Static code analysis can find hundreds of opportunities in VBA code.
Rubberduck builds its own internal representation of the code, and then proceeds to analyze it. Each individual inspection can easily be disabled, or configured to issue inspection results at different severity levels ranging from Hint to Error.
Use the Inspection Results toolwindow to review Rubberduck’s findings, search, filter, regroup results by inspection, location, type, or severity. Each inspection result comes with a detailed description of what’s being flagged and why, so you can make an enlightened decision.
Unless configured otherwise, Rubberduck automatically runs inspections after the a parser/resolver cycle completes (regardless of whether the inspection results toolwindow is displayed or not).
For the best experience, it would be recommended to first try Rubberduck with an empty project, add a new module, and write, say, a loop that counts 1 to 10 and outputs to the debug pane - then to parse that and review the inspection results; carefully review the inspection settings, and consider disabling the inspections that irreconcilably clash with your preferences: use meaningful names alone can easily produce hundreds upon hundreds of results if you’re not that much into using vowels, or if you, say, prefix all your variable names; these inspections can be re-enabled anytime you’re ready!
These inspections look for a wide range of potential issues, from missing Option Explicit to undeclared or unused variables.
Flags Rubberduck annotations used in a component type that is incompatible with that annotation.
Default severity: Warning
Some annotations can only be used in a specific type of module; others cannot be used in certain types of modules.
Warns about late-bound WorksheetFunction calls made against the extended interface of the Application object.
Default severity: Suggestion
An early-bound, equivalent function exists in the object returned by the Application.WorksheetFunction property; late-bound member calls will fail at run-time with error 438 if there is a typo (a typo fails to compile for an early-bound member call); given invalid inputs, these late-bound member calls return a Variant/Error value that cannot be coerced into another type. The equivalent early-bound member calls raise a more VB-idiomatic, trappable runtime error given the same invalid inputs: trying to compare or assign a Variant/Error to another data type will throw error 13 "type mismatch" at run-time. A Variant/Error value cannot be coerced into any other data type, be it for assignment or comparison.
This inspection will only run if the Excel library is referenced.
Locates arguments passed to functions or procedures for object parameters which the do not have a compatible declared type.
Default severity: Warning
The VBA compiler does not check whether different object types are compatible. Instead there is a runtime error whenever the types are incompatible.
Warns about parameters passed by value being assigned a new value in the body of a procedure.
Default severity: Warning
Debugging is easier if the procedure's initial state is preserved and accessible anywhere within its scope. Mutating the inputs destroys the initial state, and makes the intent ambiguous: if the calling code is meant to be able to access the modified values, then the parameter should be passed ByRef; the ByVal modifier might be a bug.
Warns about a local variable that is assigned and never read. Or, warns about a local variable that is assigned and then re-assigned before using the previous value.
Default severity: Suggestion
An assignment that is never used is a meaningless statement that was likely used by an execution path in a prior version of the module.