Code inspection: Parameter output value is always discarded (non-private accessibility)
Category: Redundancies in Symbol Declarations
ID: OutParameterValueIsAlwaysDiscarded.Global
EditorConfig: resharper_out_parameter_value_is_always_discarded_global_highlighting=[error|warning|suggestion|hint|none]
Language: C#, VB.NET
Requires SWA: Yes
tip
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.
ReSharper 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.
Suboptimal code
class SetOfThings{ private Dictionary<string, int> _set = new(); // Parameter 'value' output value is always discarded public bool HasKey(string key, out int value) { return _set.TryGetValue(key, out value); }}class Test{ public void Usage(SetOfThings set) { if (set.HasKey("abc", out _)) Console.WriteLine("do something"); }}
After the quick-fix
class SetOfThings{ private Dictionary<string, int> _set = new(); public bool HasKey(string key) { int value; return _set.TryGetValue(key, out value); }}class Test{ public void Usage(SetOfThings set) { if (set.HasKey("abc")) Console.WriteLine("do something"); }}
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 Code Inspection | Settings page of ReSharper options Alt+R, O.
Solution-wide analysis: select Enable solution-wide analysis on the Code Inspection | Settings page of ReSharper options Alt+R, O.
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.