.net 如何在C#中从DBF文件中读取Memo类型字段？

yv5phkfx  于 2023-02-17  发布在  .NET
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我找不到如何从DBF文件中读取Memo类型字段的解决方案。

根据我目前的RnD，我发现“备忘录”数据存储在一个单独的文件中，与DBF文件同名，但文件扩展名为“.fpt”。“备忘录”数据文件由一系列块组成，每个块包含固定数量的字节（通常为512字节），每个块由块编号标识。
DBF文件中的“备注”列包含一个10字节的指针，用于标识相应记录的第一个数据块的块编号。如果特定记录的数据超过一个块的大小，则根据需要分配额外的块，并使用存储在第一个块中的指针将其链接在一起。
此外，我使用的源代码提供here

using System;
using System.Collections.Generic;
using System.Data;
using System.Globalization;
using System.IO;
using System.Linq;
using System.Reflection;
using System.Runtime.InteropServices;
using System.Text;

namespace System.IO
{
    /// <summary>
    /// This class reads a dbf files
    /// </summary>
    public class DBFReader : IDisposable
    {
        private BinaryReader reader;
        private Encoding encoding;

        public DBFReader(Stream stream, Encoding encoding)
        {
            this.encoding = encoding;
            this.reader = new BinaryReader(stream, encoding);

            ReadHeader();
        }

        public DBFReader(string filename, Encoding encoding)
        {
            if (File.Exists(filename) == false)
                throw new FileNotFoundException();

            this.encoding = encoding;
            var bs = new BufferedStream(File.OpenRead(filename));
            this.reader = new BinaryReader(bs, encoding);

            ReadHeader();
        }

        private void ReadHeader()
        {
            byte[] buffer = reader.ReadBytes(Marshal.SizeOf(typeof(DBFHeader)));

            // Marshall the header into a DBFHeader structure
            GCHandle handle = GCHandle.Alloc(buffer, GCHandleType.Pinned);
            this.header = (DBFHeader)Marshal.PtrToStructure(handle.AddrOfPinnedObject(), typeof(DBFHeader));
            handle.Free();

            fields = new List<DBFFieldDescriptor>();
            while (reader.PeekChar() != 13)
            {
                buffer = reader.ReadBytes(Marshal.SizeOf(typeof(DBFFieldDescriptor)));
                handle = GCHandle.Alloc(buffer, GCHandleType.Pinned);
                var fieldDescriptor = (DBFFieldDescriptor)Marshal.PtrToStructure(handle.AddrOfPinnedObject(), typeof(DBFFieldDescriptor));
                if ((fieldDescriptor.Flags & DBFFieldFlags.System) != DBFFieldFlags.System )
                {
                    fields.Add(fieldDescriptor);
                }
                handle.Free();
            }

            byte headerTerminator = reader.ReadByte();
            byte[] backlink = reader.ReadBytes(263);
        }

        private void ReadRecords()
        {
            records = new List<Dictionary<DBFFieldDescriptor, object>>();

            // Skip back to the end of the header. 
            reader.BaseStream.Seek(header.HeaderLenght, SeekOrigin.Begin);
            for (int i = 0; i < header.NumberOfRecords; i++)
            {
                if (reader.PeekChar() == '*') // DELETED
                {
                    continue;
                }

                var record = new Dictionary<DBFFieldDescriptor, object>();
                var row = reader.ReadBytes(header.RecordLenght);

                foreach (var field in fields)
                {
                    byte[] buffer = new byte[field.FieldLength];
                    Array.Copy(row, field.Address, buffer, 0, field.FieldLength);
                    string text = (encoding.GetString(buffer) ?? String.Empty).Trim();

                    switch ((DBFFieldType)field.FieldType)
                    {
                        case DBFFieldType.Character:
                            record[field] = text;
                            break;

                        case DBFFieldType.Currency:
                            if (String.IsNullOrWhiteSpace(text))
                            {
                                if ((field.Flags & DBFFieldFlags.AllowNullValues) == DBFFieldFlags.AllowNullValues)
                                {
                                    record[field] = null;
                                }
                                else
                                {
                                    record[field] = 0.0m;
                                }
                            }
                            else
                            {
                                record[field] = Convert.ToDecimal(text);
                            }
                            break;

