free,freeandnil

Question

try
      tmpquery:=tadoquery.Create(nil);
      with tmpquery do
      begin
        connection:=sysdata.klerp;
        close;
        sql.Clear;
        sql.Add('select yuangmc from man_reny where bumbm=:bumbm');
        Parameters[0].Value:=str1;
        open;
        first;
        combobox2.Items.Clear;
        combobox2.Text:='';
        while not eof do
        begin
          combobox2.Items.Add(fieldbyname('yuangmc').asstring);
          next;
        end;
      end;
    finally
       freeandnil(tmpquery);这里如果改为tmpquery.free会报一个警告错误
    end;
  end;
  错误如下：

Answer 1

free在释放对象要检查对象的引用是否指向为空,如为空会引发异常
freeandnil在释放对象后会将引用该对象的变量变为nil

Answer 2

一般要这样写:
tmpquery:=tadoquery.Create(nil);
with tmpquery do
      try 
      begin
        connection:=sysdata.klerp;
        close;
        sql.Clear;
        sql.Add('select yuangmc from man_reny where bumbm=:bumbm');
        Parameters[0].Value:=str1;
        open;
        first;
        combobox2.Items.Clear;
        combobox2.Text:='';
        while not eof do
        begin
          combobox2.Items.Add(fieldbyname('yuangmc').asstring);
          next;
        end;
      end;
    finally
       freeandnil(tmpquery);这里如果改为tmpquery.free会报一个警告错误
    end;
  end;

Answer 3

这样写应该free or freeandnill都没问题。