Why is my jagged Array null once I initialize it again?

Question

I've been creating a Sudoku Solver in C# using Wpf as View. In the Constructor of the MainViewModel I have this code.

public MainViewModel()
{
    SudokuTable = new char[9][];
    for (int i = 0; i < 9; i++)
    {
        SudokuTable[i] = new char[9]
          { ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ' };
     }
     Solve = new RelayCommand(o => SudokuStart());
     Reset = new RelayCommand(o => SudokuReset());
}

The RelayCommands are linked to Commands of 2 Buttons inside my View. The SudokuTable is bound to an Grid of Textboxes. Each Textbox is linked to a different jagged array Element. Example:

 <TextBox Grid.Row = "0" Grid.Column = "0" Style = "{StaticResource GridTextBox}" Text = "{Binding Path=SudokuTable[0][0]}" />

As people might want to solve multiple Sudokus, I implemented a "Reset" Button. Once it's clicked it does this:

private void SudokuReset()
{
    SudokuTable = new char[9][];
    for (int i = 0; i < 9; i++)
    {
        SudokuTable[i] = new char[9] 
          { ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ' };
     }
 }

So basically it does everything it would when starting to Application. But for some reason all textboxes are empty as soon as I click on the Reset-Button. They don't contain ' ' which causes a NullReferenceException. Can anyone tell me the difference between the first Initialization and the reset of it? What am I doing wrong?

I'm thankful for all answers.

Edit as someone asked about my Property:

        private char[][] sudokuTable;
        public char[][] SudokuTable
        {
            get { return sudokuTable; }
            set
            {
                if (sudokuTable != value)
                {
                    sudokuTable = value;
                    OnPropertyChanged("SudokuTable");
                }
            }
        }

You should probably check the Designer.cs file and see what C# code is being generated from your designer. I suspect that the binding doesn't do what you think it does. Or it will need an updatetrigger because the char you are referencing isn't doing any notifypropertychange stuff. — Tormod
– Tormod, Commented May 13, 2020 at 9:05

Sven-Michael Stübe · Accepted Answer · 2020-05-13 09:20:24Z

1

This is because the PropertyChanged event is triggered in the line SudokuTable = new char[9][]; Everything you do with the array afterwards is not noticed by the UI / binding.

If you first initialize the jagged array and then set it, it should behave as you expect.

var newTable = new char[9][];
for (int i = 0; i < 9; i++)
{
    newTable[i] = new char[9]
      { ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ' };
}

SudokuTable = newTable;

Why does it behave different to the constructor?

This is because your constructor is executed and then your view model gets set as the BindingContext. And changing a binding context triggers reevaluation of the bindings.

answered May 13, 2020 at 9:20

Sven-Michael Stübe

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1 Comment

Papriker Over a year ago

Thank you! I actually didn't know this.

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