问题描述

更改 .NET DateTimePicker 控件以允许用户输入 null 值的最简单、最可靠的方法是什么?

What's the easiest and most robust way of altering the .NET DateTimePicker control, to allow users to enter null values?

推荐答案

这是 CodeProject 文章中关于创建 可以为空的 DateTimePicker.

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

我已覆盖 Value 属性以接受 Null 值作为 DateTime.MinValue，同时保持 MinValue 的验证标准控件的code>和MaxValue.

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.

这是文章中自定义类组件的一个版本

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 (Control.MouseButtons == MouseButtons.None)
        {
            if (isNull)
            {
                this.Format = originalFormat;
                this.CustomFormat = originalCustomFormat;
                isNull = false;
            }
        }
        base.OnCloseUp(eventargs);
    }

    protected override void OnKeyDown(KeyEventArgs e)
    {
        base.OnKeyDown(e);

        // on delete key press, set to min value (null)
        if (e.KeyCode == Keys.Delete)
        {
            this.Value = DateTime.MinValue;
        }
    }
}