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.

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 Editor | Inspection Settings page of JetBrains Rider settings Control+Alt+S.
Solution-wide analysis: select Enable solution-wide analysis on the Editor | Inspection Settings page of JetBrains Rider settings Control+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.

Last modified: 04 January 2022