保存文件对话框未处理的异常

Question

我有一些关于 savefiledialog c# 的问题，我在调试时遇到类型为 System.NullReferenceException 的未处理异常，这是代码：

private void saveToolStripMenuItem_Click(object sender, System.EventArgs e)
        {
            switch (fileName)
            {
                case "":
                    {
                        saveFileDialog1 = new SaveFileDialog
                        {
                            Filter = @"Image files (*.bmp)|*.bmp|All files (*.*)|*.*",
                            FileName = "MyPicture.bmp"
                        };
                        if (saveFileDialog1.ShowDialog() != DialogResult.OK) return;
                        fileName = saveFileDialog1.FileName;
                        bitmap.Save(saveFileDialog1.FileName, ImageFormat.Bmp);
                    }
                    break;
                default:
                    {
                        bitmap.Save(fileName, ImageFormat.Bmp);
                    }
                    break;
            }

        }

这是我的声明：

    private string fileName = "";
    private Bitmap bitmap;
    private Bitmap curBitmap;

这是我的完整代码：

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    //Bitmap


    //Graphics

    Graphics g;
    Pen p = new Pen(Color.Black, 8);
    Point start = new Point(0, 0);
    Point end = new Point(0, 0);
    bool drawing = false;

    //private int x1;
    //private int x2;
    //private int y1;
    //private int y2;
    //private int d1;
    //private int d2;
    private string fileName = "";
    private Bitmap bitmap;
    private Bitmap curBitmap;
    private Size fullSize;

    private void btnColorPicker_Click(object sender, EventArgs e)
    {
        //Get colors from colordialog
        DialogResult r = colorDialog1.ShowDialog();
        if (r == DialogResult.OK)
        {
            p.Color = colorDialog1.Color;
        }

    }


    private void PanelDrawing_MouseUp(object sender, MouseEventArgs e)
    {
        drawing = false;
    }

    private void PanelDrawing_MouseMove(object sender, MouseEventArgs e)
    {

        if (drawing && !earaser)
        {
            p.Width = PenSize.Value;
            p.Color = colorDialog1.Color;
            end = e.Location;
            g = PanelDrawing.CreateGraphics();
            g.DrawLine(p, start, end);
            PanelDrawing.Cursor = Cursors.HSplit;
        }
        else if (drawing && earaser)
        {
            end = e.Location;
            g = PanelDrawing.CreateGraphics();
            g.DrawLine(p, start, end);
            PanelDrawing.Cursor = Cursors.Cross;
        }
        else if (!drawing)
        {
            PanelDrawing.Cursor = Cursors.Default;
        }
        start = end;
    }

    private void PanelDrawing_MouseDown(object sender, MouseEventArgs e)
    {
        start = e.Location;
        if (e.Button == MouseButtons.Left)
        {
            drawing = true;
        }
    }

    private void PenSize_Scroll(object sender, EventArgs e)
    {
        p.Width = PenSize.Value;
        label1.Text = "" + PenSize.Value;
    }

    bool earaser = false;

    private void btnEaraser_Click(object sender, EventArgs e)
    {
        p.Color = Color.White;
        p.Width = 10;
        earaser = true;
    }

    private void btnBrush_Click(object sender, EventArgs e)
    {
        earaser = false;
    }

    private void btnClear_Click(object sender, EventArgs e)
    {
        g.Clear(PanelDrawing.BackColor);
    }

    private void saveToolStripMenuItem_Click(object sender, System.EventArgs e)
    {
        if (string.IsNullOrWhiteSpace(fileName))
        {
            saveFileDialog1 = new SaveFileDialog
            {
                Filter = @"Image files (*.bmp)|*.bmp|All files (*.*)|*.*",
                FileName = "MyPicture.bmp"
            };

            if (saveFileDialog1.ShowDialog() != DialogResult.OK)
                return;

            fileName = saveFileDialog1.FileName;
            bitmap.Save(saveFileDialog1.FileName, ImageFormat.Bmp);
        }
        else
        {
            bitmap.Save(fileName, ImageFormat.Bmp);
        }

    }
    private void Form1_Load(object sender, EventArgs e)
    {
        fullSize = SystemInformation.PrimaryMonitorMaximizedWindowSize;
        bitmap = new Bitmap(fullSize.Width, fullSize.Height);
        g = Graphics.FromImage(bitmap);
        g.SmoothingMode = SmoothingMode.AntiAlias;
        g.Clear(BackColor);
    }


