Simple C# Wrapper for SQLite

[DllImport("sqlite3")]
private static extern IntPtr sqlite3_free(IntPtr error);

string errorMessage = PointerToString(sqlite3_errmsg(error));
sqlite3_free(error);
throw new SqlQueryException("Error with executing non-query: \"" + query + "\"!\n" + errorMessage);

using System;
using System.Collections;
using System.Data;
using System.Runtime.InteropServices;
using System.Text;

namespace SQLiteWrapper
{
    /// <summary>
    /// SQLite wrapper with functions for opening, closing and executing queries.
    /// </summary>
    public class SQLiteBase
    {
        // imports system functions for work with pointers
        [DllImport("kernel32")]
        private extern static IntPtr HeapAlloc(IntPtr heap, UInt32 flags, UInt32 bytes);

		[DllImport("kernel32")]
		static extern bool HeapFree(IntPtr heap, UInt32 flags, IntPtr block);

        [DllImport("kernel32")]
        private extern static IntPtr GetProcessHeap();

        [DllImport("kernel32")]
        private extern static int lstrlen(IntPtr str);

        // imports SQLite functions
        [DllImport("sqlite3")]
        private static extern int sqlite3_open(IntPtr fileName, out IntPtr database);

        [DllImport("sqlite3")]
        private static extern int sqlite3_close(IntPtr database);

        [DllImport("sqlite3")]
        private static extern int sqlite3_exec(IntPtr database, IntPtr query, IntPtr callback, IntPtr arguments, out IntPtr error);

        [DllImport("sqlite3")]
        private static extern IntPtr sqlite3_errmsg(IntPtr database);

        [DllImport("sqlite3")]
        private static extern int sqlite3_prepare_v2(IntPtr database, IntPtr query, int length, out IntPtr statement, out IntPtr tail);

        [DllImport("sqlite3")]
        private static extern int sqlite3_step(IntPtr statement);

        [DllImport("sqlite3")]
        private static extern int sqlite3_column_count(IntPtr statement);

        [DllImport("sqlite3")]
        private static extern IntPtr sqlite3_column_name(IntPtr statement, int columnNumber);

        [DllImport("sqlite3")]
        private static extern int sqlite3_column_type(IntPtr statement, int columnNumber);

        [DllImport("sqlite3")]
        private static extern int sqlite3_column_int(IntPtr statement, int columnNumber);

        [DllImport("sqlite3")]
        private static extern double sqlite3_column_double(IntPtr statement, int columnNumber);

        [DllImport("sqlite3")]
        private static extern IntPtr sqlite3_column_text(IntPtr statement, int columnNumber);

        [DllImport("sqlite3")]
        private static extern IntPtr sqlite3_column_blob(IntPtr statement, int columnNumber);

        [DllImport("sqlite3")]
        private static extern IntPtr sqlite3_column_table_name(IntPtr statement, int columnNumber);

        [DllImport("sqlite3")]
        private static extern int sqlite3_finalize(IntPtr handle);

		[DllImport("sqlite3")]
		private static extern IntPtr sqlite3_free(IntPtr error);

        // SQLite constants 
        private const int SQL_OK = 0;
        private const int SQL_ROW = 100;
        private const int SQL_DONE = 101;

        /// <summary>
        /// SQLite data types.
        /// </summary>
        public enum SQLiteDataTypes { 
            /// <summary>
            /// Integer numbers.
            /// </summary>
            INT = 1, 
            /// <summary>
            /// Decimal numbers.
            /// </summary>
            FLOAT,
            /// <summary>
            /// All kinds of texts.
            /// </summary>
            TEXT, 
            /// <summary>
            /// Blob objects - binary large objects.
            /// </summary>
            BLOB, 
            /// <summary>
            /// Nothing.
            /// </summary>
            NULL };

        // pointer to database
        private IntPtr database;

        /// <summary>
        /// Creates new instance of SQLiteBase class with no database attached.
        /// </summary>
        public SQLiteBase()
        {
            database = IntPtr.Zero;
        }

        /// <summary>
        /// Creates new instance of SQLiteBase class and opens database with given name.
        /// </summary>
        /// <param name="baseName">Name (and path) to SQLite database file</param>
        public SQLiteBase(String baseName)
        {
            OpenDatabase(baseName);
        }

