C＃中的字典WinForms应用程序导致空引用 [英] Dictionary in C# WinForms App causing null reference

问题描述

我正在使用Visual Studio 2013创建一个Visual C＃Windows窗体应用程序，我没有使用设计器来设置窗体。

I'm using Visual Studio 2013 to create a Visual C# Windows Forms Application and I'm not using the Designer to setup the form.

我正在尝试使用字典来存储位图，以便稍后可以通过名称调用它们。但是当我调试脚本时，我得到错误：

I'm trying to use a Dictionary to store Bitmaps so that I can call them later by name. But when I debug the script I get the error:

An unhandled exception of type 'System.NullReferenceException' occurred in SimpleForm.exe
Additional information: Object reference not set to an instance of an object.

从行：

width = imgLetters["a"].Width;

任何帮助将不胜感激。

删除仍然产生错误的代码版本：

Cut down version of code which still produces the error:

using System;
using System.Drawing;
using System.Collections;
using System.Collections.Generic;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;

namespace SimpleForm
{

    public class Test : Form
    {

        static Bitmap bmpLetterA;
        static Bitmap bmpLetterB;
        static Bitmap bmpLetterC;
        private Dictionary<string, Bitmap> imgLetters;

        public Test()
        {

            ImgInitialize();
            ImgWidth();

        }

        private void ImgInitialize()
        {

            Dictionary<string, Bitmap> imgLetters;

            bmpLetterA = new Bitmap("a.png");
            bmpLetterB = new Bitmap("b.png");
            bmpLetterC = new Bitmap("c.png");

            imgLetters = new Dictionary<string, Bitmap>();

            imgLetters.Add("a", bmpLetterA);
            imgLetters.Add("b", bmpLetterB);
            imgLetters.Add("c", bmpLetterC);

        }

        private void ImgWidth()
        {

            int width = 0;
            width = imgLetters["a"].Width;

        }


    }

}

推荐答案

删除行 Dictionary< string，Bitmap> imgLetters; 从 ImgInitialize 。这将创建一个与成员变量名称相同的局部变量。然后填充，但从未使用，而成员变量保持未初始化。

Remove the line Dictionary<string, Bitmap> imgLetters; from ImgInitialize. This creates a local variable with the same name as the member variable. It is then filled, but never used, while the member variable remains uninitialized.

提示避免这样的问题：

1. 您可以以特殊方式命名实例成员，以清除变量是一个实例成员（例如 m_member 而不是 member ）。


2. 您可以使用这个清除您要访问的变量。


3. 您可以尝试避免将局部变量命名为与实例成员相同。

1. You could name instance members in a special way to make clear the variable is an instance member (for example m_member instead of member).
2. You could prefix access to an instance member with this. to make clear which variable you want to access.
3. You could try to avoid naming local variables the same as instance members.

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