How to alter a .NET DateTimePicker control to allow enter null values?

You don't need to modify it to do this.

The DateTimePicker in .net actually has a checkbox built-in.

Set the ShowCheckBox property to true.

Then you can use the Checked property to see if the user has entered a value.

http://msdn.microsoft.com/en-us/library/system.windows.forms.datetimepicker.showcheckbox(VS.80).aspx

Here's an approach from this CodeProject article on creating a Nullable DateTimePicker.

I have overridden the Value property to accept Null value as DateTime.MinValue, while maintaining the validation of MinValue and MaxValue of the standard control.

Here's a version of the custom class component from the article

public class NullableDateTimePicker : System.Windows.Forms.DateTimePicker
{
    private DateTimePickerFormat originalFormat = DateTimePickerFormat.Short;
    private string originalCustomFormat;
    private bool isNull;

    public new DateTime Value
    {
        get => isNull ? DateTime.MinValue : base.Value;
        set
        {
            // incoming value is set to min date
            if (value == DateTime.MinValue)
            {
                // if set to min and not previously null, preserve original formatting
                if (!isNull)
                {
                    originalFormat = this.Format;
                    originalCustomFormat = this.CustomFormat;
                    isNull = true;
                }

                this.Format = DateTimePickerFormat.Custom;
                this.CustomFormat = " ";
            }
            else // incoming value is real date
            {
                // if set to real date and previously null, restore original formatting
                if (isNull)
                {
                    this.Format = originalFormat;
                    this.CustomFormat = originalCustomFormat;
                    isNull = false;
                }

                base.Value = value;
            }
        }
    }

    protected override void OnCloseUp(EventArgs eventargs)
    {
        // on keyboard close, restore format
        if