delphi - DataSnap 代理问题

Question

以下错误发生在我在下面指出的行中。我不明白为什么会出现此错误。

Project ChirpSR.exe raised exception class $C0000005 with message 'access violation at 0x00e8d088: read of address 0x00000000'.

下面的代码来自为 DataSnap Server 自动生成的代理类。

interface

uses Data.DBXCommon, Data.DBXClient, Data.DBXDataSnap, Data.DBXJSON, Datasnap.DSProxy, System.Classes, System.SysUtils, Data.DB, Data.SqlExpr, Data.DBXDBReaders, Data.DBXCDSReaders, Data.DBXJSONReflect;

....
type
  TServerMethods1Client = class(TDSAdminClient)
  private
    FEchoStringCommand: TDBXCommand;
    FReverseStringCommand: TDBXCommand;
    FGetValleysCommand: TDBXCommand;
    FUpdateUserCommand: TDBXCommand;
  public
    constructor Create(ADBXConnection: TDBXConnection); overload;
    constructor Create(ADBXConnection: TDBXConnection; AInstanceOwner: Boolean); overload;
    destructor Destroy; override;
    function EchoString(Value: string): string;
    function ReverseString(Value: string): string;
    function GetValleys: TJSONValue;
    function UpdateUser(jsonobj: TJSONObject): Integer;
  end;
....
// This function runs fine
function TServerMethods1Client.GetValleys: TJSONValue;
begin
  if FGetValleysCommand = nil then
  begin
    FGetValleysCommand := FDBXConnection.CreateCommand;
    FGetValleysCommand.CommandType := TDBXCommandTypes.DSServerMethod;
    FGetValleysCommand.Text := 'TServerMethods1.GetValleys';
    FGetValleysCommand.Prepare;
  end;
  FGetValleysCommand.ExecuteUpdate;
  Result := TJSONValue(FGetValleysCommand.Parameters[0].Value.GetJSONValue(FInstanceOwner));
end;

// This function errors at the highlighted line
function TServerMethods1Client.UpdateUser(jsonobj: TJSONObject): Integer;
begin
  if FUpdateUserCommand = nil then
  begin
    FUpdateUserCommand := FDBXConnection.CreateCommand;  <============= Error Here
    FUpdateUserCommand.CommandType := TDBXCommandTypes.DSServerMethod;
    FUpdateUserCommand.Text := 'TServerMethods1.UpdateUser';
    FUpdateUserCommand.Prepare;
  end;
  FUpdateUserCommand.Parameters[0].Value.SetJSONValue(jsonobj, FInstanceOwner);
  FUpdateUserCommand.ExecuteUpdate;
  Result := FUpdateUserCommand.Parameters[1].Value.GetInt32;
end;
....

服务器正在运行，否则第一个函数会出错。

我被难住了。

我也是 DataSnap 的新手。

Answer 1

Project ChirpSR.exe raised exception class $C0000005 with message 'access violation at 0x00e8d088: read of address 0x00000000'.

错误消息提示您的情况FDBXConnection = nil。由于CreateCommand是虚拟方法，调用TDBXConnection(nil).CreateCommand将产生您提出的异常。