相対ディレクトリパスから絶対ディレクトリパスを取得する

using System;
using System.Windows.Forms;

using System.IO; // これを忘れないように.

namespace aaa
{
	public partial class Form1 : Form
	{
		public Form1()
		{
			InitializeComponent();
		}

		private void button0_Click( object sender, EventArgs e )
		{

			DirectoryInfo di0 = new DirectoryInfo( Application.StartupPath );
			String strA = di0.FullName;

			String strB = Path.GetFullPath( "./" );

			const String CAPTION = "ドットひとつ";
			MessageBox.Show( $"{strA}\r\n{strB}", CAPTION );

		}

		private void button1_Click( object sender, EventArgs e )
		{

			DirectoryInfo di0 = new DirectoryInfo( Application.StartupPath );
			DirectoryInfo di1 = di0.Parent;
			String strA = di1.FullName;

			String strB = Path.GetFullPath( "../" );

			String CAPTION = "ドットふたつが1回";
			MessageBox.Show( $"{strA}\r\n{strB}", CAPTION );

		}

		private void button2_Click( object sender, EventArgs e )
		{

			DirectoryInfo di0 = new DirectoryInfo( Application.StartupPath );
			DirectoryInfo di1 = di0.Parent;
			DirectoryInfo di2 = di1.Parent;
			String strA = di2.FullName;

			String strB = Path.GetFullPath( "../../" );

			String CAPTION = "ドットふたつが2回";
			MessageBox.Show( $"{strA}\r\n{strB}", CAPTION );

		}

		private void button3_Click( object sender, EventArgs e )
		{

			DirectoryInfo di0 = new DirectoryInfo( Application.StartupPath );
			DirectoryInfo di1 = di0.Parent;
			DirectoryInfo di2 = di1.Parent;
			DirectoryInfo di3 = di2.Parent;
			String strA = di3.FullName;

			String strB = Path.GetFullPath( "../../../" );

			String CAPTION = "ドットふたつが3回";
			MessageBox.Show( $"{strA}\r\n{strB}", CAPTION );

		}

	}
}

C:\tmp\sss\aaa\bin\Debug
C:\tmp\sss\aaa\bin\Debug\

C:\tmp\sss\aaa\bin
C:\tmp\sss\aaa\bin\

C:\tmp\sss\aaa
C:\tmp\sss\aaa\

C:\tmp\sss
C:\tmp\sss\