Null check error in Generated toJson methods in dart having nullable DateTime values

[anugrahkora]

When model for a json having a nullable field is generated, the generated toJson Method doesn't recognise nullable DateTime value and returns an exception Null check operator used on a null value.

Null Safety

Output

Input Format: Json
Output Language: Dart

When we parse a large Json with multiple nullable date time object, the toJson Method causes the null check error and it is a lot of manual work to fix this issue

Use a null check before parsing the date on the toJson methods

Input Data

{
"debitNoteDate": "2023-01-01",
"dueDate": "2023-01-01"
}

Expected Behaviour / Output

Map<String, dynamic> toJson() => {

    "debitNoteDate": "${debitNoteDate!.year.toString().padLeft(4, '0')}-${debitNoteDate!.month.toString().padLeft(2, '0')}-${debitNoteDate!.day.toString().padLeft(2, '0')}",
   
    "dueDate": "${dueDate!.year.toString().padLeft(4, '0')}-${dueDate!.month.toString().padLeft(2, '0')}-${dueDate!.day.toString().padLeft(2, '0')}"
 };

Map<String, dynamic> toJson() => {

    "debitNoteDate": debitNoteDate==null? null:"${debitNoteDate!.year.toString().padLeft(4, '0')}-${debitNoteDate!.month.toString().padLeft(2, '0')}-${debitNoteDate!.day.toString().padLeft(2, '0')}",
   
    "dueDate":dueDate==null? null: "${dueDate!.year.toString().padLeft(4, '0')}-${dueDate!.month.toString().padLeft(2, '0')}-${dueDate!.day.toString().padLeft(2, '0')}"
 };

Current Behaviour / Output

Steps to Reproduce

Possible Solution

[schani]

Confirmed reproducible on current master (v23.3.0, 68ecb28), with default options.

Root cause: packages/quicktype-core/src/language/Dart/DartRenderer.ts:599-615. The nullable-date-transformed-string case emits the null-assertion operator (!) unconditionally instead of guarding for null:

"debitNoteDate": "${debitNoteDate!.year.toString().padLeft(4, '0')}-${debitNoteDate!.month.toString().padLeft(2, '0')}-${debitNoteDate!.day.toString().padLeft(2, '0')}",

The adjacent nullable date-time case is handled correctly with a null-conditional: createdAt?.toIso8601String(). Only the date (plain yyyy-MM-dd) transformed-string kind is missing the guard, so serializing (toJson()) any instance where that field is actually null throws Dart's "Null check operator used on a null value" at runtime, even though the generated code compiles fine and fromJson handles null correctly.

Assessment

• Severity: high — happens with default options on a very common shape (a nullable date field that is sometimes null in real data), compiles without warning, and only crashes at runtime when you go to serialize a null value.
• Difficulty: easy — wrap the expression the same way the date-time case already is, e.g. debitNoteDate == null ? null : "${debitNoteDate!.year...}", mirroring the existing pattern one branch up.

Reproduction

node dist/index.js --lang dart --src-lang json sample.json

sample.json:

[
  { "debitNoteDate": "2023-01-01", "dueDate": "2023-01-01", "createdAt": "2023-01-01T10:00:00Z" },
  { "debitNoteDate": null, "dueDate": null, "createdAt": null }
]

Generated toJson():

Map<String, dynamic> toJson() => {
    "debitNoteDate": "${debitNoteDate!.year.toString().padLeft(4, '0')}-${debitNoteDate!.month.toString().padLeft(2, '0')}-${debitNoteDate!.day.toString().padLeft(2, '0')}",
    "dueDate": "${dueDate!.year.toString().padLeft(4, '0')}-${dueDate!.month.toString().padLeft(2, '0')}-${dueDate!.day.toString().padLeft(2, '0')}",
    "createdAt": createdAt?.toIso8601String(),
};

debitNoteDate and dueDate (plain dates) crash when null; createdAt (date-time) does not.

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Posted from a systematic triage sweep of the issue backlog (assisted by Claude); reproduced against a local build of current master.