Why does my ds_list become empty after loading a saved game?

ds_list stores an integer handle, not the data. Saving the handle saves just the number — the list data lives separately. After load the handle may reference a different (or no) list. Use ds_list_write() to serialize the contents to a string, save the string, and ds_list_read() on load.

Should I use ds_list or an array in GameMaker?

For most cases use arrays. They are garbage collected, copyable, and JSON-serializable via json_stringify. ds_list was the old-school option and still has some use cases (fixed-size performance), but modern GameMaker games should default to arrays and structs.

How do I save a ds_grid in GameMaker?

ds_grid_write() converts the grid to a string for saving. ds_grid_read() restores it. Same pattern as ds_list. For modern projects, consider using an array of arrays instead which serializes natively with json_stringify.

Fix: GameMaker DS List Not Persisting Save

Quick answer: ds_list is a handle, not data. Serialize with ds_list_write() to get a string, save the string, then use ds_list_read() on load. Or switch to arrays/structs and use json_stringify(), which handles lists natively.

Here is how to fix GameMaker ds_list not persisting save. You store a player’s inventory in global.inventory = ds_list_create(). You add items. You save the game (file_text_write or similar). You load the game. The inventory is empty. Or it crashes trying to read invalid items. GameMaker’s DS structures are handles to memory-backed data, and saving the handle saves just an integer.

The Symptom

After a save/load cycle, a ds_list (or ds_map, ds_grid) is:

Empty (no items)
Contains garbage data from another list
Points to an invalid handle (ds_list_size errors)
Works on the current run but fails across game sessions

What Causes This

DS types are handles. ds_list_create() returns an integer handle. The actual data lives in memory managed by GameMaker’s DS system. If you save the handle (e.g. global.inventory) as an integer and restore it later, the integer may reference a different list or nothing at all.

Memory lifecycle. DS structures persist until explicitly destroyed with ds_list_destroy(). They do not survive closing the game. The handle from a previous session is meaningless.

ds_list_write/read require explicit calls. GameMaker provides ds_list_write() to serialize the list’s contents to a string, and ds_list_read() to deserialize. Without these explicit calls, saving the handle saves nothing useful.

Nested DS structures. A ds_list containing ds_map handles requires serializing each nested structure too. ds_list_write does not recursively serialize — it only captures raw values. Nested handles produce garbage on load.

The Fix

Step 1: Use ds_list_write and ds_list_read.

// Save
var data = ds_list_write(global.inventory);
var file = file_text_open_write("save.dat");
file_text_write_string(file, data);
file_text_close(file);

// Load
if (file_exists("save.dat")) {
    var file = file_text_open_read("save.dat");
    var data = file_text_read_string(file);
    file_text_close(file);

    global.inventory = ds_list_create();
    ds_list_read(global.inventory, data);
}

ds_list_write produces a string containing all values. ds_list_read rebuilds the list from that string. Works for simple value lists (numbers, strings).

Step 2: Prefer arrays for new code. GameMaker’s modern approach uses arrays and structs. They serialize natively with JSON:

// Modern approach
global.inventory = ["sword", "potion", "key"];

// Save
var data = json_stringify({
    inventory: global.inventory,
    gold: global.gold,
    level: global.level
});
var file = file_text_open_write("save.json");
file_text_write_string(file, data);
file_text_close(file);

// Load
var file = file_text_open_read("save.json");
var raw = file_text_read_string(file);
file_text_close(file);

var save_data = json_parse(raw);
global.inventory = save_data.inventory;
global.gold = save_data.gold;
global.level = save_data.level;

Arrays + structs + JSON handle nested structures cleanly. Modern GameMaker projects should default to this approach.

Step 3: For complex DS structures, serialize manually. If you cannot migrate off DS types, walk the list and serialize each entry:

function save_inventory(list) {
    var items = [];
    for (var i = 0; i < ds_list_size(list); i++) {
        items[i] = list[| i]; // extract each value
    }
    return json_stringify(items);
}

function load_inventory(text) {
    var items = json_parse(text);
    var list = ds_list_create();
    for (var i = 0; i < array_length(items); i++) {
        ds_list_add(list, items[i]);
    }
    return list;
}

Step 4: Clean up DS structures on exit. Always call ds_list_destroy when done with a list to prevent memory leaks, even if the game is quitting:

// Room End or Game End event
if (ds_exists(global.inventory, ds_type_list)) {
    ds_list_destroy(global.inventory);
}

Combined with ds_exists checks, you avoid double-destroy crashes.

Migration Strategy

For a project using DS heavily, migrating to arrays/structs piece by piece is viable:

Wrap DS access in accessor functions (get_inventory_item, set_inventory_item)
Switch the underlying storage from ds_list to array
Update save/load to use json_stringify/json_parse

The accessor pattern lets you migrate without touching every call site.

Understanding the issue

Save data is forever. Once a save format ships, players will have that format in their files; you can't take it back. Bugs in save serialization compound over releases.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

Bugs of this class are particularly easy to ship past internal QA because they often depend on specific runtime conditions - hardware combinations, network states, or asset configurations that QA didn't reproduce. Players hit them in the wild, file reports that are hard to repro, and the bug accumulates negative reviews while engineering tries to recreate the failure mode.

At the engine level, the behavior comes from a deliberate design decision in GameMaker. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

For shipping games, the safest verification is a staged rollout. Apply the fix to 1% of players for 24 hours; watch the affected metric; expand if green. Skipping the staged rollout means the verification is the entire player base, which is too high a stakes for most fixes.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

There's almost always a less obvious case where the same problem applies. The reported case is the one a player hit; the related cases hide because they're rarer or affect fewer players. After fixing the reported case, search the codebase for the pattern - one fix often unlocks several.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

Live games surface this bug class at scale. What's a rare edge case in development becomes a daily occurrence once you have a few thousand concurrent players. The class isn't 'this player has a unique setup'; it's 'one in N thousand sessions will trigger this exact combination'.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

If this issue manifests under high load (many actors, many particles, many network connections), profile the post-fix code path with realistic counts. The original cost was a bug; the new cost is real work, and real work has a budget.

Diagnostic approach

Diagnosing this class of bug benefits from a structured approach: confirm the symptom, isolate the variables, hypothesize the cause, and verify the hypothesis before writing fix code. Skipping the isolation step is the most common mistake; without it, fixes often address symptoms while the underlying cause continues to produce other variations.

For GameMaker-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

Modern engine versions ship better tooling for this kind of issue than older versions. If you're on an older release, the diagnostic step may take significantly longer because the tools you'd want don't exist yet. Sometimes the right answer is upgrading rather than fighting through limited tooling.

Within GameMaker, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Platform-specific edge cases are worth enumerating explicitly. iOS handles backgrounding differently than Android; Windows handles focus changes differently than macOS. A fix that works on the development platform may not work on every target. Test on each shipping platform deliberately.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

When this bug class affects multiple teams (often the case for cross-system issues), early communication prevents duplicate work. The team that owns the symptom may not own the cause. A 15-minute conversation at the start of triage often saves hours of independent investigation.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“DS structures are handles, not data. Saving a handle saves an integer. Serialize explicitly, or use arrays that serialize themselves.”

Related Issues

For other GameMaker issues, see GameMaker Alarm Not Firing and GameMaker Surface Lost After Resize.

Arrays + json_stringify for all new save code. DS for niche performance cases only.