DataTableReader.GetChars(Int32, Int64, Char[], Int32, Int32) 方法

定义

命名空间:

System.Data

程序集:

System.Data.Common.dll

Source:

DataTableReader.cs

以字符数组的形式返回指定列的值。

public:
 override long GetChars(int ordinal, long dataIndex, cli::array <char> ^ buffer, int bufferIndex, int length);

public override long GetChars (int ordinal, long dataIndex, char[]? buffer, int bufferIndex, int length);

override this.GetChars : int * int64 * char[] * int * int -> int64

Public Overrides Function GetChars (ordinal As Integer, dataIndex As Long, buffer As Char(), bufferIndex As Integer, length As Integer) As Long

参数

ordinal

Int32

从零开始的列序号。

dataIndex

Int64

字段中作为读取操作起始位置的索引。

buffer

Char[]

要读入字节流的缓冲区。

bufferIndex

Int32

缓冲区中的索引，从该索引处开始放置数据。

length

Int32

复制到缓冲区中的最大长度。

返回

Int64

读取的实际字符数。

例外

ArgumentOutOfRangeException

传递的索引超出了 0 到 FieldCount -1 的范围。

DeletedRowInaccessibleException

尝试从已删除的行中检索数据。

InvalidOperationException

尝试读取或访问已关闭的 DataTableReader 中的列。

InvalidCastException

指定的列不包含字符数组。

示例

下面的示例演示 GetChars 方法。 方法 TestGetChars 需要传递一个 DataTableReader 填充了两列数据的 ：第一列是文件名，第二列是字符数组。 此外， TestGetChars 允许指定在从 中的 DataTableReader字符数组读取数据时要使用的缓冲区大小。 TestGetChars使用 的第一列中DataTableReader提供的数据作为文件名，创建一个对应于 中DataTableReader每行数据的文件。

此过程演示如何使用 GetChars 方法读取作为字符数组存储在 中的数据 DataTable 。 任何其他类型的数据都会导致 GetChars 方法引发 InvalidCastException。

using System;
using System.Data;
using System.IO;

class Class1
{
    static void Main()
    {
        DataTable table = new DataTable();
        table.Columns.Add("FileName", typeof(string));
        table.Columns.Add("Data", typeof(char[]));
        table.Rows.Add(new object[] { "File1.txt", "0123456789ABCDEF".ToCharArray() });
        table.Rows.Add(new object[] { "File2.txt", "0123456789ABCDEF".ToCharArray() });

        DataTableReader reader = new DataTableReader(table);
        TestGetChars(reader, 7);
    }

    private static void TestGetChars(DataTableReader reader, int bufferSize)
    {
        // The filename is in column 0, and the contents are in column 1.
        const int FILENAME_COLUMN = 0;
        const int DATA_COLUMN = 1;

        char[] buffer;
        long offset;
        int charsRead = 0;
        string fileName;
        int currentBufferSize = 0;

        while (reader.Read())
        {
            // Reinitialize the buffer size and the buffer itself.
            currentBufferSize = bufferSize;
            buffer = new char[bufferSize];
            // For each row, write the data to the specified file.
            // First, verify that the FileName column isn't null.
            if (!reader.IsDBNull(FILENAME_COLUMN))
            {
                // Get the file name, and create a file with
                // the supplied name.
                fileName = reader.GetString(FILENAME_COLUMN);
                // Start at the beginning.
                offset = 0;

                using (StreamWriter outputStream =
                           new StreamWriter(fileName, false))
                {
                    try
                    {
                        // Loop through all the characters in the input field,
                        // incrementing the offset for the next time. If this
                        // pass through the loop reads characters, write them to
                        // the output stream.
                        do
                        {
                            charsRead = (int)reader.GetChars(DATA_COLUMN, offset,
                                buffer, 0, bufferSize);
                            if (charsRead > 0)
                            {
                                outputStream.Write(buffer, 0, charsRead);
                                offset += charsRead;
                            }
                        } while (charsRead > 0);
                    }
                    catch (Exception ex)
                    {
                        Console.WriteLine(fileName + ": " + ex.Message);
                    }
                }
            }
        }
        Console.WriteLine("Press Enter key to finish.");
        Console.ReadLine();
    }
}

Imports System.Data
Imports System.IO

Module Module1

   Private Sub TestGetChars( _
      ByVal reader As DataTableReader, ByVal bufferSize As Integer)

      ' The filename is in column 0, and the contents are in column 1.
      Const FILENAME_COLUMN As Integer = 0
      Const DATA_COLUMN As Integer = 1

      Dim buffer() As Char
      Dim offset As Integer
      Dim charsRead As Integer
      Dim fileName As String
      Dim currentBufferSize As Integer

      While reader.Read
         ' Reinitialize the buffer size and the buffer itself.
         currentBufferSize = bufferSize
         ReDim buffer(bufferSize - 1)

         ' For each row, write the data to the specified file.

         ' First, verify that the FileName column isn't null.
         If Not reader.IsDBNull(FILENAME_COLUMN) Then
            ' Get the file name, and create a file with 
            ' the supplied name.
            fileName = reader.GetString(FILENAME_COLUMN)

            ' Start at the beginning.
            offset = 0

            Using outputStream As New StreamWriter(fileName, False)
               Try

                  ' Loop through all the characters in the input field,
                  ' incrementing the offset for the next time. If this
                  ' pass through the loop reads characters, write them to 
                  ' the output stream.
                  Do
                     charsRead = Cint(reader.GetChars(DATA_COLUMN, offset, _
                        buffer, 0, bufferSize))
                     If charsRead > 0 Then
                        outputStream.Write(buffer, 0, charsRead)
                        offset += charsRead
                     End If
                  Loop While charsRead > 0
               Catch ex As Exception
                  Console.WriteLine(fileName & ": " & ex.Message)
               End Try
            End Using
         End If
      End While
      Console.WriteLine("Press Enter key to finish.")
      Console.ReadLine()
   End Sub

   Sub Main()
      Dim table As New DataTable
      table.Columns.Add("FileName", GetType(System.String))
      table.Columns.Add("Data", GetType(System.Char()))
      table.Rows.Add("File1.txt", "0123456789ABCDEF".ToCharArray)
      table.Rows.Add("File2.txt", "0123456789ABCDEF".ToCharArray)

      Dim reader As New DataTableReader(table)
      TestGetChars(reader, 7)
   End Sub
End Module

注解

GetChars 返回字段中的可用字符数。 大多数情况下，这是字段的确切长度。 但是，如果 GetChars 已用于从字段获取字符，则返回的数字可能小于字段的真实长度。

如果到达字段的末尾，读取的实际字符数可以小于请求的长度。 如果在 Visual Basic) 中传递为 null (Nothing 的缓冲区， GetChars 则 返回整个字段的长度（以字符为单位），而不是基于缓冲区偏移量参数的剩余大小。

不执行任何转换;因此，要检索的数据必须已是字符数组或可强制转换为字符数组。