Delphi Basics : File command

DelphiBasics

File
Keyword

Defines a typed or untyped file

1	keyword File(var UntypedFile : File;
2	var TypedFile : File of some type;

Description

The File keyword is used to define a variable as a binary file, normally as written to and read from a storage device.

Version 1

Gives a base untyped file. Such a file can only be read from and written to using BlockRead and BlockWrite. The basic data type is a byte.

AssignFile must be used to get a file handle.

Reset or ReWrite must then be used to open the file for read and/or write access. These specify the number of bytes that comprise one 'record' as seen by the BlockRead and BlockWrite routines.

Version 2

Defines the file with a base data type. For example, a simple type such as a char or a complex type such as a Record.

AssignFile must be used to get a file handle.

Reset or ReWrite must then be used to open the file for read and/or write access. Read and Write must be used to access the file.

In all cases, the type must be of fixed size, and access to the file must be in units of that type.

Notes

When using Records, make sure that they are Packed to prevent alignment problems.

Related commands

Append		Open a text file to allow appending of text to the end
AssignFile		Assigns a file handle to a binary or text file
CloseFile		Closes an open file
Reset		Open a text file for reading, or binary file for read/write
ReWrite		Open a text or binary file for write access
TextFile		Declares a file type for storing lines of text

Download this web site as a Windows program.

Example code : Read and write access to an untyped binary file

var
  myFile    : File;
  byteArray : array[1..8] of byte;
  oneByte   : byte;
  i, count  : Integer;

begin
  // Try to open the Test.byt file for writing to
  AssignFile(myFile, 'Test.byt');
  ReWrite(myFile, 4);  // Define a single 'record' as 4 bytes

  // Fill out the data array
  for i := 1 to 8 do
    byteArray[i] := i;

  // Write the data array to the file
  BlockWrite(myFile, byteArray, 2);  // Write 2 'records' of 4 bytes

  // Close the file
  CloseFile(myFile);

  // Reopen the file for reading
  Reset(myFile, 1);  // Now we define one record as 1 byte

  // Display the file contents
  while not Eof(myFile) do
  begin
    BlockRead(myFile, oneByte, 1);  // Read and display one byte at a time
    ShowMessage(IntToStr(oneByte));
  end;

  // Close the file for the last time
  CloseFile(myFile);
end;

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Example code : Writing to a typed binary file

var
  myWord, myWord1, myWord2 : Word;
  myFile : File of Word;

begin
  // Try to open the Test.bin binary file for writing to
  AssignFile(myFile, 'Test.cus');
  ReWrite(myFile);

  // Write a couple of lines of Word data to the file
  myWord1 := 234;
  myWord2 := 567;
  Write(myFile, myWord1, myWord2);

  // Close the file
  CloseFile(myFile);

  // Reopen the file in read only mode
  Reset(myFile);

  // Display the file contents
  while not Eof(myFile) do
  begin
    Read(myFile, myWord);
    ShowMessage(IntToStr(myWord));
  end;

  // Close the file for the last time
  CloseFile(myFile);
end;

Show full unit code

234
567