Dart Else Clause

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Syntax

bool booleanExpression = false;

if (booleanExpression) {
  // Statements executed if booleanExpression is true
} else {
  // Statements executed if booleanExpression is false
}

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Technical Mechanics

• Unconditional Execution: Unlike if and else if, the else clause does not accept or evaluate a boolean expression. Its execution is entirely dependent on the failure of all preceding conditions in the chain.
• Placement Constraints: The else clause must be the final block in an if statement chain. Placing an else if or another else after an else clause results in a compile-time syntax error.
• Lexical Scoping: Dart enforces block scope. Any variables declared within the curly braces {} of an else clause are strictly local to that block. Once execution exits the block, these variables fall out of lexical scope, and the underlying objects they reference become eligible for garbage collection.
• Single-Statement Omission: Dart’s parser allows the omission of curly braces if the else clause contains only a single statement. However, the Dart analyzer and Effective Dart guidelines strongly recommend always using braces to prevent the “dangling else” problem and to avoid logical errors during refactoring.

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Structural Example

int value = 10;

if (value > 20) {
  // Evaluated first. If true, execution exits the chain.
} else if (value < 0) {
  // Evaluated second. If true, execution exits the chain.
} else {
  // Terminal branch: executes strictly because value <= 20 AND value >= 0.
  int scopedVariable = 42; 
}

// scopedVariable is inaccessible here due to lexical block scoping.

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The Dangling Else Ambiguity

bool conditionA = true;
bool conditionB = false;

// Ambiguous syntax (discouraged)
if (conditionA) 
  if (conditionB) 
    print('A and B'); 
else 
  print('Not B'); // Binds to conditionB, not conditionA

// Explicit syntax (recommended)
if (conditionA) {
  if (conditionB) {
    print('A and B');
  } else {
    print('Not B');
  }
}