this.connexionButton.Click += new System.EventHandler(this.simpleButton1_Click);

private void simpleButton1_Click(object sender, EventArgs e)
{
     Boolean allowCnx = false;

     foreach (var row in myClass.ds.Tables["Users"].AsEnumerable())
     {
         if (row[1].ToString().ToLower() == idBox.Text.ToLower() && row[2].ToString().ToLower() == mdpBox.Text.ToLower())
         {
            allowCnx = true;
         }
     }

     if (allowCnx)
     {
         connexionButton.DialogResult = DialogResult.OK;
     }
     else
       XtraMessageBox.Show("Invalide Information", "Erreur", MessageBoxButtons.OK, MessageBoxIcon.Error);
}

using (login loginForm = new login())
{
     var result = loginForm.ShowDialog();
     if (result == DialogResult.OK)
        this.Show();
     else
        this.Close();
}

// Points to a different method than the one you posted
// (simpleButton1_Click_1 instead of simpleButton1_Click)
this.simpleButton1.Click += new System.EventHandler(this.simpleButton1_Click_1);

// This isn't simpleButton1_Click_1
private void simpleButton1_Click(object sender, EventArgs e)

// When you launch the login form, I do not know what you intended to do with your
// calls to Show() and Close() but so long as you don't instend for them to do
// anything to the loginForm, that's fine.
using (login loginForm = new login())
{
    if (loginForm.ShowDialog() == DialogResult.OK)
        // Do stuff if logged in
    else
        // Do stuff if failed
}

private void simpleButton1_Click(object sender, EventArgs e)
{
    Boolean allowCnx = false;

    foreach (var row in myClass.ds.Tables["Users"].AsEnumerable())
        if (row[1].ToString().ToLower() == idBox.Text.ToLower() && row[2].ToString().ToLower() == mdpBox.Text.ToLower())
        {
            allowCnx = true;
        }

    if (allowCnx)
    {
        this.DialogResult = DialogResult.OK; // Don't set the DialogResult of a button.  Set it for the form.
    }
    else
    {
        this.DialogResult = DialogResult.Abort; // Because we didn't succeed
        XtraMessageBox.Show("Invalide Information",
            "Erreur",
            MessageBoxButtons.OK,
            MessageBoxIcon.Error);
    }
}

this.simpleButton1.Click += new System.EventHandler(this.simpleButton1_Click);

public bool AllowConnection = false;
private void simpleButton1_Click(object sender, EventArgs e)
{
     foreach (var row in myClass.ds.Tables["Users"].AsEnumerable())
     {
         if (row[1].ToString().ToLower() == idBox.Text.ToLower() && row[2].ToString().ToLower() == mdpBox.Text.ToLower())
            AllowConnection = true;
     }

     if (!AllowConnection)
         XtraMessageBox.Show("Invalide Information", "Erreur", MessageBoxButtons.OK, MessageBoxIcon.Error);
}

using (login loginForm = new login())
{
     loginForm.ShowDialog();
     if (loginForm.AllowConnection)
        this.Show();
     else
        this.Close();
}

foreach (var row in myClass.ds.Tables["Users"].AsEnumerable())

foreach (var row in myClass.ds.Tables["Users"].Rows)

connexionButton.DialogResult = DialogResult.OK;

this.DialogResult = DialogResult.OK;