System.invalidcastexception：'指定的强制转换无效。'标签 [英] System.invalidcastexception: 'specified cast is not valid.' with tags

问题描述

我正在尝试使用此标记属性但由于强制转换无法正常工作



我将所有标记放到property = Borderstyle.None



我尝试了什么：



这在这里工作

I'm trying to make use of this tag property but it isn't working properly because of cast is not valid

I put all tags to property = Borderstyle.None

What I have tried:

This works here

private void PictureBoxes_MouseEnter(object sender, EventArgs e)
        {
            PictureBox p = sender as PictureBox;
            if (p != null)
            {
                p.BorderStyle = BorderStyle.Fixed3D;
            }

        }

private void PictureBoxes_MouseClick(object sender, MouseEventArgs e)
        {


            PictureBox pBox = sender as PictureBox;

            

            if (e.Button == MouseButtons.Left)
            {
                pBox.MouseLeave -= PictureBoxes_MouseLeave;
                pBox.ImageLocation = (startupPath + @"img\sd0.png");
            }
            else if (e.Button == MouseButtons.Right)
            {
                pBox.MouseLeave -= PictureBoxes_MouseLeave; // prevent double subscription
                pBox.MouseLeave += PictureBoxes_MouseLeave;
            }

        }

这不起作用

This doesn't work

private void PictureBoxes_MouseLeave(object sender, EventArgs e)
        {
            PictureBox p = sender as PictureBox;
            
            if (p != null)
            {
                try
                {
                    p.BorderStyle = (BorderStyle)p.Tag; // <--- here comes the error
                }
                catch (Exception)
                {

                    
                }
                
            }
        }

推荐答案

在某个地方，你很可能没有设置Tag属性。

使用调试器在错误发生时准确找出Tag中的内容：在catch语句中放置一个断点并查看p.Tag - 如果它是一个有效的BorderStyle，那么它将是一个包含0,1的整数，或2.任何其他值不可转换：参考源 [ ^ ]

一旦你知道它是什么，你可以看看为什么它是什么。

Somewhere, you haven't set the Tag property, most likely.
Use the debugger to find out exactly what is in the Tag when the error occurs: put a breakpoint in the catch statement and look at p.Tag - if it's a valid BorderStyle, then it'll be an integer containing 0, 1, or 2. Any other value is not castable: Reference Source[^]
Once you know what it is, you can look at why it is what it is.

OriginalGriff是正确的：你没有设置标签; BorderStyle的演员表是有效的。所以，投票给他的答案，而不是这一个:)



我认为你不需要'枚举因为......但是，首先，几句话关于'枚举：



所有C＃枚举都存储一些数字类型的内部值：枚举的批准类型是byte，sbyte，short，ushort，int， uint，long或ulong：[ ^ ]。



当您设置WinForm Control的'Tag属性时，您将设置为的内容将被转换为一个物体（即盒装）。如果将其设置为可空或引用，则可以使用As运算符检索该值，这很方便，因为如果转换不起作用，它将返回null。 '枚举值不是引用类型，不可为空：所以，你不能使用'As。



现在，关于你的代码：考虑：

OriginalGriff is correct: you have not set the Tag; the cast to 'BorderStyle is valid. So, vote his answer up, not this one :)

I don't think you need an 'enum here because ... but, first, a few words about 'enum:

All C# Enumerations store an internal value of some numeric type: "The approved types for an enum are byte, sbyte, short, ushort, int, uint, long, or ulong:" [^].

When you set a WinForm Control's 'Tag property, what you set it to is cast to an object (i.e., boxed). If you set it to a nullable, or reference, Type, you can retrieve the value using the 'As operator, which is convenient because it will return 'null if the cast did not work. An 'enum value is not a reference Type, not nullable: so, you can't use 'As.

Now, about your code: consider:

PictureBox pb;

private void PictureBoxes_MouseEnter(object sender, System.EventArgs e)
{
    pb = sender as PictureBox;

    pb.BorderStyle = BorderStyle.Fixed3D;
}

private void PictureBoxes_MouseLeave(object sender, System.EventArgs e)
{
    // you can be sure that 'pb is current here
    if(pb.ImageLocation == null)
    {
        pb.BorderStyle = BorderStyle.None;
    }
}

然后你的'Click事件处理程序简化为只在左键下载时设置'ImageLocation属性。

Then your 'Click event handler simplifies to only setting the 'ImageLocation property on left-mouse down.

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