Change null value appearance on DataGrid when binding to DataTable

Question

I have DataGrid, which binds to DataTable, because i don't know the amount of columns at compile time.

XAML

<Window.Resources>
    <wpfApplication1:MainWindowViewModel x:Key="ViewModel" />
</Window.Resources>

<Grid DataContext="{StaticResource ViewModel}">
    <DataGrid x:Name="MyDataGrid" 
              ItemsSource="{Binding Path=DataTable}" />
</Grid>

MainWindowViewModel.cs

class MainWindowViewModel : INotifyPropertyChanged
{
    private DataTable dt;
    public event PropertyChangedEventHandler PropertyChanged;

    public MainWindowViewModel()
    {
        DataTable table = new DataTable();
        table.Columns.Add("Foo", typeof(int));
        table.Columns.Add("Bar", typeof(string));
        table.Rows.Add(1, "first");
        table.Rows.Add(2, null);
        DataTable = table;
    }

    private void OnPropertyChanged(string s)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(s));
        }
    }

    public DataTable DataTable
    {
        get { return dt; }
        set
        {
            dt = value;
            OnPropertyChanged("DataTable");
        }
    }
}

The result is:

simple table

There is nothing in the cell with null value. My question is: How can i change all the cells with null value to '-' without changing the source DataTable.

I can't use converter in binding because it will try to convert all the DataTable, not the values. I had no luck with DataGridCell styles also because i don't know how to get the data value and change it.

Edit

My final solution is based on one of the answers. I create MyDataGrid class and override OnAutoGeneratingColumn method.

class MyDataGrid : DataGrid
{
    protected override void OnAutoGeneratingColumn(DataGridAutoGeneratingColumnEventArgs e)
    {
        var textColumn = e.Column as DataGridTextColumn;

        if (textColumn != null)
        {
            var binding = textColumn.Binding;
            binding.TargetNullValue = "-";
        }

        base.OnAutoGeneratingColumn(e);
    }
}

I am just guessing here...can we inherit DataTable and use this new DataTable type for binding? — Savaratkar, Apr 02 '14 at 06:43
oh sry...the answer below is the best approach...i went too far for this solution :P my bad... — Savaratkar, Apr 02 '14 at 09:19

score 2 · Answer 1 · edited Apr 02 '14 at 07:16

2

Try to change the null value in datatable as shown below:

if (dt != null)
{
    foreach (DataRow row in dt.Tables[0].Rows)
    {
        foreach (DataColumn col in dt.Tables[0].Columns)
        {
            string type = col.DataType.Name;

            if (row.IsNull(col))
            {
                if (type == "String") //for string values
                {
                    row.SetField(col, "-");
                }
                else // for integer
                {
                    row.SetField(col, 0);
                }
            }

        }
    }
}

edited Apr 02 '14 at 07:16

Martin Prikryl

188,800
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490
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answered Apr 02 '14 at 06:24

Saritha.S.R

800
1
6
19

Thanks, this is one way to do it, but i was trying to find a solution without changing the DataTable if it is possible. – Dmitrii Dovgopolyi Apr 02 '14 at 06:29
You can do the same in grid cell value and replace – Saritha.S.R Apr 02 '14 at 06:31
I would like to stay inside MVVM pattern, and not use some manual editing – Dmitrii Dovgopolyi Apr 02 '14 at 06:46

Anatoliy Nikolaev · Answer 2 · 2014-04-02T09:12:48.980

You can try use this function:

private string SomeString = null;

...

table.Rows.Add(2, ReturnNullValue(SomeString, "---"));

private string ReturnNullValue(string Value, string NullValue) 
{
    if (String.IsNullOrEmpty(Value) == true)
    {
        return NullValue;
    }

    return Value;
}

Or you can add the property with additional logic:

private string _someString = "";

public string SomeString
{
    get
    {
        return _someString;
    }

    set
    {
        if (String.IsNullOrEmpty(value) == false)
        {
            _someString = value;
        }
        else
        {
            _someString = "---";
        }
    }
}

And use like this:

SomeString = "";
table.Rows.Add(2, SomeString);

I think we can do it without editing DataTable, that is no simple and quick solution, because Converter, Style and TargetNullValue powerless here.

Thanks, this is one way to do it, but i was trying to find a solution without changing the DataTable if it is possible. — Dmitrii Dovgopolyi, Apr 02 '14 at 06:29

score 1 · Accepted Answer · edited May 23 '17 at 11:57

1

You could subscribe to the AutoGeneratingColumn-Event and replace the Binding with one, which has FallbackValue/TargetNullValue set to your liking. For cloning i used this.

    private void OnAutoGeneratingColumn(object sender, DataGridAutoGeneratingColumnEventArgs e)
    {
        DataGridTextColumn textColumn;
        if ((textColumn = e.Column as DataGridTextColumn) != null)
        {
            var binding = textColumn.Binding;
            var clone = binding.CloneBinding();
            clone.FallbackValue = "-----";
            clone.TargetNullValue = "-----";
            textColumn.Binding = clone;
        }
    }

edited May 23 '17 at 11:57

Community

1
1

answered Apr 02 '14 at 08:24

user3411327

1,031
8
14

That was a great idea, thanks. I create `MyDataTable:DataTable` and modified `OnAutoGeneratingColumn` method based on your approach. – Dmitrii Dovgopolyi Apr 02 '14 at 09:00
1

Yet i have one question: why do you clone the binding object? – Dmitrii Dovgopolyi Apr 02 '14 at 09:06
1

Ah, right. In this scenario this is not necessary. If the binding was obtained by BindingOperations.GetBinding() it would throw an exception when trying to change a value of the binding. Since the binding here is just set to a CLR-Property you don't need to clone it. – user3411327 Apr 02 '14 at 09:42

score 0 · Answer 4 · edited Apr 02 '14 at 07:18

0

Maybe you can create a ViewModel class that inherits DataTable or just keeps a reference of that DataTable class. That ViewModel class is what you bind to the DataGrid's ItemSource. The ViewModel should then know what data it should or shouldn't convert (nulls to "-") and return to the DataGrid. That way the DataTable remains unchanged.

edited Apr 02 '14 at 07:18

The Heist

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answered Apr 02 '14 at 06:53

bjutus

11
3

Change null value appearance on DataGrid when binding to DataTable

4 Answers4