SonarLint for Visual Studio: Useless Assignment warning for value captured in local function

  • SonarLint for Visual Studio 2019, v4.26.0.22454
  • Sample code:
/// <summary>
/// Stupid example, but enough to demonstrate issue.
/// Actual use case was using Dapper to map 1:N relationship of actual POCO types
/// An example of this (but with an anonymous function) is found at
/// But I didn't want my example to have dependencies!
/// </summary>
/// <returns></returns>
public List<List<TValue>> GroupValues<TKey, TValue>(IEnumerable<ValueTuple<TKey, TValue>> inputs)
    // example input: new List<(string, int)> { ("A", 1), ("A", 2), ("B", 3), };
    // example output: new List<List<int>> { new List<int> { 1, 2, }, new List<int> { 3, } };

    // Below line has S1854 warning ("useless assignment"), but it is actally captured and used in MapTuple function below
    var workingMap = new Dictionary<TKey, List<TValue>>();
    return inputs

    // Obvious workaround (other than ignoring) would be to make this static and pass in workingMap, but that's not a clean option when passing as a Func<...> parameter expecting a certain signature
    List<TValue> MapTuple((TKey key, TValue value) input)
        if (!workingMap.TryGetValue(input.key, out var workingValue))
            workingValue = new List<TValue>();
            workingMap.Add(input.key, workingValue);

        return workingValue;

Not suggesting this is the cleanest way to implement something like this, but the S1854 warning is definitely invalid since the value assigned to workingMap is used.

Hi @hikarikuen,

Welcome to our community! Thank you for reporting this case, I can confirm it’s a False Positive.

We already have similar case reported in this issue. You can subscribe to it to track the progress.

Thanks for identifying that, Pavel! I will follow that issue.

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