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 Test
  {
    private Dictionary<string, int> _set = new();

    // Parameter 'value' output value is always discarded
    private bool HasKey(string key, out int value)
    {
      return _set.TryGetValue(key, out value);
    }

    private void Usage(SetOfThings set)
    {
      if (set.HasKey("abc", out _))
        Console.WriteLine("do something");
    }
  }

        

  class Test
  {
    private Dictionary<string, int> _set = new();

    private bool HasKey(string key)
    {
      int value;
      return _set.TryGetValue(key, out value);
    }

    private void Usage(SetOfThings set)
    {
      if (set.HasKey("abc"))
        Console.WriteLine("do something");
    }
  }

        

Last modified: 04 January 2022