A data-bound multi-column combobox

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Introduction

I had to write a multi-column combobox at work that supported generic data binding, and I thought it might prove useful for others too.

MultiColumnComboBox

Class usage

Using the class is fairly straightforward. Once you have your data source you just need to set the DataSource property of the

MultiColumnComboBox

ComboBoxStyle.Simple

DrawMode

The class has been tested on Windows XP SP2 as well as on Windows Vista (Ultimate Edition) both from a Visual Studio design perspective as well as from a runtime perspective. Here are some examples of populating the control with various types of data sources. In this first example, we populate it using a DataTable.

// Populate using a DataTable

DataTable dataTable = new DataTable("Employees");

dataTable.Columns.Add("Employee ID", typeof(String));
dataTable.Columns.Add("Name", typeof(String));
dataTable.Columns.Add("Designation", typeof(String));

dataTable.Rows.Add(new String[] { "D1", "Natalia", "Developer" });
dataTable.Rows.Add(new String[] { "D2", "Jonathan", "Developer" });
dataTable.Rows.Add(new String[] { "D3", "Jake", "Developer" });
dataTable.Rows.Add(new String[] { "D4", "Abraham", "Developer" });
dataTable.Rows.Add(new String[] { "T1", "Mary", "Team Lead" });
dataTable.Rows.Add(new String[] { "PM1", "Calvin", "Project Manager" });
dataTable.Rows.Add(new String[] { "T2", "Sarah", "Team Lead" });
dataTable.Rows.Add(new String[] { "D12", "Monica", "Developer" });
dataTable.Rows.Add(new String[] { "D13", "Donna", "Developer" });

multiColumnComboBox1.DataSource = dataTable;
multiColumnComboBox1.DisplayMember = "Employee ID";
multiColumnComboBox1.ValueMember = "Name";

The DisplayMember property will dictate the value that's visible in the edit box part of the combobox. And the ValueMember property will dictate which of the columns will show up in bold. If you look at the screenshots, you can clearly see this in action. In the next example, we use an array of a custom type.

public class Student
{
    public Student(String name, int age)
    {
        this.name = name;
        this.age = age;
    }

    String name;

    public String Name
    {
        get { return name; }
    }

    int age;

    public int Age
    {
        get { return age; }
    }
}

// Populate using a collection

Student[] studentArray = new Student[] 
{ new Student("Andrew White", 10), new Student("Thomas Smith", 10), 
  new Student("Alice Brown", 11), new Student("Lana Jones", 10), 
  new Student("Jason Smith", 9), new Student("Amamda Williams", 11)
};

multiColumnComboBox2.DataSource = studentArray;
multiColumnComboBox2.DisplayMember = multiColumnComboBox2.ValueMember = "Name";

Notice how we've set both DisplayMember and ValueMember to the same column field - this is perfectly okay to do. By the way if you don't set the ValueMember it will use the first column by default. You must set the DisplayMember though, else you'll see some odd strings depending on how a specific type's ToString is implemented. I decided not to provide a default as it would most likely result in non-ideal columns getting displayed. I've used a drop-down list style combobox for my 3rd example and also used a List<> object - though by now it must be pretty obvious to anyone reading this that you can basically use any standard data source.

// Drop-down list (non-editable)

List<Student> studentList = new List<Student>(studentArray);

The main difference in using a drop-down list will be that you'll see the multiple columns even when the combobox is not dropped down. Note that those who want to prevent this behavior can check if DrawItemEventArgs.State has the ComboBoxEdit flag (in the OnDrawItem method) and change the behavior accordingly. For our purposes this behavior was pretty good and I personally thought it to be the more intuitive way to do it. And finally, you can use it without data-binding, though I can't think of any reason why you'd want to do that.

// Trying to use as a regular combobox

multiColumnComboBox4.Items.Add("Cat");
multiColumnComboBox4.Items.Add("Tiger");
multiColumnComboBox4.Items.Add("Lion");
multiColumnComboBox4.Items.Add("Cheetah");
multiColumnComboBox4.SelectedIndex = 0;

Implementation details

One of the first things I did was to hide both the DrawMode and the DropDownStyle properties to prevent users from inadvertently setting unsupported values.

public new DrawMode DrawMode 
{ 
    get
    {
        return base.DrawMode;
    } 
    set
    {
        if (value != DrawMode.OwnerDrawVariable)
        {
            throw new NotSupportedException("Needs to be DrawMode.OwnerDrawVariable");
        }
        base.DrawMode = value;
    }
}

public new ComboBoxStyle DropDownStyle
{ 
    get
    {
        return base.DropDownStyle;
    } 
    set
    {
        if (value == ComboBoxStyle.Simple)
        {
            throw new NotSupportedException("ComboBoxStyle.Simple not supported");
        }
        base.DropDownStyle = value;
    } 
}

