C#

public partial class Input : Form
{
    public Input()
    {
        InitializeComponent();
    }

    public string StringValue
    {
        get { return textBox1.Text; }
        set { textBox1.Text = value; }
    }
}

public static class InputBox
{
    private static Input _input = new Input();

    public static DialogResult ShowDialog(string caption, ref string value)
    {
        _input.Text = caption;
        _input.StringValue = value; //Populates the textbox with the provided value as a default

        if (_input.ShowDialog() == DialogResult.OK)
        {
            value = _input.StringValue;
            return DialogResult.OK;
        }
        return DialogResult.Cancel;
    }
}

private void button1_Click(object sender, EventArgs e)
{
    string x = "Some default text";
    if (InputBox.ShowDialog("Test", ref x) == DialogResult.OK)  MessageBox.Show(x);
}

private void button1_Click(object sender, EventArgs e)
{
    Input inBox = new Input();
    inBox.StringValue = "Some default text";
    if (inBox.ShowDialog() == DialogResult.OK)  MessageBox.Show(inBox.StringValue);
}

public partial class Input : Form
{
    public Input()
    {
        InitializeComponent();
    }

    public Input(string caption, string defaultValue) : this()
    {
        this.Text = caption;
        textBox1.Text = defaultValue;
    }

    public string StringValue
    {
        get { return textBox1.Text; }
        set { textBox1.Text = value; }
    }
    }

public static class InputBox
{
    public static DialogResult ShowDialog(string caption, ref string value)
    {
        Input inForm = new Input(caption, value);

        if (inForm.ShowDialog() == DialogResult.OK)
        {
            value = inForm.StringValue;
            return DialogResult.OK;
        }
        return DialogResult.Cancel;
    }
}

private void button1_Click(object sender, EventArgs e)
{
    new Thread(new ParameterizedThreadStart(doMessage)).Start("one");
    new Thread(new ParameterizedThreadStart(doMessage)).Start("two");
}

private void doMessage(object msg)
{
    string x = msg.ToString();
    if (InputBox.ShowDialog("Test", ref x) == DialogResult.OK) MessageBox.Show(x);
}