JSON

Read json file:

var Json: TJson;

with TJsonParser.Create do
try
  Json := ParseUtf8File('c:\data.json');
  try
    ShowMessage(Json.ToString);
  finally
    Json.Free;
  end;
finally
  Free;
end;

Create new json file:

var
  MyArray: TJsonArray;
  MyObject: TJsonObject;

with TJsonObject.Create do
try
  SetNull('Null');
  SetTrue('True');
  SetFalse('False');
  SetNumber('Number', 1.23);
  SetString('String', 'hello');
  SetDateTime('DateTime', Now);

  MyArray := SetArray('Array', 3);
  MyArray.SetString(0, 'First item');
  MyArray.SetString(1, 'Second item');
  MyArray.SetNumber(2, 3.21);

  MyObject := SetObject('Object');
  MyObject.SetString('First Member', 'Hello');
  MyObject.SetString('Second Member', 'World');
  MyObject.SetFalse('Third Member');
  MyObject.SetTrue('Fourth Member');
  MyObject.SetNumber('Other Member', 1.11);

  ShowMessage(ToString);
  ToUtf8File('c:\new.json');
finally
  Free;
end;

Use fast, streaming TJsonWriter to create json file without in-memory overhead of TJson objects:

with TJsonWriter.Create('c:\data.json') do
try
  BeginObject;
    WriteNull('Null');
    Write('True', True);
    Write('False', False);
    Write('Number', 1.23);
    Write('String', 'hello');
    Write('DateTime', Now, 0);

    BeginArray('Array');
      Write('First item');
      Write('Second item');
      Write(3.21);
    EndArray;

    BeginObject('Object');
      Write('First Member', 'Hello');
      Write('Second Member', 'World');
      Write('Third Member', False);
      Write('Fourth Member', True);
      Write('Other Member', 1.11);
    EndObject;
  EndObject;

  Check;
finally
  Free;
end;

Use fast, streaming TJsonReader to read json file without in-memory overhead of TJson objects:

var
  JsonItem: TJsonItem;
  Prefix: WideString;

with TJsonReader.Create('c:\data.json', True, True) do
try
  repeat
    JsonItem := Read;

    Prefix := '(' + IntToStr(Row) + ', ' + IntToStr(Column) + '): ';
    if IsMember then
      Prefix := Prefix + '"' + MemberName + '" : ';

    case JsonItem of
      itNull:        ShowMessage(Prefix + 'null');
      itFalse:       ShowMessage(Prefix + 'false');
      itTrue:        ShowMessage(Prefix + 'true');
      itNumber:      ShowMessage(Prefix + FloatToStr(NumberValue));
      itString:      ShowMessage(Prefix + '"' + StringValue + '"');
      itBeginArray:  ShowMessage(Prefix + '[');
      itEndArray:    ShowMessage(Prefix + ']');
      itBeginObject: ShowMessage(Prefix + '{');
      itEndObject:   ShowMessage(Prefix + '}');
    end;
  until JsonItem = itEof;
finally
  Free;
end;