        /// <summary>
        /// Opens database. 
        /// </summary>
        /// <param name="baseName">Name of database file</param>
        public void OpenDatabase(String baseName)
        {
			IntPtr ptr = StringToPointer(baseName);
            // opens database 
            if (sqlite3_open(ptr, out database) != SQL_OK)
            {
                // if there is some error, database pointer is set to 0 and exception is throws
                database = IntPtr.Zero;
				HeapFree(GetProcessHeap(), 0, ptr);
                throw new Exception("Error with opening database " + baseName + "!");
            }
			HeapFree(GetProcessHeap(), 0, ptr);
		}

        /// <summary>
        /// Closes opened database.
        /// </summary>
        public void CloseDatabase()
        {
            // closes the database if there is one opened
            if (database != IntPtr.Zero)
            {
                sqlite3_close(database);
            }
        }

        /// <summary>
        /// Returns the list of tables in opened database.
        /// </summary>
        /// <returns></returns>
        public ArrayList GetTables()
        {
            // executes query that select names of all tables and views in master table of every database
            String query = "SELECT name FROM sqlite_master " +
                                        "WHERE type IN ('table','view') AND name NOT LIKE 'sqlite_%'" +
                                        "UNION ALL " +
                                        "SELECT name FROM sqlite_temp_master " +
                                        "WHERE type IN ('table','view') " +
                                        "ORDER BY 1";
            DataTable table = ExecuteQuery(query);

            // when table is generater, it writes all table names in list that is returned
            ArrayList list = new ArrayList();
            foreach (DataRow row in table.Rows)
            {
                list.Add(row.ItemArray[0].ToString());
            }
            return list;
        }

        /// <summary>
        /// Executes query that does not return anything (e.g. UPDATE, INSERT, DELETE).
        /// </summary>
        /// <param name="query"></param>
        public void ExecuteNonQuery(String query)
        {
            // calles SQLite function that executes non-query
            IntPtr error;
			IntPtr ptr = StringToPointer(query);
            sqlite3_exec(database, ptr, IntPtr.Zero, IntPtr.Zero, out error);
            // if there is error, excetion is thrown
            if (error != IntPtr.Zero) {
				HeapFree(GetProcessHeap(), 0, ptr);
				string errorMessage = PointerToString(sqlite3_errmsg(error));
				sqlite3_free(error);
				throw new SystemException("Error with executing non-query: \"" + query + "\"!\n" + errorMessage);
			}
        	HeapFree(GetProcessHeap(), 0, ptr);
        }

        /// <summary>
        /// Executes query that does return something (e.g. SELECT).
        /// </summary>
        /// <param name="query"></param>
        /// <returns></returns>
        public DataTable ExecuteQuery(String query)
        {
            // processed query
            IntPtr statement;

            // excess data, it has no use
            IntPtr excessData;

            // process query and make statement
			IntPtr ptr = StringToPointer(query);
            sqlite3_prepare_v2(database, ptr, GetPointerLenght(ptr), out statement, out excessData);
            // table for result of function
            DataTable table = new DataTable();

            // reads first row - it is different from next rows because it also creates table columns
            // result - returns SLQ_ROW while there is next row
            int result = ReadFirstRow(statement, ref table);

            // reads rows
            while (result == SQL_ROW)
            {
                result = ReadNextRow(statement, ref table);
            }

            // finalize executing this query
            sqlite3_finalize(statement);

            HeapFree(GetProcessHeap(), 0, ptr);

            // returns table
            return table;
        }

        // private function for reading firs row and creating DataTable
        private int ReadFirstRow(IntPtr statement, ref DataTable table)
        {
            // create new instance of DataTable with name "resultTable"
            table = new DataTable("resultTable");

            // evaluates statement 
            int resultType = sqlite3_step(statement);

            // if result of statement is SQL_ROW, create new table and write row in it
            if (resultType == SQL_ROW)
            {
                // returns number of columns returned by statement
                int columnCount = sqlite3_column_count(statement);

                // declartaion of variables for reading first row
                String columnName = "";
                int columnType = 0;
                object[] columnValues = new object[columnCount];

                // reads columns one by one
                for (int i = 0; i < columnCount; i++)
                {
                    // returns the name of current column
                    columnName = PointerToString(sqlite3_column_name(statement, i));

