Home > Articles > Data

Client Dataset Basics

This chapter is from the book

This chapter is from the book

Populating and Manipulating Client Datasets

After you create a client dataset (either at design-time or at runtime), you want to populate it with data. There are several ways to populate a client dataset: You can populate it manually through code, you can load the dataset's records from another dataset, or you can load the dataset from a file or a stream. The following sections discuss these methods, as well as how to modify and delete records.

Populating Manually

The most basic way to enter data into a client dataset is through the Append and Insert methods, which are supported by all datasets. The difference between them is that Append adds the new record at the end of the dataset, but Insert places the new record immediately before the current record.

I always use Append to insert new records because it's slightly faster than Insert. If the dataset is indexed, the new record is automatically sorted in the correct order anyway.

The following code snippet shows how to add a record to a client dataset:

cdsEmployee.Append; // You could use cdsEmployee.Insert; here as well
cdsEmployee.FieldByName('ID').AsInteger := 5;
cdsEmployee.FieldByName('FirstName').AsString := 'Eric';

Modifying Records

Modifying an existing record is almost identical to adding a new record. Rather than calling Append or Insert to create the new record, you call Edit to put the dataset into edit mode. The following code changes the first name of the current record to Fred.

cdsEmployee.Edit; // Edit the current record
cdsEmployee.FieldByName('FirstName').AsString := 'Fred';

Deleting Records

To delete the current record, simply call the Delete method, like this:


If you want to delete all records in the dataset, you can use EmptyDataSet instead, like this:


Populating from Another Dataset

dbExpress datasets are unidirectional and you can't scroll backward through them. This makes them incompatible with bidirectional, data-aware controls such as TDBGrid. However, TClientDataSet can load its data from another dataset (including dbExpress datasets, BDE datasets, or other client datasets) through a provider. Using this feature, you can load a client dataset from a unidirectional dbExpress dataset, and then connect a TDBGrid to the client dataset, providing bidirectional support.

Indeed, this capability is so powerful and important that it forms the basis for Delphi's multitier database support.

Populating from a File or Stream: Persisting Client Datasets

Though client datasets are located in RAM, you can save them to a file or a stream and reload them at a later point in time, making them persistent. This is the third method of populating a client dataset.

To save the dataset to a file, use the SaveToFile method, which is defined like this:

procedure SaveToFile(const FileName: string = ''; 
 Format: TDataPacketFormat = dfBinary);

Similarly, to save the dataset to a stream, you call SaveToStream, which is defined as follows:

procedure SaveToStream(Stream: TStream; Format: TDataPacketFormat = dfBinary);

SaveToFile accepts the name of the file that you're saving to. If the filename is blank, the data is saved using the FileName property of the client dataset.

Both SaveToFile and SaveToStream take a parameter that indicates the format to use when saving data. Client datasets can be stored in one of three file formats: binary, or either flavor of XML. Table 3.1 lists the possible formats.

Table 3.1 Data Packet Formats for Loading and Saving Client Datasets




Data is stored using a proprietary, binary format.


Data is stored in XML format. Extended characters are represented using an escape sequence.


Data is stored in XML format. Extended characters are represented using UTF8.

When client datasets are stored to disk, they are referred to as MyBase files. MyBase stores one dataset per file, or per stream, unless you use nested datasets.


If you're familiar with Microsoft ADO, you recall that ADO enables you to persist datasets using XML format. The XML formats used by ADO and MyBase are not compatible. In other words, you cannot save an ADO dataset to disk in XML format, and then read it into a client dataset (or vice versa).

Sometimes, you need to determine how many bytes are required to store the data contained in the client dataset. For example, you might want to check to see if there is enough room on a floppy disk before saving the data there, or you might need to preallocate the memory for a stream. In these cases, you can check the DataSize property, like this:

if ClientDataSet1.DataSize > AvailableSpace then
 ShowMessage('Not enough room to store the data');

DataSize always returns the amount of space necessary to store the data in binary format (dfBinary). XML format usually requires more space, perhaps twice as much (or even more).


One way to determine the amount of space that's required to save the dataset in XML format is to save the dataset to a memory stream, and then obtain the size of the resulting stream.

Example: Creating, Populating, and Manipulating a Client Dataset

The following example illustrates how to create, populate, and manipulate a client dataset at runtime. Code is also provided to save the dataset to disk and to load it.

Listing 3.1 shows the complete source code for the CDS (ClientDataset) application.

