Question

sqlite是一个数据库 - 一个描述符，包含表列表，域列表，字段列表，限制列表（主键和外键），索引列表。我在Delphi XE3的帮助下，在内置组件的帮助下连接到此基础。有一个单独的模块，其中描述了类TTableSpec，TFieldSpec，TConstraintSpec，TConstraintDetSpec，TDomainSpec。这些类对应于上述sqlite的基础记录。在TTableSpec类中有这样的FFields字段：TComponent成为像TFieldSpec这样的对象的所有者，它们也是从基础上卸载的（例如，DBSchema也包含FTables字段是所有表的所有者），并且在TFieldSpec类中有FDomainSpec字段：TDomainSpec。实际上我给出了一个连接错误的类代码

type
TDataTypeId = (DataTypeId_String, DataTypeId_SmallInt, DataTypeId_Integer, DataTypeId_Word,
               DataTypeId_Boolean, DataTypeId_Float, DataTypeId_Currency,
               DataTypeId_BCD, DataTypeId_FmtBCD, DataTypeId_Date,
               DataTypeId_Time, DataTypeId_DateTime, DataTypeId_TimeStamp,
               DataTypeId_Bytes, DataTypeId_VarBytes, DataTypeId_Blob,
               DataTypeId_Memo, DataTypeId_Graphic, DataTypeId_fmtMemo,
               DataTypeId_FixedChar, DataTypeId_WideChar, DataTypeId_LargeInt,
               DataTypeId_Array, DataTypeId_FixedWideChar, DataTypeId_WideMemo, DataTypeId_Default);
TDBSchemaSpec=class(Tcomponent)
  private
    FDomains: TComponent;
    FTables : TComponent;
  public
    procedure Setup();
    destructor Destroy; override;
    property Domains: TComponent read FDomains;
    property Tables : TComponent read FTables;
end;
TDomainSpec = class(TComponent)
  private
    FName: string;
    FDescription: String;
    FDataTypeId: TDataTypeId;
    FLength: Cardinal;
    FCharLength: Cardinal;
    FPrecision: Cardinal;
    FScale: Cardinal;
    FWidth: Word;
    FAlign: TAlignSpec;
    FShowNull: Boolean;
    FShowLeadNulls: Boolean;
    FThousandsSeparator: Boolean;
  public
    procedure Setup(FName: string; FDescription: String; FDataTypeId: TDataTypeId;
    FLength: Cardinal;FCharLength: Cardinal;FPrecision: Cardinal;FScale: Cardinal;
    FWidth: Word;FAlign: TAlignSpec;FShowNull: Boolean;FShowLeadNulls: Boolean;
    FThousandsSeparator: Boolean);
    destructor Destroy; override;
    property Name: String read FName;
    property Description: String read FDescription;
    property DataTypeId: TDataTypeId read FDataTypeId;
    property Length: Cardinal read FLength;
    property CharLength: Cardinal read FCharLength;
    property Precision: Cardinal read FPrecision;
    property Scale: Cardinal read FScale;
    property Width: Word read FWidth;
    property Align: TAlignSpec read FAlign;
    property ShowNull: Boolean read FShowNull;
    property ShowLeadNulls: Boolean read FShowLeadNulls;
    property ThousandsSeparator: Boolean read FThousandsSeparator;
  end;

TTableSpec= class(TComponent)
  private
    FFields : TComponent;
    FIndices: TComponent;
    FConstraints : TComponent;
    FName : string;
    FDescription: string;
    FCanAdd:  boolean;
    FCanEdit: boolean;
    FCanDelete: boolean;
  public
    procedure Setup(FName : string; FDescription:string;
    FCanAdd:  boolean; FCanEdit: boolean; FCanDelete: boolean);
    destructor Destroy; override;
    property Description : string read FDescription;
    property Name : string read FName;
    property CanAdd:  boolean read FCanAdd;
    property CanEdit: boolean read FCanEdit;
    property CanDelete: boolean read FCanDelete;
    property Fields : TComponent read FFields;
    property Indices: TComponent read FIndices;
    property Constraints : TComponent read FConstraints;
  end;