}

Answer 1

由于您没有指定 "where"，System.NullReferenceException 正在发生。我会假设以下2种情况

1.fileName 为空。如果是这种情况，请在需要 null 的 switch 语句中再添加 1 个 case（如下所示）考虑在内。

switch (fileName)
        {
            case  "":
            case null:
                {
                    saveFileDialog1 = new SaveFileDialog
                    {
                        Filter = @"Image files (*.bmp)|*.bmp|All files (*.*)|*.*",
                        FileName = "MyPicture.bmp"
                    };
                    if (saveFileDialog1.ShowDialog() != DialogResult.OK) return;
                    fileName = saveFileDialog1.FileName;
                    bitmap.Save(saveFileDialog1.FileName, ImageFormat.Bmp);
                }
                break;
            default:
                {
                    bitmap.Save(fileName, ImageFormat.Bmp);
                }
                break;
        }

2。您在保存时使用的是相同的图像流，用于构造它。如果是这种情况，请使用如下所示的新位图对象。

 var newBitmap = new Bitmap(bitmap);
        switch (fileName)
        {

            case  "":
            case null:
                {
                    saveFileDialog1 = new SaveFileDialog
                    {
                        Filter = @"Image files (*.bmp)|*.bmp|All files (*.*)|*.*",
                        FileName = "MyPicture.bmp"
                    };
                    if (saveFileDialog1.ShowDialog() != DialogResult.OK) return;
                    fileName = saveFileDialog1.FileName;
                    newBitmap.Save(saveFileDialog1.FileName, ImageFormat.Bmp);
                }
                break;
            default:
                {
                    newBitmap.Save(fileName, ImageFormat.Bmp);
                }
                break;
        }

Answer 2

首先，我认为 switch 不是处理字符串的好方法。

string.IsNullorEmpty 和 string.IsNullorWhiteSpace 可能对您有用。

你能在控制台上跟踪位图吗？您确定您在代码中正确地声明和初始化了它吗？

这应该可以解决任何可能性：

private void saveToolStripMenuItem_Click(object sender,System.EventArgs e)
{
    var _Bitmap = new Bitmap(bitmap);
    //when we quit here we don't need this anymore; declaring it here helps with memory management
    if(_Bitmap == null)
        return;

    if(string.IsNullorEmpty(fileName)||string.IsNullorWhiteSpace(fileName)
    {
        Filter = @"Image files (*.bmp)|*.bmp|All files (*.*)|*.*",
        FileName = "MyPicture.bmp"
    }
    else
    {
        _Bitmap.Save(fileName,ImageFormat.Bmp);
    }
}

您在 Form1_Load 上分配位图，尝试在单独的方法上进行，并在需要时调用它。

Answer 3

如果变量为 null 或空字符串，它可能没有捕捉到变量。

所以不要使用 switch 语句，而是尝试在 If-Else 语句中进行。

Answer 4

这不是使用 switch 语句的地方。这更容易重写为 if:

private void saveToolStripMenuItem_Click(object sender, System.EventArgs e)
{
    if (string.IsNullOrWhiteSpace(fileName))
    {
        saveFileDialog1 = new SaveFileDialog
        {
            Filter = @"Image files (*.bmp)|*.bmp|All files (*.*)|*.*",
            FileName = "MyPicture.bmp"
        };

        if (saveFileDialog1.ShowDialog() != DialogResult.OK) 
            return;

        fileName = saveFileDialog1.FileName;
        bitmap.Save(saveFileDialog1.FileName, ImageFormat.Bmp);
    }
    else
    {
        bitmap.Save(fileName, ImageFormat.Bmp);
    }
}

这个开关不合适，因为它非常混乱，你必须测试多种情况（null、空字符串、空格）。您可以堵塞代码并使其工作，但这更简单。

如果不查看其余代码，就无法判断 bitmap 是否为空。如果它为空，则调用 .Save 将导致 NullReferenceException。了解为什么它为空的最好方法是学习如何使用调试器。在位图或使用位图的位置设置断点，看看为什么不写入位图或首先创建对象。

保存文件对话框未处理的异常

Save file dialog An unhandled exception

c#

savefiledialog