Making Sudoku in Unity3D. It's more of a quick project so I've started with the backend before going on to the UI. This code hasn't been fully tested but I'll be updating it as time goes on.

SudokuGameManager.cs

using System.Collections;
using System.Collections.Generic;
using System.Linq;
using UnityEngine;

public class SudokuGameManager : MonoBehaviour
{
    public Transform display;
    public GameObject numberPrefab;
    
    public List<List<BoardTile>> board= new List<List<BoardTile>>();
    private List<List<BoardTile>> m_flippedBoard = new List<List<BoardTile>>(); //for checking 
    private  List<List<BoardTile>> m_squaresToArrays = new List<List<BoardTile>>(); //for checking

    public int boardSize = 9;
    public float prefabSize = 100;


    // Start is called before the first frame update
    void Start()
    {
        StartGame();
    }

    public void StartGame()
    {
        CreateBoard();        
    }

    void CreateBoard()
    {

        for(int x = 0; x < boardSize; x++)
        {
            for(int y = 0; y < boardSize; y++)
            {
                //data storage
                board[x][y].value = 0;

                //physical board
                GameObject num = Instantiate(numberPrefab, display);
                num.transform.position = new Vector2(x, y) * prefabSize;
                board[x][y].displayObj = num;
            }
        }

        display.transform.localPosition = new Vector2(450, 58);
    }

    void FlipBoard()
    {
        //flip the board for easy checking
        m_flippedBoard = board;    
        for(int i =0; i < board.Count; i++)
        {
            for(int j = 0; j < board[i].Count; j++)
            {
                m_flippedBoard[j][i] = board[i][j];
            }
        }
    }

    void TurnSquaresToArrays()
    {
        //TODO: turn 3x3 squares into arrays for helper functions
    }

    void CheckAll()
    {    
        FlipBoard();
        TurnSquaresToArrays();

        // check the sum on each row
        CheckSumsOfRows();

        // check the sum on each column
        CheckSumsOfColumns();

        // check for sum on each box
        // TODO: box to list

        // check for duplicate numbers on each row
        CheckRowsForDuplicates();

        // check for duplicate numbers on each column
        CheckColumnsForDuplicates();

        // check for duplicate numbers on each box
        // TODO: box to list

    }

    bool CheckSumsOfRows()
    {
        for(int i =0; i < board.Count; i++)
        {
            if(CheckSum(board[i]) == false)
            {
                return false;
            }
        }
        return true;
    }

    bool CheckSumsOfColumns()
    {
        for(int i =0; i < m_flippedBoard.Count; i++)
        {
            if(CheckSum(m_flippedBoard[i]) == false)
            {
                return false;
            }
        }
        return true;
    }

    bool CheckRowsForDuplicates()
    {
        for(int i =0; i < board.Count; i++)
        {
            if(CheckForDuplicates(board[i]) == false)
            {
                return false;
            }
        }
        return true;
    }

    bool CheckColumnsForDuplicates()
    {
        for(int i =0; i < m_flippedBoard.Count; i++)
        {
            if(CheckForDuplicates(m_flippedBoard[i]) == false)
            {
                return false;
            }
        }
        return true;
    }

    //checks if the sum of array is 45
    bool CheckSum(List<BoardTile> _list)
    {
        int sum = 0;
        foreach(BoardTile i in _list) sum += i.value; 
        return sum != 45 ? false : true;
    }

    //checks if there is a duplicate value in the array
    bool CheckForDuplicates(List<BoardTile> _list)
    {
        List<int> values = new List<int>();
        foreach(BoardTile b in _list)
            values.Add(b.value);

        List<int> distinct = values.Distinct().ToList();
        return (distinct.Count != _list.Count) ? false : true;
    }
}


[System.Serializable]
public class BoardTile
{
    public int value;
    public GameObject displayObj;
}