Converting jagged array to 2D array C#

Question

I'm trying to convert this function from Jagged Array to 2D array, and I'm not able to convert everything Original Function:

public static double[][] InvertMatrix(double[][] A)
{
    int n = A.Length;
    //e will represent each column in the identity matrix
    double[] e;
    //x will hold the inverse matrix to be returned
    double[][] x = new double[n][];
    for (int i = 0; i < n; i++)
    {
        x[i] = new double[A[i].Length];
    }
    /*
    * solve will contain the vector solution for the LUP decomposition as we solve
    * for each vector of x.  We will combine the solutions into the double[][] array x.
    * */
    double[] solve;

    //Get the LU matrix and P matrix (as an array)
    Tuple<double[][], int[]> results = LUPDecomposition(A);

    double[][] LU = results.Item1;
    int[] P = results.Item2;

    /*
    * Solve AX = e for each column ei of the identity matrix using LUP decomposition
    * */
    for (int i = 0; i < n; i++)
    {
        e = new double[A[i].Length];
        e[i] = 1;
        solve = LUPSolve(LU, P, e);
        for (int j = 0; j < solve.Length; j++)
        {
            x[j][i] = solve[j];
        }
    }
    return x;
}

What I have converted until now:

public static double[,] InvertMatrix(double[,] A)
{
    int n = A.Length;
    //e will represent each column in the identity matrix
    double[] e;
    //x will hold the inverse matrix to be returned
    double[,] x = new double[n][];
    for (int i = 0; i < n; i++)
    {
        //how to convert this line?
        x[i] = new double[A[i].Length];
    }
    /*
    * solve will contain the vector solution for the LUP decomposition as we solve
    * for each vector of x.  We will combine the solutions into the double[][] array x.
    * */
    double[] solve;

    //Get the LU matrix and P matrix (as an array)
    Tuple<double[,], int[]> results = LUPDecomposition(A);

    double[,] LU = results.Item1;
    int[] P = results.Item2;

    /*
    * Solve AX = e for each column ei of the identity matrix using LUP decomposition
    * */
    for (int i = 0; i < n; i++)
    {
        //This one too?!
        e = new double[A[i].Length];
        e[i] = 1;
        solve = LUPSolve(LU, P, e);
        for (int j = 0; j < solve.Length; j++)
        {
            x[j,i] = solve[i,j];
        }
    }
    return x;
}

How to convert x[i] = new double[A[i].Length] to 2D array?

Answer 1

This snippet can be helpful

static T[,] To2D<T>(T[][] source)
{
    try
    {
        int FirstDim = source.Length;
        int SecondDim = source.GroupBy(row => row.Length).Single().Key; // throws InvalidOperationException if source is not rectangular

        var result = new T[FirstDim, SecondDim];
        for (int i = 0; i < FirstDim; ++i)
            for (int j = 0; j < SecondDim; ++j)
                result[i, j] = source[i][j];

        return result;
    }
    catch (InvalidOperationException)
    {
        throw new InvalidOperationException("The given jagged array is not rectangular.");
    } 
}

Usage:

double[][] array = { new double[] { 52, 76, 65 }, new double[] { 98, 87, 93 }, new double[] { 43, 77, 62 }, new double[] { 72, 73, 74 } };
double[,] D2 = To2D(array);

UPD: For those scenarios where unsafe context is acceptable there is a faster solution, thanks Styp: https://stackoverflow.com/a/51450057/3909293