Add 18 community AL/BC patterns across style, data-modeling, web-services, appsource, breaking-changes, performance, and testing

Contributed by CURABIS ApS, generalized from patterns observed across real AppSource/PTE development. Each article follows the knowledge file format (frontmatter, Description/Best Practice/Anti Pattern, sibling .good.al/.bad.al samples).

microsoft/knowledge/appsource/file-datatype-saas.bad.al

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+codeunit 50104 "Import File Reader"
+{
+    procedure ImportFile()
+    var
+        ImportFile: File;
+        InStream: InStream;
+    begin
+        ImportFile.WriteMode(false);
+        ImportFile.TextMode(true);
+        ImportFile.Open('C:\Import\data.txt'); // fails in SaaS — no local filesystem
+        ImportFile.CreateInStream(InStream);
+    end;
+}

microsoft/knowledge/appsource/file-datatype-saas.good.al

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+codeunit 50104 "Import File Reader"
+{
+    procedure ImportFile()
+    var
+        TempBlob: Codeunit "Temp Blob";
+        InStream: InStream;
+        FileName: Text;
+    begin
+        if UploadIntoStream('Import file', '', 'All Files (*.*)|*.*', FileName, InStream) then
+            ParseStream(InStream);
+    end;
+
+    local procedure ParseStream(var InStream: InStream)
+    begin
+        // parse InStream content here
+    end;
+}

microsoft/knowledge/appsource/file-datatype-saas.md

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+---
+bc-version: [all]
+domain: appsource
+keywords: [file-datatype, saas, onprem, uploadintostream, downloadfromstream, instream, outstream, streaming]
+technologies: [al]
+countries: [w1]
+application-area: [all]
+---
+
+# The File data type's direct I/O methods are OnPrem-only
+
+> Contributions welcome — open a PR to refine or extend this article.
+
+## Description
+
+The classic `File` variable type — `Open`/`Create`/`Read`/`Write`/`Close` against a path on the local or server filesystem — only works on-premises, because there is no accessible filesystem in the SaaS/cloud sandbox. Any extension meant to run in Business Central Online must not rely on `File.Open`, `File.Create`, `File.Read`, or `File.Write` for its core functionality: code built this way compiles but fails, or is silently skipped, in the cloud.
+
+## Best Practice
+
+Use the stream-based equivalents: `UploadIntoStream` to read user-selected file content into an `InStream`, and `DownloadFromStream` to write an `OutStream`'s content to a file the user saves. Stage the content in a `TempBlob` between the stream and the rest of the parsing/formatting code.
+
+See sample: `file-datatype-saas.good.al`.
+
+## Anti Pattern
+
+Opening a hardcoded or user-supplied filesystem path with the `File` variable type. This is a strong signal the code was written for on-premises only, or copied from material that predates the cloud-first streaming APIs.
+
+See sample: `file-datatype-saas.bad.al`.