How to insert 'Empty' field in ComboBox bound to DataTable

Question

This is frustrating, as I am sure I have done this before without issue.

I have a combo box on a WinForms app in which an item may be selected, but it is not mandatory. I therefore need an 'Empty' first item to indicate that no value has been set.

The combo box is bound to a DataTable being returned from a stored procedure (I offer no apologies for Hungarian notation on my UI controls :p ):

 DataTable hierarchies = _database.GetAvailableHierarchies(cmbDataDefinition.SelectedValue.ToString()).Copy();//Calls SP
 cmbHierarchies.DataSource = hierarchies;
 cmbHierarchies.ValueMember = "guid";
 cmbHierarchies.DisplayMember = "ObjectLogicalName";

How can I insert such an empty item?

I do have access to change the SP, but I would really prefer not to 'pollute' it with UI logic.

Update: It was the DataTable.NewRow() that I had blanked on, thanks. I have upmodded you all (all 3 answers so far anyway). I am trying to get the Iterator pattern working before I decide on an 'answer'

Update: I think this edit puts me in Community Wiki land, I have decided not to specify a single answer, as they all have merit in context of their domains. Thanks for your collective input.

Answer 1

Cant you add a new DataRow to the DataTable before you bind it to your DataSource?

You can use the NewRow function of the DataTable to achieve this:

http://msdn.microsoft.com/en-us/library/system.data.datatable.newrow.aspx

Answer 2

insert a blank row in your datatable, and check for it in validation/update/create

Answer 3

I usually create an iterator for this type of thing. It avoids polluting your data, and works well with data-binding:

DataTable hierarchies = _database.GetAvailableHierarchies(cmbDataDefinition.SelectedValue.ToString()).Copy();//Calls SP
cmbHierarchies.DataSource = GetDisplayTable(hierarchies);
cmbHierarchies.ValueMember = "guid";
cmbHierarchies.DisplayMember = "ObjectLogicalName";

...

private IEnumerable GetDisplayTable(DataTable tbl)
{
    yield return new { ObjectLogicalName = string.Empty, guid = Guid.Empty };

    foreach (DataRow row in tbl.Rows)
        yield return new { ObjectLogicalName = row["ObjectLogicalName"].ToString(), guid = (Guid)row["guid"] };
}

Disclaimer: I have not compiled this code, but have used this pattern many times.

Note: I have been in WPF and ASP.Net land for the last couple of years. Apparently the Winforms combo box wants an IList, not an IEnumerable. A more costly operation would be to create a list. This code is really stream-of-conciseness and I really, really have not compiled it:

DataTable hierarchies = _database.GetAvailableHierarchies(cmbDataDefinition.SelectedValue.ToString()).Copy();
List<KeyValuePair<string, Guid>> list = new List<KeyValuePair<string, Guid>>(hierarchies.Rows.Cast<DataRow>().Select(row => new KeyValuePair<string, Guid>(row["Name"].ToString(), (Guid)row["Guid"])));
list.Insert(0, new KeyValuePair<string,Guid>(string.Empty, Guid.Empty));
cmbHierarchies.DataSource = list;
cmbHierarchies.ValueMember = "Value";
cmbHierarchies.DisplayMember = "Key";

Answer 4

There are two things you can do:

1. Add an empty row to the DataTable that is returned from the stored procedure.

   DataRow emptyRow = hierarchies.NewRow();
   emptyRow["guid"] = "";
   emptyRow["ObjectLogicalName"] = "";
   hierarchies.Rows.Add(emptyRow);
   

   Create a DataView and sort it using ObjectLogicalName column. This will make the newly added row the first row in DataView.

   DataView newView =           
        new DataView(hierarchies,       // source table
        "",                             // filter
        "ObjectLogicalName",            // sort by column
        DataViewRowState.CurrentRows);  // rows with state to display
   

   Then set the dataview as DataSource of the ComboBox.

2. If you really don't want to add a new row as mentioned above. You can allow the user to set the ComboBox value to null by simply handling the "Delete" keypress event. When a user presses Delete key, set the SelectedIndex to -1. You should also set ComboBox.DropDownStyle to DropDownList. As this will prevent user to edit the values in the ComboBox.

Answer 5

Er, can't you just add a default item to the ComboBox after data binding?

Answer 6

I would bind the data then insert an blank item at position 0 using the ComboxBox.Items.Insert method. Similar to what flipdoubt suggested, but it adds the item to the top.

Answer 7

cmbHierarchies.SelectedIndex = -1;

Answer 8

I wrote this method based on the suggestions here by Jason Jackson:

private IEnumerable<KeyValuePair<object,object>> GetDisplayTable(DataTable dataTable,  DataColumn ValueMember, string sep,params DataColumn[] DisplayMembers)
{
    yield return new KeyValuePair<object,object>("<ALL>",null);

    if (DisplayMembers.Length < 1)
        throw new ArgumentException("At least 1 DisplayMember column is required");

    foreach (DataRow r in dataTable.Rows)
    {
        StringBuilder sbDisplayMember = new StringBuilder();
        foreach(DataColumn col in DisplayMembers)
        {
            if (sbDisplayMember.Length > 0) sbDisplayMember.Append(sep);
            sbDisplayMember.Append(r[col]);
        }
        yield return new KeyValuePair<object, object>(sbDisplayMember.ToString(), r[ValueMember]);
    }
}

Usage:

bindingSource1.DataSource = GetDisplayTable(
            /*DataTable*/typedDataTable, 
            /*ValueMember*/typedDataTable.IDColumn, 
            /*DisplayColumn Seperator*/" - ",
            /*List of Display Columns*/
            typedDataTable.DB_CODEColumn,
            typedDataTable.DB_NAMEColumn);

comboBox1.DataSource = bindingSource1;
comboBox1.DisplayMember = "Key";
comboBox1.ValueMember = "Value";

//another example without multiple display data columns:
bindingSource2.DataSource = GetDisplayTable(
            /*DataTable*/typedDataTable, 
            /*ValueMember*/typedDataTable.IDColumn, 
            /*DisplayColumn Seperator*/null,
            /*List of Display Columns*/
                typedDataTable.DESCColumn );

further down, where the Selected Value is consumed:

if (comboBox1.SelectedValue != null)
     // Do Something with SelectedValue   
else 
     // All was selected (all is my 'empty')

This will allow to display several columns concatenated in the ComboBox, while keeping the Value member to the single identifier + it uses the iterator block with the BindingSource, BindingSource might be overkill for your situation.

Сomments and suggestions are welcome.