一个form调用的问题

[alexsten]

有三个form：form1,form2,form3,
form1为主窗口，form2和form3是用来动态生成的
现在在form1中点一个button，动态创建form2
点form2的一个button，动态创建form3
在form3中要调用或者修改form2中声明的一个public变量

程序运行出错了，form2和form3都是nil
如何解决？

[alexsten]

unit Unit1;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls;

type
TForm1 = class(TForm)
Button1: TButton;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
aaa : String;
end;

var
Form1: TForm1;

implementation
uses Unit2,Unit3;

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
var
F2:TForm2;

begin
F2:=Tform2.create(nil);
F2.show;
end;

end.




unit Unit2;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls;

type
TForm2 = class(TForm)
Edit1: TEdit;
Button2: TButton;
procedure Button2Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
a:String;
end;

var
Form2:Tform2;
implementation

uses Unit1,Unit3;
{$R *.dfm}

procedure TForm2.Button2Click(Sender: TObject);
var
form3:TForm3;
begin
Form3:=TForm3.Create(self);
Form3.Show;
end;

end.




unit Unit3;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls;

type
TForm3 = class(TForm)
Button1: TButton;
Edit1: TEdit;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;

var
Form3: TForm3;

implementation

uses Unit2;

{$R *.dfm}

procedure TForm3.Button1Click(Sender: TObject);
begin
form2.Edit1.Text := form3.Edit1.text;
form2.a := form3.edit1.text;
end;

end.

[yansea]

你是怎么创建的？如果变量在form的public里声明，可以用form2.变量名引用~~~~~似乎不应该有问题

[alexsten]

底下是代码：

unit Unit1;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls;

type
TForm1 = class(TForm)
Button1: TButton;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
aaa : String;
end;

var
Form1: TForm1;

implementation
uses Unit2,Unit3;

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
var
F2:TForm2;

begin
F2:=Tform2.create(nil);
F2.show;
end;

end.


unit Unit2;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls;

type
TForm2 = class(TForm)
Edit1: TEdit;
Button2: TButton;
procedure Button2Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
a:String;
end;

var
Form2:Tform2;
implementation

uses Unit1,Unit3;
{$R *.dfm}

procedure TForm2.Button2Click(Sender: TObject);
var
form3:TForm3;
begin
Form3:=TForm3.Create(self);
Form3.Show;
end;

end.


unit Unit3;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls;

type
TForm3 = class(TForm)
Button1: TButton;
Edit1: TEdit;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;

var
Form3: TForm3;

implementation

uses Unit2;

{$R *.dfm}

procedure TForm3.Button1Click(Sender: TObject);
begin
form2.Edit1.Text := form3.Edit1.text;
form2.a := form3.edit1.text;
end;

end.

[gxyc76]

现在在form1中点一个button，动态创建form2
var
myform2 :TForm2;

myform2 :=TForm2.Create(nil);

点form2的一个button，动态创建form3
var
myform3 :TForm3;

myform3 :=TForm3.Create(nil);

在form3中要调用或者修改form2中声明的一个public变量
因为form2与form3要进行单元互调,所以uses unit2,uses unit3必需放在interface部分.

[yansea]

unit3 没有use unit2

[alexsten]

在unit2的interface部分uses unit3
在unit3的interface部分uses unit2

编译无法通过
错误：Circular unit reference to "unit2"

[superljj]

同意楼上.

[alexsten]

to yansea：
有啊，unit3里面有uses unit2

[superljj]

procedure TForm1.Button1Click(Sender: TObject);
begin
Application.CreateForm(TForm2, Form2);
Form2.show;
end;

end.


procedure TForm2.Button2Click(Sender: TObject);
begin
Application.CreateForm(TForm3, Form3);
Form3.Show;
end;

end.

[alexsten]

to superljj:
试了一下，是可以执行了！

可以说说这是为了什么原因呢？
为什么会出这样的错误呢？

[knock]

在form2和form3中互相调用那是肯定不对的，那会造成循环！
***在unit2的interface部分uses unit3
在unit3的inplementation部分uses unit2***//表示局部引用
这样是可以的。也可以都放在inplementation区。