C# 为什么我的变量不能在表单之间正确传递？

Question

我有几个表格，其中一些依赖于每个表格之间的信息。在这种情况下，Form 2（AKA testSelect）中所选选项的所选索引是确定Form 3（AKA testPresent）中将发生什么的关键。这被放入一个名为index 的整数中。在关闭form 2 时，index 的值绝对是列表框的selectedindex。

但是，在form 3 打开并应用它时，它一直重置为 0，我不知道为什么。下面是在form 2 中使用/确定index 的代码以及在form 3 中调用它的代码。还有，它是一个在form 2开头定义的公共int；

     private void lstExams_SelectedIndexChanged(object sender, EventArgs e)
    {
        try
        {
            //Create a connection object to the DB via string location
            con = new OleDbConnection(connectionString);
            //Open the connection to the DB
            con.Open();

            String sql = "SELECT typeID, description FROM TestConfiguration WHERE examID = " +(lstExams.SelectedIndex +1);
            OleDbDataAdapter da = new OleDbDataAdapter(sql, con);
            //DataSet ds = new DataSet();
            DataTable testConfig = new DataTable();
            //Set the SQL command text

            da.Fill(testConfig);
            lstTestType.DisplayMember = "description";
            lstTestType.ValueMember = "typeID";

            lstTestType.DataSource = testConfig;

            index = lstExams.SelectedIndex + 1;

            MessageBox.Show("INDEX = " + index);

        }
        catch (Exception err)
        {
            //If cannot connect to DB, display the error;
            MessageBox.Show("A Database error has occurred: " + Environment.NewLine + err.Message);
        }
    }

    private void btnStart_Click(object sender, EventArgs e)
    {

        //var testPresent = new testPresent(this);

        testPresent testForm = new testPresent();
        testForm.Show();


        //testForm.difficulty = lstTestType.ValueMember;
        this.Close();

        MessageBox.Show("INDEX IS " + index);
        testForm.eID = (index);


    }

对于form 3

    public partial class testPresent : Form
{
    public int eID = 0;
    public String difficulty;
    testSelect select = new testSelect();


    //Get the connection path to the DB from the static class
    String connectionString = staticConnectionString.connectionString;
    //Declare connection object
    OleDbConnection con;

    public testPresent()
    {
        InitializeComponent();
    }


    public void testPresent_Load(object sender, EventArgs e)
    {
        try
        {

            int qID;
            Random random = new Random();
            int examID;
            bool valid = false;
            String theQuestion;

            eID += select.index;

            //Create a connection object to the DB via string location
            con = new OleDbConnection(connectionString);
            //Open the connection to the DB
            con.Open();

            MessageBox.Show("eID = " + select.index);

            if (eID == 1)
            {
                ...

            } 

            if (eID == 2)
            {
                ...

            } 

            if (eID == 3)
            {
                ...

            }



        }
        catch (Exception err)
        {
            //If cannot connect to DB, display the error;
            MessageBox.Show("A Database error has occurred: " + Environment.NewLine + err.Message);
        }
    }

哦，是的，这也使用 Microsoft Access 数据库来填充列表框。

Answer 1

你设置的值：

testForm.eID = (index);

但是你这样做是在表单已经加载之后，所以testPresent_Load已经已经使用了它的默认值：

testPresent testForm = new testPresent();
testForm.Show();  // this is where it uses the value
// ...
testForm.eID = (index);  // this is where you set it

如果 testPresent 表单完全需要该值，并且它立即需要该值，请尝试在构造函数中包含该值：

public testPresent(int eid)
{
    InitializeComponent();
    eID = eid;
}

然后，当您创建表单的实例时，您必须将值传递给它：

testPresent testForm = new testPresent(index);
testForm.Show();

此时不需要在外部设置该值，因此它应该是私有的。 （无论如何，数据成员在对象上应该是私有的。）：

private int eID;