Code Inspection: Parameter output value is always discarded (non-private accessibility)
The C# 6.0 "out var" greatly reduces the amount of code needed to use methods with
out parameters — now we almost always declare variables inline and often use type inference to omit variable type:
dictionary.TryGetValue(key, out var value).
Another aspect of
out parameters that C# 6.0 has improved is the ability to ignore the output parameter values that we don't need with
M(out _) or
M(out var _) syntax. So it's not uncommon to see invocations like
DoSomething(arg, out _, out _) in modern C# codebase.
JetBrains Rider analyzes such invocations and reports
out parameters that are discarded on all call sites. This helps identify potential problems of a value being always discarded and simplify the signature by removing the unused parameters.
For the solution-wide inspection to work, you need to enable at least one of the following:
Simplified global usage checking: select Show unused non-private type members when solution-wide analysis is off on the page of JetBrains Rider settings Ctrl+Alt+S.
Solution-wide analysis: select Enable solution-wide analysis on the page of JetBrains Rider settings Ctrl+Alt+S.
Note that even if the reported parameter has no direct usages in your solution, there could be cases where it is used indirectly — for example, via reflection — or it could just be designed as public API. In all those cases you would want to suppress the usage-checking inspection for the parameter in one of the following ways:
The recommended way is to decorate the implicitly used parameter with a code annotation attribute. There are two attributes for this purpose: [UsedImplicitly] and [PublicAPI], which are functionally similar, but let you and your teammates understand how the parameter is actually used.
You can also suppress usage-checking inspections with any custom attribute. To do so, mark the definition of that attribute with the [MeansImplicitUse] attribute.
And finally, you can suppress a specific usage-checking inspection as any other code inspection with a suppression comment or a suppression attribute.