                        case DBFFieldType.Numeric:
                        case DBFFieldType.Float:
                            if (String.IsNullOrWhiteSpace(text))
                            {
                                if ((field.Flags & DBFFieldFlags.AllowNullValues) == DBFFieldFlags.AllowNullValues)
                                {
                                    record[field] = null;
                                }
                                else
                                {
                                    record[field] = 0.0f;
                                }
                            }
                            else
                            {
                                record[field] = Convert.ToSingle(text);
                            }
                            break;

                        case DBFFieldType.Date:
                            if (String.IsNullOrWhiteSpace(text))
                            {
                                if ((field.Flags & DBFFieldFlags.AllowNullValues) == DBFFieldFlags.AllowNullValues)
                                {
                                    record[field] = null;
                                }
                                else
                                {
                                    record[field] = DateTime.MinValue;
                                }
                            }
                            else
                            {
                                record[field] = DateTime.ParseExact(text, "yyyyMMdd", CultureInfo.InvariantCulture);
                            }
                            break;

                        case DBFFieldType.DateTime:
                            if (String.IsNullOrWhiteSpace(text) || BitConverter.ToInt64(buffer, 0) == 0)
                            {
                                if ((field.Flags & DBFFieldFlags.AllowNullValues) == DBFFieldFlags.AllowNullValues)
                                {
                                    record[field] = null;
                                }
                                else
                                {
                                    record[field] = DateTime.MinValue;
                                }
                            }
                            else
                            {
                                record[field] = JulianToDateTime(BitConverter.ToInt64(buffer, 0));
                            }
                            break;

                        case DBFFieldType.Double:
                            if (String.IsNullOrWhiteSpace(text))
                            {
                                if ((field.Flags & DBFFieldFlags.AllowNullValues) == DBFFieldFlags.AllowNullValues)
                                {
                                    record[field] = null;
                                }
                                else
                                {
                                    record[field] = 0.0;
                                }
                            }
                            else
                            {
                                record[field] = Convert.ToDouble(text);
                            }
                            break;

                        case DBFFieldType.Integer:
                            if (String.IsNullOrWhiteSpace(text))
                            {
                                if ((field.Flags & DBFFieldFlags.AllowNullValues) == DBFFieldFlags.AllowNullValues)
                                {
                                    record[field] = null;
                                }
                                else
                                {
                                    record[field] = 0;
                                }
                            }
                            else
                            {
                                record[field] = BitConverter.ToInt32(buffer, 0);    
                            }
                            break;

                        case DBFFieldType.Logical:
                            if (String.IsNullOrWhiteSpace(text))
                            {
                                if ((field.Flags & DBFFieldFlags.AllowNullValues) == DBFFieldFlags.AllowNullValues)
                                {
                                    record[field] = null;
                                }
                                else
                                {
                                    record[field] = false;
                                }
                            }
                            else
                            {
                                record[field] = (buffer[0] == 'Y' || buffer[0] == 'T');    
                            }
                            break;
                        
                        case DBFFieldType.Memo:
                        case DBFFieldType.General:
                        case DBFFieldType.Picture:
                        default:
                            record[field] = buffer;
                            break;
                    }
                }

                records.Add(record);
            }
        }

        public DataTable ReadToDataTable()
        {
            ReadRecords();

            var table = new DataTable();

            // Columns
            foreach (var field in fields)
            {
                var colType = ToDbType(field.FieldType);
                var column = new DataColumn(field.FieldName, colType ?? typeof(String));
                table.Columns.Add(column);
            }

            // Rows
            foreach (var record in records)
            {
                var row = table.NewRow();
                foreach (var column in record.Keys)
                {
                    row[column.FieldName] = record[column] ?? DBNull.Value;
                }
                table.Rows.Add(row);
            }

            return table;
        }

        public IEnumerable<Dictionary<string, object>> ReadToDictionary()
        {
            ReadRecords();
            return records.Select(record => record.ToDictionary(r => r.Key.FieldName, r => r.Value)).ToList();
        }

        public IEnumerable<T> ReadToObject<T>()
            where T : new()
        {
            ReadRecords();

            var type = typeof(T);
            var list = new List<T>();

