The BidirectionalMatrixGraph<TVertex, TEdge> provides an efficient data structure to access the out edges and the in edges of a vertex of dense directed graphs with known number of vertices. This class is mutable, serializable, cloneable and can be constructed in many different ways. Internally, the data structure keeps a 2D fixed size array of edges. Does not support multi-edges.

int vertexCount = ...; // must be known a-priori
var graph = new BidirectionalMatrixGraph<int, Edge<int>>(vertexCount);

foreach(var vertex in graph.Vertices)
    foreach(var edge in graph.InEdges(vertex))
        Console.WriteLine(edge);

For sparse graphs, consider using AdjacencyGraph or BidirectionalGraph.