New rule idea: Prefer .Substring() over CopyStr()

[Arthurvdv]

Proposal

A new analyzer rule that suggests replacing CopyStr(text, startIndex, length) with text.Substring(startIndex, length) when the project's app.json has an "application" version of "27.1.0.0" or higher.

Why?

1. Cross-language familiarity. .Substring() is the standard method name in C#, Java, JavaScript, Python, and most other languages. CopyStr is a BC/NAV-specific built-in that developers coming from other ecosystems won't recognize.

2. Fluent, object-oriented style. Calling .Substring() directly on the text variable reads naturally left-to-right (MyText.Substring(1, 5)) instead of wrapping it in a function (CopyStr(MyText, 1, 5)). This improves code readability and makes intent clearer.

3. AI-assisted development. AI coding tools (Copilot, ChatGPT, etc.) are trained predominantly on mainstream language patterns. They are far more likely to suggest and correctly reason about .Substring() than the BC-specific CopyStr. Using the common method name reduces friction when working with AI assistants.

4. Discoverability. New AL developers can look up .Substring() behavior based on general programming knowledge, whereas CopyStr requires BC-specific documentation.

Prerequisites

This rule should only fire when the "application" property in app.json is "27.1.0.0" or higher. The runtime error that previously made .Substring() unsafe (when length exceeds the string) was fixed in BC 27.0 platform (online) and guaranteed in BC 27.1 (all deployment types). The "runtime" property and AL Language version are not relevant here, only the application version matters.

Current state in ALCops

Analyzer: PC0022 (PossibleOverflowAssigning) already handles both

The analyzer already recognizes both CopyStr and Substring when calculating result lengths. No changes needed on the diagnostic side.

CodeFix: PossibleOverflowAssigningApplyCopyStr needs an update

The code fix for PC0022 currently always wraps with CopyStr, generating code like:

// Assignment
MyText := Text.CopyStr(MyLabelLbl, 1, Text.MaxStrLen(MyText));

// Validate
MyTable.Validate(MyField, Text.CopyStr(MyLabelLbl, 1, Text.MaxStrLen(MyTable.MyField)));

// Exit
exit(Text.CopyStr(MyLabelLbl, 1, 10));

When the app targets application 27.1.0.0 or higher, the code fix should instead generate:

// Assignment
MyText := MyLabelLbl.Substring(1, Text.MaxStrLen(MyText));

// Validate
MyTable.Validate(MyField, MyLabelLbl.Substring(1, Text.MaxStrLen(MyTable.MyField)));

// Exit
exit(MyLabelLbl.Substring(1, 10));

This makes the generated code consistent with modern AL style and avoids the Text. static class prefix.

TODO

• Update PossibleOverflowAssigningApplyCopyStr code fix to emit .Substring() when app.json application >= 27.1.0.0
• Add HasFixSubstring/ test fixture variants alongside existing HasFixCopyStr/
• Investigate whether a standalone "prefer Substring over CopyStr" diagnostic rule (informational severity) is worth adding

Reference

Blog post by Natalie Karolak explaining the change and when it is safe to use:
https://nataliekarolak.wordpress.com/2026/04/23/using-substring-to-replace-copystr/