                    // returns the type of current column
                    columnType = sqlite3_column_type(statement, i);

                    // checks type of columns - neccessary because different functions are required for different types
                    switch (columnType)
                    {
                            // in case of integer column
                        case (int)SQLiteDataTypes.INT:
                            {
                                // adds new integer column to table
                                table.Columns.Add(columnName, Type.GetType("System.Int32"));

                                // writes column value in object array
                                columnValues[i] = sqlite3_column_int(statement, i);
                                break;
                            }
                            // same as for integer, this one is for float
                        case (int)SQLiteDataTypes.FLOAT:
                            {
                                table.Columns.Add(columnName, Type.GetType("System.Single"));
                                columnValues[i] = sqlite3_column_double(statement, i);
                                break;
                            }
                            // ... for text
                        case (int)SQLiteDataTypes.TEXT:
                            {
                                table.Columns.Add(columnName, Type.GetType("System.String"));
                                columnValues[i] = PointerToString(sqlite3_column_text(statement, i));
                                break;
                            }
                            // ... for blob - blob are written in table as strings!!
                        case (int)SQLiteDataTypes.BLOB:
                            {
                                table.Columns.Add(columnName, Type.GetType("System.String"));
                                columnValues[i] = PointerToString(sqlite3_column_blob(statement, i));
                                break;
                            }
                            // in case of something other, value is read as string
                        default:
                            {
                                table.Columns.Add(columnName, Type.GetType("System.String"));
                                columnValues[i] = "";
                                break;
                            }
                    }
                }

                // writes column values to table
                table.Rows.Add(columnValues);
            }

            // evalute statemnet for next results
            return sqlite3_step(statement);
        }

        // private function for reading rows other than first
        // it' same like first row, only without creating table and columns
        private int ReadNextRow(IntPtr statement, ref DataTable table)
        {
            int columnCount = sqlite3_column_count(statement);

            int columnType = 0;
            object[] columnValues = new object[columnCount];

            for (int i = 0; i < columnCount; i++)
            {
                columnType = sqlite3_column_type(statement, i);

                switch (columnType)
                {
                    case (int)SQLiteDataTypes.INT:
                        {
                            columnValues[i] = sqlite3_column_int(statement, i);
                            break;
                        }
                    case (int)SQLiteDataTypes.FLOAT:
                        {
                            columnValues[i] = sqlite3_column_double(statement, i);
                            break;
                        }
                    case (int)SQLiteDataTypes.TEXT:
                        {
                            columnValues[i] = PointerToString(sqlite3_column_text(statement, i));
                            break;
                        }
                    case (int)SQLiteDataTypes.BLOB:
                        {
                            columnValues[i] = PointerToString(sqlite3_column_blob(statement, i));
                            break;
                        }
                    default:
                        {
                            columnValues[i] = "";
                            break;
                        }
                }
            }
            table.Rows.Add(columnValues);
            return sqlite3_step(statement);
        }

        // converts string to pointer
        private IntPtr StringToPointer(String str)
        {
            // if string is null, pointer is 0
            if (str == null)
            {
                return IntPtr.Zero;
            }
            else
            {
                // else, convert it to pointer
                Encoding encoding = Encoding.UTF8;
                Byte[] bytes = encoding.GetBytes(str);
                int length = bytes.Length + 1;
                IntPtr pointer = HeapAlloc(GetProcessHeap(), 0, (UInt32)length);
                Marshal.Copy(bytes, 0, pointer, bytes.Length);
                Marshal.WriteByte(pointer, bytes.Length, 0);
                return pointer;
            }
        }

        // convert pointer to string
        private String PointerToString(IntPtr ptr)
        {
            if (ptr == IntPtr.Zero)
                return null;

            Encoding encoding = Encoding.UTF8;

            int length = GetPointerLenght(ptr);
            Byte[] bytes = new Byte[length];
            Marshal.Copy(ptr, bytes, 0, length);
            return encoding.GetString(bytes, 0, length);
        }

        // returns length of pointer
        private int GetPointerLenght(IntPtr ptr)
        {
            if (ptr == IntPtr.Zero)
                return 0;
            return lstrlen(ptr);
        }
    }
}