由于保护级别而无法访问的地址？

Question

在某个地方，由于保护级别，cb.Items.AddRange(数据库)是不可访问的，但是所有变量都被设置为public。这里写着：

C：\Users\Shulz\exer3\showdb.cs\MySql\Oliver exer3\showdb.cs(77,18)：error CS0122：‘`Sys CS0122由于其保护级别C:\PROGRA~2\MONO-3~1.3\lib\mono\4.5\System.Windows.Forms.dll (与先前错误相关的sy mbol的位置)而无法访问:1错误，0警告

    public ComboBox cb;
    public Label label;
    public object[] databases;

    public MForm() {

       string connectionString =
          "Server=localhost;" +
          "Database=information_schema;" +
          "User ID=root;" +
          "Password=root;" +
          "Pooling=false";
       IDbConnection dbcon;
       dbcon = new MySqlConnection(connectionString);
       dbcon.Open();
       IDbCommand dbcmd = dbcon.CreateCommand();

        string sql = "SELECT COUNT(*) as count FROM information_schema.SCHEMATA"; //count the databases(string) and names it count
        dbcmd.CommandText = sql;                        //sends the string to sql
        IDataReader reader = dbcmd.ExecuteReader();     //assign the function to reader
        reader.Read();                                  //uses its getter(Read)
        int count = Convert.ToInt32(reader["count"]);   //converts the "count"(column) to integer

        reader.Close();
        reader = null;
        dbcmd.Dispose();
        dbcmd = null;



        dbcmd = dbcon.CreateCommand();
        sql = "show databases";
        dbcmd.CommandText = sql;
        reader = dbcmd.ExecuteReader();

        var databases = new List<string>();

        var excludeDatabases = new List<string> { "information_schema","sakila","enrollmentsystem","mysql","world","performance_schema" };

        while(reader.Read())
        {
            var data = reader["Database"].ToString();

            if(!excludeDatabases.Contains(data)){
            databases.Add(data);
            }
        }

       reader.Close();
       reader = null;
       dbcmd.Dispose();
       dbcmd = null;
       dbcon.Close();
       dbcon = null;




        Text = "School Year";
        Size = new Size(340, 240);

        cb = new ComboBox();
        cb.Parent = this;
        cb.Location = new Point(50, 30);

        cb.Items.AddRange(databases);

        cb.SelectionChangeCommitted += new EventHandler(OnChanged);

        label = new Label();
        label.Location = new Point(80, 170);
        label.Parent = this;
        label.Text = "...";

        CenterToScreen();




    }

    void OnChanged(object sender, EventArgs e) {
         ComboBox combo = (ComboBox) sender;
         label.Text = combo.Text;
    }
}

Answer 1

List<string>不是object[]，因此它与AddRange不兼容，您将得到一个编译器错误。

在.net上，此错误是：

'System.Windows.Forms.ComboBox.ObjectCollection.AddRange(object[])'的最佳重载方法匹配有一些无效的参数 参数1:无法从'System.Collections.Generic.List<string>'转换为'object[]'

Mono似乎有另一个内部重载，需要一个IList。如果这个过载是可以访问的，它将与你的呼叫相匹配。因此，建议的错误原因有点不同。

一般来说，编译器首先注意到没有匹配的重载。为了帮助你，它试图猜测可能的原因。这种猜测可能会受到内部方法存在的影响。

Answer 2

我想出了解决这个问题的办法。因此，我所做的是db.ToArray()，然后对我的数据库进行分析，这是一个对象。就这么简单。

    var db = new List<string>();

    var excludeDatabases = new List<string> { "information_schema","sakila","enrollmentsystem","mysql","world","performance_schema" };

    while(reader.Read())
    {
        var data = reader["Database"].ToString();

        if(!excludeDatabases.Contains(data)){
        db.Add(data);

        }
    }

   reader.Close();
   reader = null;
   dbcmd.Dispose();
   dbcmd = null;
   dbcon.Close();
   dbcon = null;




    Text = "School Year";
    Size = new Size(340, 240);

    cb = new ComboBox();
    cb.Parent = this;
    cb.Location = new Point(50, 30);


    databases = db.ToArray();
    cb.Items.AddRange(databases);