How do I allow a null value when binding to a numeric field in a datatable?

Question

I have a winforms application with a numeric field on a form. This starts out null then if I set it to a number, change focus then clear again the textbox changes back to the number previously entered. If I change the number eg. from 4 to 5 it is updated correctly but I want the user to be able to clear what has been entered.

Here is some sample code that demonstrates this behaviour:

public partial class Form1 : Form
{
    DataTable table = new DataTable("TableName");
    public Form1()
    {
        DataColumn column = new DataColumn("NumericColumnName", typeof(Double));
        column.AllowDBNull = true;
        table.Columns.Add(column);
        object[] rowData = new object[1];
        rowData[0] = DBNull.Value;
        table.Rows.Add(rowData);

        InitializeComponent();

        BindingSource bindingSource = new BindingSource();
        bindingSource.DataSource = table;
        Binding binding = new Binding("Text", bindingSource, "NumericColumnName");
        textBox1.DataBindings.Add(binding);
    }
}

This is in a brand new .net 3.5 forms project in Visual Studio 2008. I added two textboxes to the form.

My actual application generates the dataset differently however it has the same behaviour. Do bindings to numeric columns of a datatable allow nulls.

Solution 1

In fact You can allow NULL values, however You have to add additional parsing to the code:

binding.Parse += new ConvertEventHandler(ParseNumeric);

where ParseNumeric has the following form:

private void ParseNumeric(object sender, ConvertEventArgs e)
{
    if(e.Value.Equals(""))
        e.Value = DBNull.Value;
}

for me this code allows empty texts in text boxes which are then represented by NULL values in database.

Solution 2

By default double is a non-nullable field. So even if you put a null in it will change to 0.

You need to change the column datatype to be double?. double? is a nullable type.

UPDATE

Upon further looking, typed datasets do not support nullable types.

A solution is to use a 'loose' type (IE: object) but then you will have to check it to make sure the input is valid.

I tested with your sample and change it to object allowed me to empty the box.

Comments

[Adam Butler]

If I change the example code to say typeof(Double?) I get a "DataSet does not support System.Nullable<>." exception