# Compound assignment (+= -= *= /= %= ??=)

> Update a variable or property in place: x op= y is shorthand for x = x op y. Arithmetic forms need a numeric lvalue; ??= assigns only when the left side is null.

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## Summary        {#summary}
Compound assignment updates a variable or property in place: `x op= y` is exactly `x = x op y`. The
arithmetic forms (`+= -= *= /= %=`) require a numeric lvalue; `??=` (null-coalescing assignment) assigns
the right side only when the left is null.

## Signature      {#signature}
```osy syntax
x += y    // x = x + y   (also string concatenation when x is a string)
x -= y    // x = x - y
x *= y    // x = x * y
x /= y    // x = x / y   (int/int truncates, like SQL)
x %= y    // x = x % y   (remainder)
x ??= y   // x = x ?? y  — assign y only when x is null
```

## Description    {#description}
- The left side must be an assignable lvalue — a variable or a property.
- `+= -= *= /= %=` follow the arithmetic rules of their operator: numeric operands, numeric widening
  (`long`/`decimal`/`double`), and int÷int truncation for `/=` (SQL parity). `+=` on a string is
  concatenation.
- `??=` assigns only when the left side is null — a non-null left keeps its value; the right side is
  evaluated only when needed (short-circuit). Value-equivalent to `x = x ?? y` for variable/property
  targets.
- Decompile normalizes to the expanded `x = x op y` form.

## Examples       {#examples}
```osy title="compound assignment" test app=compound-assign
int Mod() { int x = 17; x %= 5; return x; }             // 2

string Keep() { string? s = "have"; s ??= "fallback"; return s; }   // "have" (non-null kept)
string Fill(string? s) { s ??= "fallback"; return s; }              // "fallback" when s is null

decimal RunningTotal(decimal[] amounts) {
  decimal total = 0;
  foreach (var a in amounts) { total += a; }
  return total;
}
```

## See also       {#see-also}
- [++ / -- (increment / decrement)](https://osysharp.com/reference/function/increment-decrement/) — `++`/`--`, the `± 1` special case
- [Numeric types & literal suffixes](https://osysharp.com/reference/function/numeric-literals/) — the numeric types the arithmetic forms operate on