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