How to refresh datagridview when closing child form?

There are many ways to do this, but the following is the simpliest and it will do what you want and get you started.

• Create a public method on your main form.
• Modified constructor of second form to take a main form.
• Create an instance of the second form passing the main form object.
• When closing second form, call the public method of the main form object.

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Form1

public partial class Form1 : Form {
    public Form1() {
        //'add a label and a buttom to form
        InitializeComponent();
    }
    private void button1_Click(object sender, EventArgs e) {
        Form2 oForm = new Form2(this);
        oForm.Show();
    }
    public void PerformRefresh() {
        this.label1.Text = DateTime.Now.ToLongTimeString();
    }
}

Form2

public class Form2 : Form {
    Form1 _owner;
    public Form2(Form1 owner) {
        _owner = owner;
        this.FormClosing += new System.Windows.Forms.FormClosingEventHandler(this.Form2_FormClosing);
    }
    private void Form2_FormClosing(object sender, FormClosingEventArgs e) {
        _owner.PerformRefresh();
    }
}