hello Every one help me

Are you using comboBox1_SelectedIndexChanged event...
if yes
You need use try -catch
because for first time when form loaded, this event fire ...ant throw exception

This code work properly...I Have checked

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            try
            {
                
                SqlDataAdapter dt = new SqlDataAdapter("Select * from " + comboBox1.SelectedValue.ToString(), con.Connection);
                DataSet ds = new DataSet();
                dt.Fill(ds);
                dataGridView1.DataSource = ds.Tables[0];
            }
            catch (Exception ex) { MessageBox.Show(ex.Message); }
        }

Both your code and the code someone gave you are a disaster. You should be writing a proper data layer, giving your controls meaningful names, and generally writing readable code.

The fact that you have no idea what your error is, proves you know nothing about C#, do a C# course, then learn ASP.NET after that. You should also learn to use the debugger, so you can see the SQL being generated. The issue is that ToString returns the class name of the object, not the SelectedValue, as someone rightly said. His answer will fix your immediate problem, but you have far deeper issues that need addressing.

you need to prepare well first and go the above answer is absolutly right
think before asking a answer