> ## Documentation Index > Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt > Use this file to discover all available pages before exploring further. # Dart Collection If Collection `if` is a declarative control flow element used directly within Dart collection literals (`List`, `Set`, and `Map`) to conditionally insert elements during instantiation. It evaluates a boolean expression and includes the subsequent element, sequence, or spread only if the expression resolves to `true`. Unlike standard `if` statements, which control program execution flow, collection `if` operates as an element generator within the lexical scope of a collection literal. ## Syntax and Mechanics The operator supports standard `if`, `if-else`, and Dart 3.0+ `if-case` pattern matching. **Standard `if` and `if-else`** ```dart theme={"dark"} final bool condition = true; final list = [ 'A', if (condition) 'B', if (!condition) 'C' else 'D', ]; ``` **Map and Set Implementation** The syntax applies identically to `Set` and `Map` literals. For maps, the conditional element must be a valid key-value pair. ```dart theme={"dark"} final bool includeAdmin = false; final bool condition = true; final userMap = { 'username': 'sys_user', if (includeAdmin) 'role': 'admin', }; final uniqueSet = { 1, if (condition) 2, }; ``` **If-Case (Pattern Matching)** Introduced in Dart 3.0, `if-case` allows conditional element inclusion based on whether a value matches a specific pattern, simultaneously extracting destructured variables. ```dart theme={"dark"} final Object result = [200, 'Success']; final logEntries = [ 'Operation started', if (result case [int code, String message]) 'Completed: $code - $message', ]; ``` ## Technical Characteristics **Omission vs. Null Insertion** When a collection `if` condition evaluates to `false` and no `else` branch is provided, the element is entirely omitted from the collection. It does not insert a `null` value, and the collection's `length` property is not incremented. **Compile-Time Constant Evaluation** Collection `if` is fully supported within `const` collection literals, provided that the condition itself is a compile-time constant expression. The Dart compiler evaluates the condition during compilation, resulting in a fixed, immutable collection without runtime overhead. ```dart theme={"dark"} const bool isProduction = true; const config = [ 'base_module', if (isProduction) 'prod_module' else 'dev_module', ]; ``` **Type Inference and Least Upper Bound (LUB)** Elements introduced via collection `if` participate in the compiler's type inference. If the collection's generic type is not explicitly declared, the Dart analyzer calculates the Least Upper Bound (LUB) of all elements, including those in both branches of an `if-else` construct. ```dart theme={"dark"} final bool condition = true; // The inferred type is List because the LUB of int (1) and double (2.5) is num. final numbers = [ 1, if (condition) 2.5 else 3, ]; ``` **Composability with Spread Operators** Collection `if` can be composed with the spread operator (`...`) to conditionally insert multiple elements from an iterable. ```dart theme={"dark"} final bool condition = true; final List extraItems = [4, 5, 6]; final combinedList = [ 1, 2, 3, if (condition) ...extraItems, ]; ``` **Evaluation Phase** For non-constant collections, collection `if` elements are evaluated synchronously at runtime during the instantiation of the collection literal. They are not lazy, nor can they be used to mutate a collection after it has been allocated in memory.

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