How To Refresh User Control From Another Form In WinForms Using C#?

I have tried several ways now to achieve this. Amongst other things using this code:

var UserControlee = new UserControl ();
UserControlee .load_data();

this however does literally nothing. From what I have read this is because I should not use a new instance of the form. But all the suggested ways like using var UserControlee = new UserControl (this); don't work.

I by the way insert my data through SQL and currently trie to use the method load(), which works and refreshes the DataGridView when used on UserControl.

Your question is How To Refresh User Control From Another Form In WinForms Using C#. The debugging details are missing in your question. Nevertheless, a general answer for one way of doing that is to have the requesting form fire an event when a refresh is needed.

Here is a minimal example of a MainForm that features a [Refresh] button to test fire this event. The main form also creates the second form that hosts the UserControl.

public partial class MainForm : Form
{
    public MainForm()
    {
        InitializeComponent();
        buttonRefresh.Click += onClickButtonRefresh;
        ucHost = new UserControlHostForm(this);
    }
    UserControlHostForm ucHost;
    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);
        ucHost.StartPosition = FormStartPosition.Manual;
        ucHost.Location = new Point(Location.X + Width + 10, Location.Y);
        ucHost.Show();
    }
    private void onClickButtonRefresh(object? sender, EventArgs e)
    {
        RefreshNeeded?.Invoke(this, EventArgs.Empty);
    }
    public event EventHandler? RefreshNeeded;
}

The second form subscribes to the main form event in its constructor method and calls myUserControl.Refresh() in response.

public partial class UserControlHostForm : Form
{
    public UserControlHostForm(Form owner)
    {
        Owner = owner;
        InitializeComponent();

        // If the UserControl hasn't already been
        // added in the Designer, add it here.
        myUserControl = new MyUserControl
        {
            BackColor = Color.LightBlue,
            Dock = DockStyle.Fill,
        };
        Controls.Add(myUserControl);

        // Subscribe to the MainForm event
        ((MainForm)Owner).RefreshNeeded += onRefreshNeeded;
    }
    MyUserControl myUserControl;
    private void onRefreshNeeded(object? sender, EventArgs e)
    {
        myUserControl.Refresh();
        Visible = true;
    }
    protected override void OnFormClosing(FormClosingEventArgs e)
    {
        base.OnFormClosing(e);
        if (e.CloseReason.Equals(CloseReason.UserClosing))
        {
            e.Cancel = true;
            Hide();
        }
    }
}

The custom UserControl implements the Control.Refresh() method in an app-specific manner, for example by calling load(). Here is a mock implementation that demonstrates that it's working.

class MyUserControl : UserControl
{
    public MyUserControl()
    {
        Controls.Add(label);
    }
    public new void Refresh()
    {
        base.Refresh();
        load();
    }
    Label label = new Label { Location = new Point(10, 100), AutoSize = true,  };
    int debugCount = 0;
    private void load()
    {
        label.Text =  $"Count = {++debugCount}: Your custom SQL load code goes here";
    }
}