            foreach (var record in records)
            {
                T item = new T();
                foreach (var pair in record.Select(s => new { Key = s.Key.FieldName, Value = s.Value }))
                {
                    var property = type.GetProperty(pair.Key, BindingFlags.IgnoreCase | BindingFlags.Public | BindingFlags.Instance);
                    if (property != null)
                    {
                        if (property.PropertyType == pair.Value.GetType())
                        {
                            property.SetValue(item, pair.Value, null);
                        }
                        else
                        {
                            if (pair.Value != DBNull.Value)
                            {
                                property.SetValue(item, System.Convert.ChangeType(pair.Value, property.PropertyType), null);
                            }
                        }
                    }
                }
                list.Add(item);
            }

            return list;
        }

        private DBFHeader header;
        private List<DBFFieldDescriptor> fields = new List<DBFFieldDescriptor>();

        private List<Dictionary<DBFFieldDescriptor, object>> records = new List<Dictionary<DBFFieldDescriptor,object>>();

        #region IDisposable

        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }

        protected void Dispose(bool disposing)
        {
            if (disposing == false) return;
            if (reader != null)
            {
                reader.Close();
                reader.Dispose();
                reader = null;
            }
        }

        ~DBFReader()
        {
            Dispose(false);
        } 

        #endregion

         /// <summary>
        /// Convert a Julian Date as long to a .NET DateTime structure
        /// Implemented from pseudo code at http://en.wikipedia.org/wiki/Julian_day
        /// </summary>
        /// <param name="julianDateAsLong">Julian Date to convert (days since 01/01/4713 BC)</param>
        /// <returns>DateTime</returns>
        private static DateTime JulianToDateTime(long julianDateAsLong)
         {
             if (julianDateAsLong == 0) return DateTime.MinValue;
            double p = Convert.ToDouble(julianDateAsLong);
            double s1 = p + 68569;
            double n = Math.Floor(4 * s1 / 146097);
            double s2 = s1 - Math.Floor(((146097 * n) + 3) / 4);
            double i = Math.Floor(4000 * (s2 + 1) / 1461001);
            double s3 = s2 - Math.Floor(1461 * i / 4) + 31;
            double q = Math.Floor(80 * s3 / 2447);
            double d = s3 - Math.Floor(2447 * q / 80);
            double s4 = Math.Floor(q / 11);
            double m = q + 2 - (12 * s4);
            double j = (100 * (n - 49)) + i + s4;
            return new DateTime(Convert.ToInt32(j), Convert.ToInt32(m), Convert.ToInt32(d));
        }

        /// <summary>
        /// This is the file header for a DBF. We do this special layout with everything
        /// packed so we can read straight from disk into the structure to populate it
        /// </summary>
        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)]
        private struct DBFHeader
        {
            /// <summary>The version.</summary>
            public readonly DBFVersion Version;

            /// <summary>The update year.</summary>
            public readonly byte UpdateYear;

            /// <summary>The update month.</summary>
            public readonly byte UpdateMonth;

            /// <summary>The update day.</summary>
            public readonly byte UpdateDay;

            /// <summary>The number of records.</summary>
            public readonly int NumberOfRecords;

            /// <summary>The length of the header.</summary>
            public readonly short HeaderLenght;

            /// <summary>The length of the bytes records.</summary>
            public readonly short RecordLenght;

            [MarshalAs(UnmanagedType.ByValArray, SizeConst = 16)]
            public readonly byte[] Reserved;

            /// <summary>Table Flags</summary>
            public readonly DBFTableFlags TableFlags;

            /// <summary>Code Page Mark</summary>
            public readonly byte CodePage;

            /// <summary>Reserved, contains 0x00</summary>
            public readonly short EndOfHeader;
        }

        public enum DBFVersion : byte
        {
            Unknown = 0,
            FoxBase = 0x02,
            FoxBaseDBase3NoMemo = 0x03,
            VisualFoxPro = 0x30,
            VisualFoxProWithAutoIncrement = 0x31,
            dBase4SQLTableNoMemo = 0x43,
            dBase4SQLSystemNoMemo = 0x63,
            FoxBaseDBase3WithMemo = 0x83,
            dBase4WithMemo = 0x8B,
            dBase4SQLTableWithMemo = 0xCB,
            FoxPro2WithMemo = 0xF5,
            FoxBASE = 0xFB
        }

        [Flags]
        public enum DBFTableFlags : byte
        {
            None = 0x00,
            HasStructuralCDX = 0x01,
            HasMemoField = 0x02,
            IsDBC = 0x04
        }

