TheAlgorithms/C-Sharp · error · InvalidOperationException

Vertex already exists

Error message

Vertex already exists: {currentEdgeWeight}

What it means

ThrowIfEdgeExists throws when AddEdge is called for a vertex pair that already has an edge. The (misleading) message interpolates the existing edge's weight, e.g. 'Vertex already exists: 5'. The library does not silently overwrite edges; callers must remove the edge first.

Solutions

1. Check first: if (!AreAdjacent(u, v)) AddEdge(u, v, w); else RemoveEdge then AddEdge.
2. Deduplicate edge input before insertion (distinct on the vertex pair).
3. Wrap in try/catch for InvalidOperationException when re-adding is expected/ignorable.

Example fix

// before
graph.AddEdge(a, b, 2.0);
graph.AddEdge(a, b, 3.0); // throws
// after
if (graph.AreAdjacent(a, b)) graph.RemoveEdge(a, b);
graph.AddEdge(a, b, 3.0);

if (!graph.AreAdjacent(u, v)) { graph.AddEdge(u, v, w); }

try { graph.AddEdge(u, v, w); }
catch (InvalidOperationException ex) when (ex.Message.StartsWith("Vertex already exists:")) { /* edge already present: ignore or update */ }

When it happens

Trigger: Calling AddEdge(u, v, w) twice for the same vertex pair; the second call finds the current matrix weight non-zero and throws.

Common situations: Re-running graph-building code without clearing state, importing edge lists containing duplicate edges, or trying to update an edge's weight via AddEdge instead of RemoveEdge-then-AddEdge.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

• Vertex does not belong to graph
• Graph contains a negative weight cycle.
• Matrix must be symmetric!
• Graph must be undirected!
• Adjacency matrix must be square!

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/de8a5eb3d74dfc53. Report an issue: GitHub.

Appendix: source

Thrown at DataStructures/Graph/DirectedWeightedGraph.cs:214

        if (capacity < 0)
        {
            throw new InvalidOperationException("Graph capacity should always be a non-negative integer.");
        }
    }

    private static void ThrowIfWeightZero(double weight)
    {
        if (weight.Equals(0.0d))
        {
            throw new InvalidOperationException("Edge weight cannot be zero.");
        }
    }

    private static void ThrowIfEdgeExists(double currentEdgeWeight)
    {
        if (!currentEdgeWeight.Equals(0.0d))
        {
            throw new InvalidOperationException($"Vertex already exists: {currentEdgeWeight}");
        }
    }

    private void ThrowIfOverflow()
    {
        if (Count == capacity)
        {
            throw new InvalidOperationException("Graph overflow.");
        }
    }

    private void ThrowIfVertexNotInGraph(Vertex<T> vertex)
    {
        if (vertex.Graph != this)
        {
            throw new InvalidOperationException($"Vertex does not belong to graph: {vertex}.");
        }
    }

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