ReSharper 2021.3 Help

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.

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.

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"); } }
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:

Last modified: 04 January 2022