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

定義

命名空間:

System.Data

組件:

System.Data.Common.dll

來源:

DataTableReader.cs

傳回指定資料行的值做為字元陣列。

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

參數

ordinal

Int32

以零為基底的資料行序數。

dataIndex

Int64

要開始讀取作業之欄位中的索引。

buffer

Char[]

要讀入字元資料流的目的緩衝區。

bufferIndex

Int32

緩衝區中的索引，從該處開始放置資料。

length

Int32

複製入緩衝區的最大長度。

傳回

Int64

讀取的實際字元數目。

例外狀況

ArgumentOutOfRangeException

傳遞的索引超出 0 到 FieldCount - 1 的範圍。

DeletedRowInaccessibleException

嘗試從已刪除的資料列擷取資料。

InvalidOperationException

嘗試在關閉的 DataTableReader 中讀取或存取資料行。

InvalidCastException

指定的資料行不包含字元陣列。

範例

下列範例示範 GetChars 方法。 方法 TestGetChars 預期會傳遞 DataTableReader 填滿兩個數據行的 ：第一個數據行中的檔名，以及第二個數據行中的字元陣列。 此外， TestGetChars 可讓您指定要用來讀取 中字元陣列 DataTableReader數據的緩衝區大小。 TestGetChars 使用 中第一個數據行中提供的數據做為檔名，建立對應至 中每個數據 DataTableReader列的 DataTableReader 檔案。

此程式示範如何使用 GetChars 方法讀取儲存在 中 DataTable 做為字元數位的數據。 任何其他資料類型都會導致 GetChars 方法擲回 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

備註

GetChars 會傳回欄位中可用的字元數。 在大部分情況下，這是欄位的確切長度。 但是，如果已使用 GetChars 從欄位取得字元數，則傳回的數字可能小於欄位的實際長度。

如果達到欄位結尾，讀取的實際字元數可以小於要求的長度。 如果您在 Visual Basic) 中傳遞 null (Nothing 的緩衝區， GetChars 則會以字元傳回整個字段的長度，而不是根據緩衝區位移參數的剩餘大小。

不會執行轉換;因此，要擷取的數據必須已經是字元陣列或可強制轉換成字元陣列。