C#

Private Sub BACKToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BACKToolStripMenuItem.Click
        Try
            Dim t As New Form
            For Each f As Form In Application.OpenForms
                If f IsNot Me.ActiveMdiChild Then
                    t = f
                Else
                    t.Activate()
                    Return
                End If
            Next
        Catch ex As Exception
            System.Windows.Forms.MessageBox.Show(ex.Message, "Error...!!!", MessageBoxButtons.OK, MessageBoxIcon.Error)
        End Try
    End Sub

    Private Sub FORWARDToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles FORWARDToolStripMenuItem.Click
        Try
            Dim t As New Form
            For i As Integer = Application.OpenForms.Count - 1 To 0 Step -1
                Dim f As Form = Application.OpenForms(i)
                If f IsNot Me.ActiveMdiChild Then
                    t = f
                Else
                    t.Activate()
                    Return
                End If
            Next
        Catch ex As Exception
            System.Windows.Forms.MessageBox.Show(ex.Message, "Error...!!!", MessageBoxButtons.OK, MessageBoxIcon.Error)
        End Try
    End Sub

private void BACKToolStripMenuItem_Click(System.Object sender, System.EventArgs e)
{
	try {
		Form t = new Form();
		foreach (Form f in Application.OpenForms) {
			if (!object.ReferenceEquals(f, this.ActiveMdiChild)) {
				t = f;
			} else {
				t.Activate();
				return;
			}
		}
	} catch (Exception ex) {
		System.Windows.Forms.MessageBox.Show(ex.Message, "Error...!!!", MessageBoxButtons.OK, MessageBoxIcon.Error);
	}
}

private void FORWARDToolStripMenuItem_Click(System.Object sender, System.EventArgs e)
{
	try {
		Form t = new Form();
		for (int i = Application.OpenForms.Count - 1; i >= 0; i += -1) {
			Form f = Application.OpenForms(i);
			if (!object.ReferenceEquals(f, this.ActiveMdiChild)) {
				t = f;
			} else {
				t.Activate();
				return;
			}
		}
	} catch (Exception ex) {
		System.Windows.Forms.MessageBox.Show(ex.Message, "Error...!!!", MessageBoxButtons.OK, MessageBoxIcon.Error);
	}
}