由於保護級別而無法訪問？

Question

由於保護級別的緣故，在cb.Items.AddRange（databases）的某個位置無法訪問，但是所有變量都設置為public。 它說：

C：\\ Users \\ Shulz \\ Documents \\ MySql \\ Oliver exer3 \\ showdb.cs（77,18）：錯誤CS0122：由於無法訪問Sys tem.Windows.Forms.ComboBox.ObjectCollection.AddRange（System.Collections.IList）'到其保護級別C：\\ PROGRA〜2 \\ MONO-3〜1.3 \\ lib \\ mono \\ 4.5 \\ System.Windows.Forms.dll（與先前錯誤相關的sy mbol的位置）編譯失敗：1個錯誤， 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[] ，因此它與AddRange不兼容，並且會出現編譯器錯誤。

在.net上，此錯誤是：

'System.Windows.Forms.ComboBox.ObjectCollection.AddRange(object[])'的最佳重載方法匹配具有一些無效的參數

參數1：無法從'System.Collections.Generic.List<string>'轉換為'object[]'

Mono似乎還有另一個內部重載采用IList 。 如果可以訪問此重載，那么它將與您的呼叫匹配。 因此，建議的錯誤原因有些不同。

通常，編譯器首先會注意到沒有匹配的重載。 為了幫助您，它會嘗試猜測可能的原因。 這種猜測可能會受到內部方法的影響。

Answer 2

我已經弄清楚了如何解決這個問題。 所以我要做的是db.ToArray（）然后分配我的數據庫對象。 就這么簡單。

    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);