RDC00112 — Implicit narrowing conversion

Severity: Warning · Family: RDC (Rubberduck Core analyzers) · Raised by: ImplicitNarrowingConversionAnalyzer

A value is assigned, passed or returned into a type that holds less than the value's own type, and nothing in the code says that this is meant.

VBA converts silently in these situations (MS-VBAL §5.5.1.2 Let-coercion), and what it does to the value is easy to miss:

A Double or Single assigned to a whole-number type is rounded, and a value that is exactly halfway rounds to the even number: 2.5 becomes 2, and 3.5 becomes 4. It is not truncated, and it is not always rounded up.
A value that does not fit the destination - 70000 into an Integer, 300 into a Byte - raises run-time error 6 (Overflow), and only when that particular value shows up. The code works with the values it was tried with, and stops with others.

Writing the conversion out - CLng(d), CInt(n), CByte(n) - changes nothing about what is converted, but it states that the loss is meant, and the next reader can tell a decision from an oversight. A conversion that is written out is not reported.

The whole module is checked, including the procedures nothing calls and the branches that never run. A value that is known - a literal, or a constant - that fits the destination and loses nothing is not reported: Dim b As Byte: b = 10 needs no conversion function. One that loses its fraction (n = 2.5) is. One that does not fit at all (b = 300) is not a warning but an error: the line fails every time it runs, and is reported as VBR00006 (Overflow) before the program runs.

How it is raised

Situation	Example	Resolution
A `Double` or `Single` is assigned to a whole-number type.	`count = ratio`, with `count As Long` and `ratio As Double`	Write the conversion: `count = CLng(ratio)`. Use `Int` or `Fix` if rounding is not what is meant.
A wider whole-number type is assigned to a narrower one.	`small = big`, with `small As Integer` and `big As Long`	Declare `small` as a `Long`, or write the conversion: `small = CInt(big)`.
An expression is wider than the variable it is assigned to.	`result = result + 1`, with `result As Byte` (the sum is an `Integer`)	Declare the variable wide enough, or write the conversion: `result = CByte(result + 1)`.
A value is passed to a `ByVal` parameter of a narrower type.	`Show total`, where `Sub Show(ByVal n As Integer)` is given a `Long`	Change the parameter's type, or convert the argument: `Show CInt(total)`.
A function returns a value of a wider type than it is declared with.	`Probability = 0.25`, in a function declared `As Single`, from a `Double` constant	Declare the function with the type the value has, or convert: `Probability = CSng(Threshold)`.

Table of Contents

RDC00112 — Implicit narrowing conversion

How it is raised

See also