        /// <summary>
        /// This is the field descriptor structure. There will be one of these for each column in the table.
        /// </summary>
        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)]
        private struct DBFFieldDescriptor
        {
            /// <summary>The field name.</summary>
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 11)]
            public readonly string FieldName;

            /// <summary>The field type.</summary>
            public readonly char FieldType;

            /// <summary>The field address.</summary>
            public readonly int Address;

            /// <summary>The field length in bytes.</summary>
            public readonly byte FieldLength;

            /// <summary>The field precision.</summary>
            public readonly byte DecimalCount;

            /// <summary>Field Flags</summary>
            public readonly DBFFieldFlags Flags;

            /// <summary>AutoIncrement next value</summary>
            public readonly int AutoIncrementNextValue;

            /// <summary>AutoIncrement step value</summary>
            public readonly byte AutoIncrementStepValue;

            /// <summary>Reserved</summary>
            [MarshalAs(UnmanagedType.ByValArray, SizeConst = 8)]
            public readonly byte[] Reserved;

            public override string ToString()
            {
                return String.Format("{0} {1}", FieldName, FieldType);
            }
        }

        [Flags]
        public enum DBFFieldFlags : byte
        {
            None = 0x00,
            System = 0x01,
            AllowNullValues = 0x02,
            Binary = 0x04,
            AutoIncrementing = 0x0C
        }

        public enum DBFFieldType : int
        {
            Character = 'C',
            Currency = 'Y',
            Numeric = 'N',
            Float = 'F',
            Date = 'D',
            DateTime = 'T',
            Double = 'B',
            Integer = 'I',
            Logical = 'L',
            Memo = 'M',
            General = 'G',
            Picture = 'P'
        }

        public static Type ToDbType(char type)
        {
            switch ((DBFFieldType)type)
            {
                case DBFFieldType.Float:
                    return typeof(float);

                case DBFFieldType.Integer:
                    return typeof(int);

                case DBFFieldType.Currency:
                    return typeof(decimal);

                case DBFFieldType.Character:
                case DBFFieldType.Memo:
                    return typeof(string);

                case DBFFieldType.Date:
                case DBFFieldType.DateTime:
                    return typeof(DateTime);

                case DBFFieldType.Logical:
                    return typeof(bool);

                case DBFFieldType.General:
                case DBFFieldType.Picture:
                    return typeof(byte[]);

                default:
                    return null;
            }
        }
    }
}
.net

1条答案

按热度按时间
o2g1uqev

o2g1uqev1#

你的问题是什么？（将是一个混乱的评论）
有关FPT文件结构的信息不完全正确，VFP帮助文件中提供了完整的详细信息。
你一定要读低层次的吗？如果你必须读，基本上你要做的是：

  • 从DBF本身收集备忘录块编号（块编号存储在DBF的相应字段中），
  • 对于备忘录条目，转到FPT中的块编号（取决于块大小）
  • 从块标题中读取大小并获取与存储大小匹配的字节。

执行此操作时，请始终读取为二进制，因为与C字符串不同，VFP字符串值不是ASCIIZ值，它们甚至可以包含'\x0'。
如果它不是一个必须阅读的方式，那么简单的方法是简单地阅读它使用VFPOLEDB OleDb驱动程序。

void Main()
{
    string cn = @"Provider=VFPOLEDB;Data source=C:\PROGRAM FILES (X86)\MICROSOFT VISUAL FOXPRO 9\SAMPLES\DATA;";
    string query = "select emp_id, first_name, last_name, notes  from Employee";

    DataTable t = new DataTable();
    new OleDbDataAdapter(query, cn).Fill(t);

    foreach (var row in t.AsEnumerable())
    {
        Console.WriteLine($"{row["emp_id"]}, {row["first_name"]}, {row["last_name"]}, ({row["notes"]})");
    }
}

请注意，VFPOLEDB驱动程序是32位的，您需要针对x86平台。正如他们所说，有来自sybaseads的64位驱动程序（从未使用过）。
PS：还搜索汤姆兄弟的Linq到VFP，Linq到EF VFP驱动程序。PS2：我很快浏览了一下您提供的源代码，并注意到这是旧的VFP版本，并不是真正正确的（即：假设Memo是一个不正确的字符串-如果你在C#中接受它们，你可能会丢失很多数据）。

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