C#/WinForm拖拽文件上传

一、首先创建一个上传文件的类，继承Control类，如下：


public class UploadControl : Control
    {
        private Image _image;
        public UploadControl()
        {
            this.SetStyle(
             ControlStyles.UserPaint |  //控件自行绘制，而不使用操作系统的绘制
             ControlStyles.AllPaintingInWmPaint | //忽略背景擦除的Windows消息，减少闪烁，只有UserPaint设为true时才能使用。
             ControlStyles.OptimizedDoubleBuffer |//在缓冲区上绘制，不直接绘制到屏幕上，减少闪烁。
             ControlStyles.ResizeRedraw | //控件大小发生变化时，重绘。
             ControlStyles.SupportsTransparentBackColor, //支持透明背景颜色
             true);

            _image =Properties.Resources.upload;
            this.Cursor = Cursors.Hand;
            this.AllowDrop = true;
        }
      }

准备好上传的图片

二、我们需要绘制圆角矩形，所以先准备一个圆角路径，如下：


private GraphicsPath GetRoundedRectPath(Rectangle rect, uint radius)
        {
            int r = (int)radius << 1;
            Rectangle arcRect = new Rectangle(rect.Location, new Size(r, r));
            GraphicsPath path = new GraphicsPath();
            path.AddArc(arcRect, 180, 90);
            // 右上圆弧
            arcRect.X = rect.Right - r;
            path.AddArc(arcRect, 270, 90);
            // 右下圆弧
            arcRect.Y = rect.Bottom - r;
            path.AddArc(arcRect, 0, 90);
            // 左下圆弧
            arcRect.X = rect.Left;
            path.AddArc(arcRect, 90, 90);
            path.CloseFigure();
            return path;
        }

三、重写OnPaint事件，绘制填充圆角背景

Rectangle outRect = new Rectangle(0, 0, this.Width, this.Height);
using var outPath = GetRoundedRectPath(outRect, Radius);
this.Region = new Region(path);
using SolidBrush b = new SolidBrush(this.BackColor);
e.Graphics.FillPath(b, outPath);

效果如下：

发现使用Region属性有锯齿，尽管开启高质量绘图也无用，我们去掉Region属性，但是新的问题出现，此控件的背景必须和父级窗体的背景一致，不然圆角效果就没有了。后续我们都是不设置Region属性，保持背景与父级控件背景一致就行。

四、在OnPaint事件，绘制圆角边框

var innerRect = Rectangle.Inflate(outRect, -(int)border, -(int)border);
using GraphicsPath innerPath = GetRoundedRectPath(innerRect, Radius - border);
using Pen pen = new Pen(ColorState switch
{
    ControlState.Hover => this.BorderHoverColor,
    ControlState.Pressed => this.BorderPressedColor,
    ControlState.DragEnter => this.BorderHoverColor,
    _ => this.BorderNormalColor,
}, border);
pen.DashStyle = BorderStyle;
e.Graphics.DrawPath(pen, innerPath);

效果如下：

四、在OnPaint事件，绘制上传图像以及文字


const int h = 5;
SizeF sizeF = SizeF.Empty;
if (!string.IsNullOrWhiteSpace(Text))
    sizeF = e.Graphics.MeasureString(Text, this.Font);
if (_image != null)
{
    int imageH = 0;
    if (sizeF != SizeF.Empty)
    {
        imageH = (int)((this.Height - _image.Height - h - sizeF.Height) / 2);
        TextRenderer.DrawText(e.Graphics, Text, this.Font, new Point((int)((this.Width - sizeF.Width) / 2), imageH + _image.Height + h), this.ForeColor);
    }
    else
        imageH = this.Height - _image.Height >> 1;
    e.Graphics.DrawImage(_image, this.Width - _image.Width >> 1, imageH);
}
else
{
    if (sizeF != SizeF.Empty)
    {
        TextRenderer.DrawText(e.Graphics, Text, this.Font, new Point((int)((this.Width - sizeF.Width) / 2), (int)((this.Height - sizeF.Height) / 2)), this.ForeColor);
    }
}

效果如下：

五、我们定义一个枚举，用来实现鼠标移、出移入、点击、文件拖动让边框变色


public enum ControlState { Hover, Normal, Pressed, DragEnter }

然后重写OnMouseEnter、OnMouseLeave、OnMouseDown、OnMouseUp事件，如下：

private ControlState ColorState { get; set; }= ControlState.Normal;
protected override void OnMouseEnter(EventArgs e)//鼠标进入时
{
    ColorState = ControlState.Hover;//Hover
    this.Invalidate();
    base.OnMouseEnter(e);
}
protected override void OnMouseLeave(EventArgs e)//鼠标离开
{
    ColorState = ControlState.Normal;//正常
    this.Invalidate();
    base.OnMouseLeave(e);
}
protected override void OnMouseDown(MouseEventArgs e)//鼠标按下
{
    if (e.Button == MouseButtons.Left && e.Clicks == 1)//鼠标左键且点击次数为1
    {
        ColorState = ControlState.Pressed;//按下的状态
        this.Invalidate();
        using OpenFileDialog openFileDialog = new OpenFileDialog();
        openFileDialog.Filter = "所有文件 (*.*)|*.*";
        if (openFileDialog.ShowDialog() == DialogResult.OK)
        {
            FilesCallback?.Invoke(new string[] { openFileDialog.FileName });
        }
    }
    base.OnMouseDown(e);
}
protected override void OnMouseUp(MouseEventArgs e)//鼠标弹起
{
    if (e.Button == MouseButtons.Left && e.Clicks == 1)
    {
        if (ClientRectangle.Contains(e.Location))//控件区域包含鼠标的位置
        {
            ColorState = ControlState.Hover;
        }
        else
        {
            ColorState = ControlState.Normal;
        }
        this.Invalidate();
    }
    base.OnMouseUp(e);
}

就是在OnPaint事件中笔的颜色代码：

ColorState switch
{
    ControlState.Hover => this.BorderHoverColor,
    ControlState.Pressed => this.BorderPressedColor,
    ControlState.DragEnter => this.BorderHoverColor,
    _ => this.BorderNormalColor,
}

当然我们在OnMouseDown事件中还实现了点击选择文件：

using OpenFileDialog openFileDialog = new OpenFileDialog();
openFileDialog.Filter = "所有文件 (*.*)|*.*";
if (openFileDialog.ShowDialog() == DialogResult.OK)
{
    FilesCallback?.Invoke(new string[] { openFileDialog.FileName });
}

其中FilesCallback是一个事件：


public event Action<string[]> FilesCallback;

六、实现文件拖动上传

protected override void OnDragEnter(DragEventArgs drgevent)
{
    base.OnDragEnter(drgevent);
    if (drgevent.Data.GetDataPresent(DataFormats.FileDrop))
        drgevent.Effect = DragDropEffects.Copy;
    else
        drgevent.Effect = DragDropEffects.None;
    ColorState = ControlState.DragEnter;
    this.Invalidate();
}
protected override void OnDragLeave(EventArgs e)
{
    base.OnDragLeave(e);
    ColorState = ControlState.Normal;
    this.Invalidate();
}
protected override void OnDragDrop(DragEventArgs drgevent)
{
    base.OnDragDrop(drgevent);
    ColorState = ControlState.Normal;
    this.Invalidate();
    object? obj = drgevent.Data?.GetData(DataFormats.FileDrop);
    if (obj == null) return;

    FilesCallback?.Invoke((string[])obj);
}

同时需要在构造函数中开启AllowDrop：