C#

static Form1 _Instance = null;

public Form1()
{
  if ( _Instance != null ) [ throw an exception or assert or something ]
  _Instance = this;
}

public static void LogMessage( string message )
{
  _Instance.log( message );
}

Form1.LogMessage( message );

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Be excellent to each other

if (InvokeRequired)<br />

if (this.lb.InvokeRequired)<br />

lb.BeginInvoke(new LogDelegate(log), new object[] { message });<br />

class Form1 : Form {
    public Form() {
        ...
        Class2.Logger+=log;    // one statement per "log client" class
    }

    public void log(string s) {...}
}

class Class2 {
    public static event Action<string> Logger;
    private void log(string s) {
        if (Logger!=null) Logger(s);
    }
}

private void log(Exception exc) {
    foreach(string s in exc.ToString().Split('\r','\n')) if (s.Length!=0) log(s);
}

private void btnSearch_Click(object sender, EventArgs e)
{
    FillListBox(textBox1.Text);
}

public void FillListBox(string searchValue)
{
    try
    {
        dataTable.Clear();

        DataAdapter =
            new OleDbDataAdapter
            ("Select [PicId], [Path] From [Pics] WHERE PicId = '" +
            searchValue + "'", conn);
        DataAdapter.Fill(dataTable);

        listBox1.DataSource = dataTable;
        listBox1.DisplayMember = "Path";
        listBox1.ValueMember = "Path";
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message.ToString());
    }
}