ordinal would be more reliable and consistent with FlatBuffers' cross-platform guarantees.

Thanks!

[timestee]

I have hundreds of tables that were automatically converted from Protobuf. I’ve already written a script to perform the replacements automatically, and so far everything seems to be working fine.

[jtdavis777]

I believe this is solved by #8547, and this is a duplicate of #8553, so closing as such.