无法在文件代码中声明 UserControl

Question

I have had a problem. I create an UserControl and I save it, it appears in ToolBox. I can drop it from ToolBox normally. However, I can not declare it as a variable in the file code. Can you help me, please? And it is file code of UserControl

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using Music_Player_Project_IT008N13.Controls;

namespace Music_Player_Project_IT008N13.Music_design_User_Control
{
    public partial class LocationPanel : UserControl
    {
        public LocationPanel()
        {
            InitializeComponent();
        }
        public delegate void DoEvent(string maSo, string tenSV, string khoa, string diemTB);
        public event DoEvent RefeshDgv;
        private void btnDelete_Click(object sender, EventArgs e)
        {
            
        }
    }
}

Answer 1

When you "drop it from the ToolBox normally", the Designer generates a member declaration for a new user control and also adds it to the forms Control collection. As I understand it, your question states that you would like to declare your user control variable in code . If this is the goal, we have to do the same thing that the Designer would do and add it to the Controls collection of the container you want to put it in.

---

Example

Add a FlowLayoutPanel using code, then add 3 CustomUserControls to it also in code.

public partial class MainForm : Form
{
    public MainForm() => InitializeComponent();
    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);

        // Declare a flow layout panel in code.
        // Add it to the main form control collection
        var flowLayoutPanel = new FlowLayoutPanel
        {
            Name = "flowLayoutPanel",
            Dock = DockStyle.Fill,
        };
        this.Controls.Add(flowLayoutPanel);

        for (char c = 'A'; c < 'D'; c++)
        {
            var userControl = new CustomUserControl
            {
                Name = $"userControl{c}",  // No space, start with lowercase
                Text = $"UserControl {c}", // Visible name
                Margin = new Padding(10, 2, 10, 2),
                Width = flowLayoutPanel.Width - 20,
            };
            flowLayoutPanel.Controls.Add(userControl);
        }

        // T E S T
        CustomUserControl? loopback = GetUserControl("userControlA");
        Debug.Assert(loopback != null, "Expecting to retrieve user control by name");
    }

Then, to use the control, retrieve it from the Controls collection by name.

    CustomUserControl? GetUserControl(string name)
    {
        var layoutPanel = Controls["flowLayoutPanel"] as FlowLayoutPanel;
        return layoutPanel.Controls[name] as CustomUserControl;
    }
}

Where:

public partial class CustomUserControl : UserControl
{
    public CustomUserControl() => InitializeComponent();
    public new string Text
    {
        get => label1.Text; 
        set => label1.Text = value;
    }
}