TFieldSpec = class(TComponent)
  private
    FDomainSpec: TDomainSpec;
    FPosition: integer;
    FFieldName: string;
    FDescription: string;
    FCanInput: boolean;
    FCanEdit: boolean;
    FShowInGrid: boolean;
    FShowInDetails: boolean;
    FIsMean: boolean;
    FAutoCalculated: boolean;
    FRequired: boolean;
  public
    procedure Setup(FDomainSpec: TDomainSpec; FPosition: integer; FFieldName: string; FDescription: string; FCanInput: boolean; FCanEdit: boolean;
    FShowInGrid: boolean; FShowInDetails: boolean; FIsMean: boolean;FAutoCalculated: boolean;
    FRequired: boolean);
    destructor Destroy; override;
    property DomainSpec: TDomainSpec read FDomainSpec;
    property Position: integer read FPosition;
    property FieldName: string read FFieldName;
    property Description: string read FDescription;
    property CanInput: boolean read FCanInput;
    property CanEdit: boolean read FCanEdit;
    property ShowInGrid: boolean read FShowInGrid;
    property ShowInDetails: boolean read FShowInDetails;
    property IsMean: boolean read FIsMean;
    property AutoCalculated: boolean read FAutoCalculated;
    property Required: boolean read FRequired;
 end;

在应用程序中有一个表单fmListOfTables，在CheckListBox中从中选择其他窗口中将在DBGrid中显示数据的表名。错误

Access violation at address 00822135 in module 'GUI.exe'.Read of address 00000040.(When debugging Project GUI.exe raised exception class $C0000005 with message 'access violation at 0x00822135:  read of address 0x00000040'.  )

出现在

行

if ((TTableSpec(MainForm.DBSchema.Tables.FindComponent(ListBox1.Items.Strings[ListBox1.ItemIndex])).Fields.Components[i] is TFieldSpec) and (TDomainSpec(TFieldSpec(TTableSpec(MainForm.DBSchema.Tables.FindComponent(ListBox1.Items.Strings[ListBox1.ItemIndex])).Fields.Components[i]).DomainSpec).DataTypeId<>DataTypeId_Blob) and (TDomainSpec(TFieldSpec(TTableSpec(MainForm.DBSchema.Tables.FindComponent(ListBox1.Items.Strings[ListBox1.ItemIndex])).Fields.Components[i]).DomainSpec).DataTypeId<>DataTypeId_WideMemo)) then

