Не могу понять в чем проблема, выводит Ошибка CS7036

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.IO;
 
namespace Strange_Math
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
public class GFG
{
int a, b;
 
public void Dannie(int N, int K)
{
this.a = N;
this.b = K;
}
 
public static void lexNumber(int a)
{
List<int> sol = new List<int>();
dfs(1, a, sol);
MessageBox.Show("[" + string.Join(",", sol) + "]");
}
 
public static void dfs(int temp, int a, List<int> sol)
{
if (temp > a)
return;
 
sol.Add(temp);
dfs(temp % 10, a, sol);
 
if (temp % 10 != 9)
dfs(temp + 1, a, sol);
 
}
}
 
private void button1_Click(object sender, EventArgs e)
{
int N = Convert.ToInt32(textBox1.Text);
int K = Convert.ToInt32(textBox2.Text);
 
GFG atv = new GFG();
atv.Dannie(N, K);
 
textBox3.Text = Convert.ToString(atv.lexNumber());
}
 
private void button2_Click(object sender, EventArgs e)
{
saveFileDialog1.Filter = "txt files(*.txt)| *.txt | All files(*.*) | *.* ";
if (saveFileDialog1.ShowDialog() == DialogResult.Cancel)
{
return;
}
else
{
File.WriteAllText(saveFileDialog1.FileName, "Числа в лексикографическом порядке: " + textBox3.Text);
}
}
}
}