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microsoft/knowledge/performance/use-textbuilder-for-loop-string-assembly.md

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+---
+bc-version: [all]
+domain: performance
+keywords: [textbuilder, string-concatenation, loop, text, allocation]
+technologies: [al]
+countries: [w1]
+application-area: [all]
+---
+
+# Use TextBuilder for loop-based string assembly
+
+## Description
+
+Repeated `Text := Text + ...` concatenation inside a loop reallocates and copies the growing string on every iteration. In AL, `TextBuilder` is the platform type for constructing larger text payloads incrementally. `StrSubstNo` remains appropriate for formatting one message; TextBuilder is for many appends, especially inside loops.
+
+## Best Practice
+
+Use `TextBuilder.Append` or `AppendLine` when assembling CSV rows, log payloads, JSON-ish diagnostic text, or other multi-line strings from repeated loop iterations. Convert to Text once, after the loop, with `ToText()`.
+
+See sample: `use-textbuilder-for-loop-string-assembly.good.al`.
+
+## Anti Pattern
+
+Appending to the same Text variable on every iteration of a large loop. Each append copies the accumulated prefix again, so the cost grows with both row count and final string length.
+
+See sample: `use-textbuilder-for-loop-string-assembly.bad.al`.