在选择表的过程中

procedure TfmListOfTables.ListBox1DblClick(Sender: TObject);
var  fmShowData : TfmTableData;
      i : integer;
      querystr : string;
begin
  MainWindow.IBDatabase1.DatabaseName:=MainWindow.dbname;
  MainWindow.IBDatabase1.Connected:=true;
  fmShowData:=TfmTableData.Create(MainWindow);
  fmShowData.Caption:=ListBox1.Items.Strings[ListBox1.ItemIndex];
  fmShowData.tname:=ListBox1.Items.Strings[ListBox1.ItemIndex];
  DisplayTable:=ListBox1.Items.Strings[ListBox1.ItemIndex];
  querystr:='select ';
  for i:= 0 to TTableSpec(DBSchema.Tables.FindComponent(ListBox1.Items.Strings[ListBox1.ItemIndex])).Fields.ComponentCount-1 do
     begin
     if ((TTableSpec(MainForm.DBSchema.Tables.FindComponent(ListBox1.Items.Strings[ListBox1.ItemIndex])).Fields.Components[i] is TFieldSpec) and (TDomainSpec(TFieldSpec(TTableSpec(MainForm.DBSchema.Tables.FindComponent(ListBox1.Items.Strings[ListBox1.ItemIndex])).Fields.Components[i]).DomainSpec).DataTypeId<>DataTypeId_Blob) and (TDomainSpec(TFieldSpec(TTableSpec(MainForm.DBSchema.Tables.FindComponent(ListBox1.Items.Strings[ListBox1.ItemIndex])).Fields.Components[i]).DomainSpec).DataTypeId<>DataTypeId_WideMemo)) then
        querystr:=querystr+TFieldSpec(TTableSpec(MainForm.DBSchema.Tables.FindComponent(ListBox1.Items.Strings[ListBox1.ItemIndex])).Fields.Components[i]).FieldName+', ';
     end;
  Delete(querystr, Length(querystr)-1, 1);
  querystr:=querystr+'from '+fmShowData.tname;
  fmShowData.IBQuery1.SQL.Clear;
  fmShowData.IBQuery1.SQL.Add(querystr);
  fmShowData.IBQuery1.Open;
  fmShowData.DBGrid1.DataSource:=fmShowData.DataSource1;
  for I := 0 to fmShowData.DBGrid1.Columns.Count-1 do
    fmShowData.DBGrid1.Columns[i].Width:=90;
  fmShowData.DragKind:=dkDock;
  fmShowData.DragMode:=dmAutomatic;
  fmShowData.tname:=ListBox1.Items.Strings[ListBox1.ItemIndex];
  TfmTableData(fmShowData).databasetable:=ListBox1.Items.Strings[ListBox1.ItemIndex];
  fmShowData.Show;
end;

Answer 1

让我们看一下导致问题的代码行：

if ((TTableSpec(MainForm.DBSchema.Tables.FindComponent(
  ListBox1.Items.Strings[ListBox1.ItemIndex])).Fields.Components[i] is TFieldSpec) 
  and (TDomainSpec(TFieldSpec(TTableSpec(MainForm.DBSchema.Tables.FindComponent(
  ListBox1.Items.Strings[ListBox1.ItemIndex])).Fields.Components[i]).DomainSpec)
  .DataTypeId<>DataTypeId_Blob) and (TDomainSpec(TFieldSpec(TTableSpec(MainForm.
  DBSchema.Tables.FindComponent(ListBox1.Items.
  Strings[ListBox1.ItemIndex])).Fields.Components[i]).DomainSpec).
  DataTypeId<>DataTypeId_WideMemo)) then

很难找到关于这行代码的任何话题。纯粹而简单，这是一种彻头彻尾的讽刺。

首先要做的是使用一些局部变量。例如：

ComponentName := ListBox1.Items.Strings[ListBox1.ItemIndex];

然后我们可以这样写：

Component1 := MainForm.DBSchema.Tables.FindComponent(ComponentName);

我们继续这样做：

Component2 := TTableSpec(Component1).Fields.Components[i];

然后代码看起来像这样：

if (Component2 is TFieldSpec) 
and (TDomainSpec(TFieldSpec(Component2).DomainSpec).DataTypeId<>DataTypeId_Blob) 
and (TDomainSpec(TFieldSpec(Component2).DomainSpec).DataTypeId<>DataTypeId_WideMemo)) then

你的第一个任务是按照我的建议，以理智的方式重新编写代码。

然后是实际的错误。您的访问权限违规，读取地址为$00000040。这是尝试取消引用空指针的标志。显然，这里的内容是nil。我说不出来。但是你的调试器会告诉你。确保将其配置为在引发异常时中断。这是IDE调试器选项中的一个设置。

FindComponent次调用中有一次可能会返回nil。或者DomainSpec可能是nil。您可能需要将单个if语句分解为多个if语句，以便于调试。

我的最后一点是重新重复我的初步建议。此代码不可行。立即将它重新纳入理智的形式。

地址00822135的模块'GUI.exe'中的访问冲突。地址00000040

1 个答案: