实现效果:

知识运用:
DateTime结构的Now属性
File类的Create方法
实现代码:
private void button1_Click(object sender, EventArgs e)
{
FolderBrowserDialog fb = new FolderBrowserDialog();
if (fb.ShowDialog() == DialogResult.OK)
{
string fileName = DateTime.Now.ToString("yyyyMMdd-hhmmss")+".txt";
File.Create(fb.SelectedPath+"\"+fileName);
}
}