如何通过从前一个（级联）中选择值来填充另一个 combobox？

Question

I'm having a bit of a problem when trying to populate cascading comboboxes. First part works great, the first combobox gets filled.

Here is the code:

private void LoadCombo()
{
        try
        {
            SqlConnection conn = new SqlConnection(@"Data source=SERVER\SOURCE;Initial Catalog=MyDatabase; Integrated security=true;");
            conn.Open();
            SqlDataAdapter da = new SqlDataAdapter("select name from sys.tables where type = 'U'", conn);
            DataTable dt = new DataTable();
            foreach (DataRow row in dt.Rows)
            {
                comboBox1.Items.Add(row["name"].ToString());
            }
        }
        catch (Exception ex)
        {
            throw ex;
        }
} 

Now, all table names are loaded in comboBox1. Now, here's the main problem: I'm trying to retrieve table name from comboBox1 and use the text to pass another query to SQL Server, so that, when I make a selection in comboBox 1, comboBox 2 gets filled with table properties of a specified column of a called table.

Here is the code:

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
        try
        {
            SqlConnection conn = new SqlConnection(@"Data source=SERVER\SOURCE;Initial Catalog=MyDatabase; Integrated security=true;");

            SqlDataAdapter da = new SqlDataAdapter("select distinct ColumnName from'" + comboBox1.Text + "'", conn);
            DataTable dt = new DataTable();
            da.Fill(dt);

            foreach (DataRow row in dt.Rows)
            {
                comboBox2.Items.Add(row["ColumnName"].ToString());
            }
        }
        catch (Exception ex)
        {
            throw ex;
        }                  
}

I've been getting an error

Syntax error near columnName

How do I fix this problem? Thank you!