What causes "ArgumentNullException: vertices" in TheAlgorithms/C-Sharp?

Pass a non-empty or empty collection instead of null (e.g. ?? Enumerable.Empty ()).

TheAlgorithms/C-Sharp · error · ArgumentNullException

ArgumentNullException: vertices

Error message

ArgumentNullException: vertices

What it means

Bridges.Find computes all bridges in an undirected graph from a vertex collection and a neighbor function. The library throws ArgumentNullException for 'vertices' when the caller passes a null vertex enumerable, because the algorithm cannot enumerate a null sequence. It is an eager argument guard at the start of the public Find method (Algorithms/Graph/Bridges.cs:26).

Solutions

Pass a non-empty or empty collection instead of null (e.g. ?? Enumerable.Empty<T>()).
Fix the upstream source that produced a null vertex set (check TryGetValue / failed lookups).
Wrap the call in a null check or throw a more descriptive exception in your own code before calling Find.

Example fix

// before
var bridges = Bridges.Find(adjacencyMap?[nodeKey], n => adjacencyMap[n]);
// after
var vertices = adjacencyMap?[nodeKey] ?? Enumerable.Empty<string>();
var bridges = Bridges.Find(vertices, n => adjacencyMap[n]);

Defensive patterns

Strategy: validation

Validate before calling

if (vertices is null)
    throw new ArgumentException("Vertex collection must not be null", nameof(vertices));
var bridges = Bridges.Find(vertices, getNeighbors);

Type guard

static bool HasVertices<T>(IEnumerable<T>? source) => source is not null;

Try / catch

try
{
    var bridges = Bridges.Find(vertices, getNeighbors);
}
catch (ArgumentNullException ex)
{
    // ex.ParamName == "vertices": fix the null vertex source before retrying
    logger.LogError(ex, "Vertex collection was null when computing bridges");
}

Prevention

Never let graph vertex sources come from nullable lookups without coalescing to Enumerable.Empty<T>().
Enable C# nullable reference types so null vertex arguments are caught at compile time.
Validate graph inputs at the boundary where the graph is loaded, not deep in algorithm calls.

When it happens

Trigger: Calling Algorithms.Graph.Bridges.Find<T>(vertices, getNeighbors) with vertices == null, e.g. Find<int>(null, v => graph[v]).

Common situations: Passing a dictionary lookup result that is null (TryGetValue failed), a factory method returning null instead of an empty collection, or wiring up a graph variable that was never initialized before computing bridges.

Related errors

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/ae3f7a812785aa1b. Report an issue: GitHub.

Appendix: source

Thrown at Algorithms/Graph/Bridges.cs:26

/// Finds bridges (cut edges) in an undirected graph.
/// A bridge is an edge whose removal increases the number of connected components.
/// </summary>
public static class Bridges
{
    /// <summary>
    /// Finds all bridges in an undirected graph.
    /// </summary>
    /// <typeparam name="T">Type of vertex.</typeparam>
    /// <param name="vertices">All vertices in the graph.</param>
    /// <param name="getNeighbors">Function to get neighbors of a vertex.</param>
    /// <returns>Set of bridges as tuples of vertices.</returns>
    public static HashSet<(T From, T To)> Find<T>(
        IEnumerable<T> vertices,
        Func<T, IEnumerable<T>> getNeighbors) where T : notnull
    {
        if (vertices == null)
        {
            throw new ArgumentNullException(nameof(vertices));
        }

        if (getNeighbors == null)
        {
            throw new ArgumentNullException(nameof(getNeighbors));
        }

        var vertexList = vertices.ToList();
        if (vertexList.Count == 0)
        {
            return new HashSet<(T, T)>();
        }

        var bridges = new HashSet<(T, T)>();
        var visited = new HashSet<T>();
        var discoveryTime = new Dictionary<T, int>();
        var low = new Dictionary<T, int>();
        var parent = new Dictionary<T, T?>();

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