How to validate user's input in a sudoku's game using C#

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C#

Windows

.NET

I am trying to implement a Sudoku game, i have been able to populate the game control with values, but i have a challenge validating input fields, i want to be able to validate and display to the user when it gets it wrong or right.

the rule of the game is that, number one to 4 must not appear more than once in a row, column and block.

Any assistance would be appreciated.

What I have tried:

private void Populate()
    {
        List<string> num = new List<string>();

        foreach (string line in File.ReadLines("Sudoku Text File.txt"))
        {
            num.Add(line);
        }
        Random rnd = new Random();
        rand = rnd.Next(1, 5);
        var number = num[rand];
        listBox1.Items.Add(number);

        for (int i = 0; i < number.Length; i++)
        {

            if (number[i] == '0')
            {
                panel1.Controls[i].Text = "";
            }
            else
            {
                panel1.Controls[i].Text = number[i].ToString();
                panel1.Controls[i].Enabled = false;
            }               
        }

    }

the Sudoku text file has these values:

4000001002004021
0200002030204001
0040010220000423
4000000220041400
0200103201200001