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

Question

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 s

Answer 1

this.recieptDateTimePicker.SelectedIndex = -1; this.dateCompoBox.SelectedIndex = -1;

Answer 2

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.

Answer 3

I had a similar challenge. As part of the form load event I set the SelectedIndex of the control to -1

ie

private void Form1_Load(object sender, EventArgs e)
{         
    this.TableAdapter.Fill(this.dsListOfCampaigns.EvolveCampaignTargetListMasterInfo);
    this.comboCampaignID.SelectedIndex = -1;
}

Effectively, the combo box is populated and the first item is selected. Then the item is unselected. May not be a viable solution for all cases.

Answer 4

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 5

Instead of adding a new row to your datatable, just bind the data to the combobox and at load set the SelectedIndex to -1. This will cause the selection to be null until the user selects an item.

I clipped this from one of my current projects.

        Attorney_List_CB.DataSource = DA_Attorney_List.BS.DataSource;
        Attorney_List_CB.DisplayMember = "Attorney Name";
        Attorney_List_CB.SelectedIndex = -1;      

In order to clear the selection I usually insert a button that sets the SelectedIndex back to -1.

    private void Clear_Selection_BTN_Click(object sender, EventArgs e)
    {
        Attorney_List_CB.SelectedIndex = -1;  // Clears user selection
    }

Finally, once I validate the data on my form, if the SelectedIndex of any combobox is -1 then it is skipped, or I will generate some type of default value such as "N/A" or whatever I need under the circumstances.

Answer 6

I have found this way:

    DataTable hierarchies = new DataTable(); 

    cmbHierarchies.BeginUpdate();
    cmbHierarchies.ValueMember = this.Value;
    cmbHierarchies.DisplayMember = this.Display;
    hierarchies = DataView.ToTable();
    cmbHierarchies.DataSource = table;
    cmbHierarchies.EndUpdate();

    //Add empty row
    DataRow row = table.NewRow();
    table.Rows.InsertAt(row, 0);
    cmbHierarchies.SelectedIndex = 0;