Topological Sorting in C# from http://tawani.blogspot.com/2009/02/topological-sorting-and-cyclic.html?m=1

topological.cs

class TopologicalSorter
{
    #region - Private Members -

    private readonly int[] _vertices; // list of vertices
    private readonly int[,] _matrix; // adjacency matrix
    private int _numVerts; // current number of vertices
    private readonly int[] _sortedArray;

    #endregion

    #region - CTors -

    public TopologicalSorter(int size)
    {
        _vertices = new int[size];
        _matrix = new int[size, size];
        _numVerts = 0;
        for (int i = 0; i < size; i++)
            for (int j = 0; j < size; j++)
                _matrix[i, j] = 0;
        _sortedArray = new int[size]; // sorted vert labels
    }

    #endregion

    #region - Public Methods -

    public int AddVertex(int vertex)
    {
        _vertices[_numVerts++] = vertex;
        return _numVerts - 1;
    }

    public void AddEdge(int start, int end)
    {
        _matrix[start, end] = 1;
    }

    public int[] Sort() // toplogical sort
    {
        while (_numVerts > 0) // while vertices remain,
        {
            // get a vertex with no successors, or -1
            int currentVertex = noSuccessors();
            if (currentVertex == -1) // must be a cycle                
                throw new Exception("Graph has cycles");

            // insert vertex label in sorted array (start at end)
            _sortedArray[_numVerts - 1] = _vertices[currentVertex];

            deleteVertex(currentVertex); // delete vertex
        }

        // vertices all gone; return sortedArray
        return _sortedArray;
    }

    #endregion

    #region - Private Helper Methods -

    // returns vert with no successors (or -1 if no such verts)
    private int noSuccessors()
    {
        for (int row = 0; row < _numVerts; row++)
        {
            bool isEdge = false; // edge from row to column in adjMat
            for (int col = 0; col < _numVerts; col++)
            {
                if (_matrix[row, col] > 0) // if edge to another,
                {
                    isEdge = true;
                    break; // this vertex has a successor try another
                }
            }
            if (!isEdge) // if no edges, has no successors
                return row;
        }
        return -1; // no
    }

    private void deleteVertex(int delVert)
    {
        // if not last vertex, delete from vertexList
        if (delVert != _numVerts - 1)
        {
            for (int j = delVert; j < _numVerts - 1; j++)
                _vertices[j] = _vertices[j + 1];

            for (int row = delVert; row < _numVerts - 1; row++)
                moveRowUp(row, _numVerts);

            for (int col = delVert; col < _numVerts - 1; col++)
                moveColLeft(col, _numVerts - 1);
        }
        _numVerts--; // one less vertex
    }

    private void moveRowUp(int row, int length)
    {
        for (int col = 0; col < length; col++)
            _matrix[row, col] = _matrix[row + 1, col];
    }

    private void moveColLeft(int col, int length)
    {
        for (int row = 0; row < length; row++)
            _matrix[row, col] = _matrix[row, col + 1];
    }

    #endregion
}