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Codes / Error Code 013

Error Code 013

Overview

This error occurs when there is an attempt to modify a variable or object declared as immutable. Immutable values cannot be altered once initialized.

Details

  • Common Causes:

    • Attempting to assign a new value to an immutable variable.
    • Modifying properties of an object that is declared as immutable.
    • Passing an immutable object to a function expecting a mutable parameter.

  • Example:

void main() {
    immutable int x = 10;
    x = 20; // Error: cannot modify immutable variable `x`
}
  • Solution:
    • Avoid assigning values to immutable variables after initialization: 
      void main() {
    immutable int x = 10;
    // x = 20; // Invalid
}
    • If modification is necessary, use const or mutable instead of immutable: 
      void main() {
    int x = 10; // Mutable variable
    x = 20; // Valid
}
    • When working with functions, ensure the correct parameter types: 
      void modifyValue(ref int x) {
    x = 42;
}

void main() {
    int y = 10;
    modifyValue(y); // Valid
}