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If I pass an object to a method and change its properties, does the caller see the change?

C# Methods & Parameters Basic Fresher1–2 Yrs Conceptual 1 min read

Quick Answer

Yes for mutating the object's members (both variables point to the same instance). No for reassigning the parameter to a new object — that only changes the local copy of the reference, unless the parameter is ref.

It depends on what you do. If you change a property on the object — o.Total = 100 — yes, the caller sees it, because you both have a reference to the same object. If you reassign the parameter itself — o = new Order() — no, because C# passed a copy of the reference and you just repointed the copy. To make a reassignment visible you'd need ref. For a value type like a struct or an int, the whole thing is copied, so nothing you do inside the method is visible unless it's a ref parameter.

  • void Update(Order o) { o.Total = 100; } — caller sees Total == 100 (same object).
  • void Replace(Order o) { o = new Order(); } — caller's variable unchanged (reassigned the copy).
  • void Replace(ref Order o) { o = new Order(); } — now the caller's variable is replaced.
  • Value types (struct, int) are copied entirely, so neither mutation nor reassignment is visible without ref.
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