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What is Option Strict in VB.NET?

VB.NET Language Basic 1–2 Yrs2–3 Yrs Conceptual 1 min read

Quick Answer

Option Strict On makes the VB.NET compiler reject implicit narrowing conversions and late binding — you must write explicit casts (CInt, CType, DirectCast), and every call is resolved at compile time. It should be on in every project; leaving it off hides type errors until run time.

Option Strict On tells the compiler to stop being lenient. It disallows implicit narrowing conversions — so you can't silently assign a Double to an Integer — and it disallows late binding, so calling a method on an Object has to resolve at compile time. The effect is that a whole class of errors that would otherwise be a run-time InvalidCastException becomes a compile error instead. It should be on everywhere. A typical job on an old VB.NET codebase is turning it on and working through everything it flags, which usually surfaces genuine bugs.

Three related options:

  • Option Explicit On (default) — every variable must be declared.
  • Option Strict On — no implicit narrowing (Object → String, Double → Integer), no late binding on Object, no implicit Object conversions in operators. Turns run-time InvalidCastExceptions into compile errors.
  • Option Infer On — allows Dim x = 5 local type inference (still statically typed).

With Option Strict Off, Dim total = value1 + value2 where the values are Object compiles and does late-bound arithmetic that can blow up at run time. Turning it on across a legacy codebase is a common hardening task — you fix the flagged conversions and often find real bugs.

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