How to program to obtain database information

Obtain database information:

        public List<string> GetDatabase(string connectionString)
        {
using (SqlDataAdapter sqlDataAdapter = new SqlDataAdapter("SELECT Name FROM Master.sys.SysDatabases WHERE dbid > 4 ORDER BY Name ", connectionString))
            {
                DataTable table = new DataTable();
try
                {
                    sqlDataAdapter.Fill(table);

                    List<string> tables = new List<string>();
foreach (DataRow row in table.Rows)
                    {
                        tables.Add(row[0].ToString());
                    }
return tables;
                }
catch
                {
throw new ApplicationException("can not connect to server");
                }
            }
        }

Get data table information:

        public List<string> GetTables(string connectionString, string db)
        {
using (SqlDataAdapter sqlDataAdapter = new SqlDataAdapter(String.Format("SELECT name FROM {0}.sys.SysObjects WHERE (xtype = 'U' OR xtype = 'V') ORDER BY name", db), connectionString))
            {
                DataTable table = new DataTable();
try
                {
                    sqlDataAdapter.Fill(table);

                    List<string> tables = new List<string>();
foreach (DataRow row in table.Rows)
                    {
                        tables.Add(row[0].ToString());
                    }
return tables;
                }
catch
                {
throw new ApplicationException(String.Format("can not connect to db {0} ", db));
                }
            }
        }

Obtain field information:

        public DataTable GetFields(string connectionString, string db, string tableName)
        {
using (SqlDataAdapter sqlDataAdapter = new SqlDataAdapter(string.Format("SELECT A.[name],[Type] =B.[name],A.[length] FROM {0}.sys.SysColumns AS A LEFT JOIN {0}.sys.SysTypes AS B ON A.[xtype]=B.[xusertype]  WHERE A.[id] = OBJECT_ID('{0}.dbo.[{1}]')", db, tableName.Replace("'", "''")), connectionString))
            {
                DataTable table = new DataTable();
try
                {
                    sqlDataAdapter.Fill(table);

return table;
                }
catch
                {
throw new ApplicationException(String.Format("can not connect to db {0}", db));
                }
            }
        }