I overrode OnDataSourceChanged so that the column names could be initialized.

protected override void OnDataSourceChanged(EventArgs e)
{
    base.OnDataSourceChanged(e);

    InitializeColumns();
}

private void InitializeColumns()
{
    PropertyDescriptorCollection propertyDescriptorCollection = 
        DataManager.GetItemProperties();

    columnWidths = new float[propertyDescriptorCollection.Count];
    columnNames = new String[propertyDescriptorCollection.Count];

    for (int colIndex = 0; colIndex < propertyDescriptorCollection.Count; colIndex++)
    {
        String name = propertyDescriptorCollection[colIndex].Name;
        columnNames[colIndex] = name;
    }
}

I use the DataManager property which returns the

CurrencyManager

OnValueMemberChanged

protected override void OnValueMemberChanged(EventArgs e)
{
    base.OnValueMemberChanged(e);

    InitializeValueMemberColumn();
}

private void InitializeValueMemberColumn()
{
    int colIndex = 0;
    foreach (String columnName in columnNames)
    {
        if (String.Compare(columnName, ValueMember, true, 
            CultureInfo.CurrentUICulture) == 0)
        {
            valueMemberColumnIndex = colIndex;
            break;
        }
        colIndex++;
    }
}

OnMeasureItem will be called once for every row in the combobox and that's where I do my width calculations.

protected override void OnMeasureItem(MeasureItemEventArgs e)
{
    base.OnMeasureItem(e);

    if (DesignMode)
        return;

    for (int colIndex = 0; colIndex < columnNames.Length; colIndex++)
    {
        string item = Convert.ToString(
            FilterItemOnProperty(Items[e.Index], columnNames[colIndex]));
        SizeF sizeF = e.Graphics.MeasureString(item, Font);
        columnWidths[colIndex] = Math.Max(columnWidths[colIndex], sizeF.Width);
    }

    float totWidth = CalculateTotalWidth();

    e.ItemWidth = (int)totWidth;
}

The interesting trick here is to use FilterItemOnProperty to get the text associated with a specific column. The width calculation is elementary and I calculate the total width using a CalculateTotalWidth method which merely adds all the individual column widths. I also add width for the vertical scrollbar (in case one shows up). We must also remember to override OnDropDown to set the drop down width appropriately (remember this is different from the width of the combobox itself).

protected override void OnDropDown(EventArgs e)
{
    base.OnDropDown(e);
    this.DropDownWidth = (int)CalculateTotalWidth();
}

Now we come to the meat of the class -the OnDrawItem override.

protected override void OnDrawItem(DrawItemEventArgs e)
{
    base.OnDrawItem(e);

    if (DesignMode)
        return;

    e.DrawBackground();

    Rectangle boundsRect = e.Bounds;
    int lastRight = 0;

    using (Pen linePen = new Pen(SystemColors.GrayText))
    {
        using (SolidBrush brush = new SolidBrush(ForeColor))
        {
            if (columnNames.Length == 0)
            {
                e.Graphics.DrawString(Convert.ToString(Items[e.Index]), 
                    Font, brush, boundsRect);
            }
            else
            {
                for (int colIndex = 0; colIndex < columnNames.Length; colIndex++)
                {
                    string item = Convert.ToString(FilterItemOnProperty(
                        Items[e.Index], columnNames[colIndex]));

                    boundsRect.X = lastRight;
                    boundsRect.Width = (int)columnWidths[colIndex] + columnPadding;
                    lastRight = boundsRect.Right;

                    if (colIndex == valueMemberColumnIndex)
                    {
                        using (Font boldFont = new Font(Font, FontStyle.Bold))
                        {
                            e.Graphics.DrawString(item, boldFont, brush, boundsRect);
                        }
                    }
                    else
                    {
                        e.Graphics.DrawString(item, Font, brush, boundsRect);
                    }

                    if (colIndex < columnNames.Length - 1)
                    {
                        e.Graphics.DrawLine(linePen, boundsRect.Right, boundsRect.Top, 
                            boundsRect.Right, boundsRect.Bottom);
                    }
                }
            }
        }
    }

    e.DrawFocusRectangle();
}

Though it's the longest function in the class (and probably exceeds the Marc Clifton approved limit for maximum number of lines in a method), it's quite straightforward. For each row, it iterates through all the columns, gets the column text and draws the text along with vertical lines that will act as column separators.

Acknowledgements

• Rama Krishna Vavilala - For some awesome suggestions on the implementation.

History

• July 27th 2007 - Article first published.