for (int i = 0; i < x; i++)
{
    for (int j = 0; j < y; j++)
    {
        Console.Write("mas[" + i + "," + j + "]: ");
        mas[i, j] = int.Parse(Console.ReadLine());
    }
}
Console.WriteLine();

using System;
using System.Linq;

int[,] array = new int[x, y];
Console.Write("Enter the matrix in one line: ")
int[] numbers = Console.ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();
for (int i = 0; i < x; i++)
    for (int j = 0; j < y; j++)
        array[i, j] = numbers[i * x + j];

using System;
using System.Linq;

int[,] array = new int[x, y];
for (int i = 0; i < x; i++)
{
    Console.Write("Enter the line (i = {0}): ", i);
    int[] numbers = Console.ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();
    for (int j = 0; j < y; j++)
        array[i, j] = numbers[j];
}