delphi6 的string和sql sever2000的char 是否匹配？？(40分)

我用adoquery对表lsntbl进行插入记录，其中要用到表lsntsk的数据，表lsntsk中有一字段
为char型，我在delphi中与之对应的是string，程序如下：

procedure tscjbkb.scjbkbp;
var
k,j,time,classroomid,teacherid,lsnnmb:integer;
classid1,classid2,classid3,classid4,mltnmb: integer;
i:extended;
typt:string;
begin
datamodule2.ADODataSetlsntbl.Open;
if datamodule2.ADODataSetlsntbl.RecordCount<>0 then
begin
showmessage('基本课程表已经生成！');
datamodule2.ADODataSetlsntbl.Close
end
else
begin
datamodule2.ADODataSetlsntbl.Close;
datamodule2.ADODataSetlsntsk.Open;
datamodule2.ADODataSetlsntsk.Sort:='rank';
datamodule2.ADODataSetlsntsk.First;
while not datamodule2.ADODataSetlsntsk.Eof do
begin
i:=datamodule2.ADODataSetlsntsk.fieldbyname('weektime').value;
k:=getint(i);
teacherid:=datamodule2.ADODataSetlsntsk.fieldbyname('teacherid').value;
classid1:=datamodule2.ADODataSetlsntsk.fieldbyname('classid1').value;
classid2:=datamodule2.ADODataSetlsntsk.fieldbyname('classid2').value;
classid3:=datamodule2.ADODataSetlsntsk.fieldbyname('classid3').value;
classid4:=datamodule2.ADODataSetlsntsk.fieldbyname('classid4').value;
mltnmb:=datamodule2.ADODataSetlsntsk.fieldbyname('mltnmb').value;
typt:=datamodule2.ADODataSetlsntsk.fieldbyname('typt').value;
lsnnmb:=datamodule2.ADODataSetlsntsk.fieldbyname('lsnnmb').Value;

datamodule2.adoqueryinsert.Parameters.ParamByName('sngdbl').value:=NuLL;
datamodule2.adoqueryinsert.Parameters.ParamByName('lsntskid').value
:=datamodule2.ADODataSetlsntsk.fieldbyname('lsntskid').Value;

for j:=1 to k do
begin
time:=gettime(j*2-1,lsnnmb);

while (not pdtimeclass(time,classid1,classid2,classid3,classid4)) or (not pdtimeteacher(time,teacherid)) do
time:=getnexttime(time,lsnnmb);

classroomid:=getclassroom(mltnmb,typt);
while not (pdtimeclassroom(time,classroomid)) do
classroomid:=getnextclassroom(classroomid,typt);

while not (pdtimeclassroom(time,classroomid)) do
begin
time:=getnexttime(time,lsnnmb);

while (not pdtimeclass(time,classid1,classid2,classid3,classid4)) or (not pdtimeteacher(time,teacherid)) do
time:=getnexttime(time,lsnnmb);

classroomid:=getclassroom(mltnmb,typt);
while not (pdtimeclassroom(time,classroomid)) do
classroomid:=getnextclassroom(classroomid,typt);
end;
datamodule2.adoqueryinsert.Parameters.ParamByName('time').Value:=time;
datamodule2.adoqueryinsert.Parameters.ParamByName('classroomid').Value:=classroomid;

datamodule2.adoqueryinsert.ExecSQL;
end;
datamodule2.adodatasetlsntsk.Next;
end;
datamodule2.ADODataSetlsntsk.close;
end;
end;
程序在 typt:=datamodule2.ADODataSetlsntsk.fieldbyname('typt').value;处出错，提示为
：invalid variant type conversion.

请问这是何故？该怎么办？

value为null值吧？ variant null无类型，不能转成字符串
用fieldbyname('typt').asstring强制转吧

有没有更详尽一点的意见呀

你能不能跟踪一下看看那里面是什么，定义个variant

呵呵,不用variant,就直接在
typt:=datamodule2.ADODataSetlsntsk.fieldbyname('typt').value
那里设置一个断点,让程序运行到那里时看看是社么值就可以了