Listing 3.1 CDS—MainForm.pas

unit MainForm;


 SysUtils, Types, IdGlobal, Classes, QGraphics, QControls, QForms, QDialogs,
 QStdCtrls, DB, DBClient, QExtCtrls, QGrids, QDBGrids, QActnList;

 MAX_RECS = 10000;

 TfrmMain = class(TForm)
 DataSource1: TDataSource;
 pnlClient: TPanel;
 pnlBottom: TPanel;
 btnPopulate: TButton;
 btnSave: TButton;
 btnLoad: TButton;
 ActionList1: TActionList;
 btnStatistics: TButton;
 Populate1: TAction;
 Statistics1: TAction;
 Load1: TAction;
 Save1: TAction;
 DBGrid1: TDBGrid;
 lblFeedback: TLabel;
 procedure FormCreate(Sender: TObject);
 procedure Populate1Execute(Sender: TObject);
 procedure Statistics1Execute(Sender: TObject);
 procedure Save1Execute(Sender: TObject);
 procedure Load1Execute(Sender: TObject);
 { Private declarations }
 FCDS: TClientDataSet;
 { Public declarations }

 frmMain: TfrmMain;


{$R *.xfm}

procedure TfrmMain.FormCreate(Sender: TObject);
 FCDS := TClientDataSet.Create(Self);
 FCDS.FieldDefs.Add('ID', ftInteger, 0, True);
 FCDS.FieldDefs.Add('Name', ftString, 20, True);
 FCDS.FieldDefs.Add('Birthday', ftDateTime, 0, True);
 FCDS.FieldDefs.Add('Salary', ftCurrency, 0, True);
 DataSource1.DataSet := FCDS;

procedure TfrmMain.Populate1Execute(Sender: TObject);
 FirstNames: array[0 .. 19] of string = ('John', 'Sarah', 'Fred', 'Beth',
 'Eric', 'Tina', 'Thomas', 'Judy', 'Robert', 'Angela', 'Tim', 'Traci',
 'David', 'Paula', 'Bruce', 'Jessica', 'Richard', 'Carla', 'James',
 LastNames: array[0 .. 11] of string = ('Parker', 'Johnson', 'Jones',
 'Thompson', 'Smith', 'Baker', 'Wallace', 'Harper', 'Parson', 'Edwards',
 'Mandel', 'Stone');
 Index: Integer;
 t1, t2: DWord;
 RandSeed := 0;
 t1 := GetTickCount;
 for Index := 1 to MAX_RECS do begin
  FCDS.FieldByName('ID').AsInteger := Index;
  FCDS.FieldByName('Name').AsString := FirstNames[Random(20)] + ' ' +
  FCDS.FieldByName('Birthday').AsDateTime := StrToDate('1/1/1950') +
  FCDS.FieldByName('Salary').AsFloat := 20000.0 + Random(600) * 100;
 t2 := GetTickCount;
 lblFeedback.Caption := Format('%d ms to load %.0n records',
 [t2 - t1, MAX_RECS * 1.0]);

procedure TfrmMain.Statistics1Execute(Sender: TObject);
 t1, t2: DWord;
 msLocateID: DWord;
 msLocateName: DWord;
 t1 := GetTickCount;
 FCDS.Locate('ID', 9763, []);
 t2 := GetTickCount;
 msLocateID := t2 - t1;

 t1 := GetTickCount;
 FCDS.Locate('Name', 'Eric Wallace', []);
 t2 := GetTickCount;
 msLocateName := t2 - t1;

 ShowMessage(Format('%d ms to locate ID 9763' +
 #13'%d ms to locate Eric Wallace' +
 #13'%.0n bytes required to store %.0n records',
 [msLocateID, msLocateName, FCDS.DataSize * 1.0, MAX_RECS * 1.0]));

procedure TfrmMain.Save1Execute(Sender: TObject);
 t1, t2: DWord;
 t1 := GetTickCount;
 t2 := GetTickCount;
 lblFeedback.Caption := Format('%d ms to save data', [t2 - t1]);

procedure TfrmMain.Load1Execute(Sender: TObject);
 t1, t2: DWord;
 t1 := GetTickCount;
 t2 := GetTickCount;
 lblFeedback.Caption := Format('%d ms to load data', [t2 - t1]);


There are five methods in this application and each one is worth investigating:

  • FormCreate creates the client dataset and its schema at runtime. It would actually be easier to create the dataset at design-time, but I wanted to show you the code required to do this at runtime. The code creates four fields: Employee ID, Name, Birthday, and Salary.

  • Populate1Execute loads the client dataset with 10,000 employees made up of random data. At the beginning of the method, I manually set RandSeed to 0 to ensure that multiple executions of the application would generate the same data.


    The Delphi Randomizer normally seeds itself with the current date and time. By manually seeding the Randomizer with a constant value, we can ensure that the random numbers generated are consistent every time we run the program.

  • The method calculates approximately how long it takes to generate the 10,000 employees, which on my computer is about half of a second.

  • Statistics1Execute simply measures the length of time required to perform a couple of Locate operations and calculates the amount of space necessary to store the data on disk (again, in binary format). I'll be discussing the Locate method later in this chapter.

  • Save1Execute saves the data to disk under the filename C:\Employee.cds. The .cds extension is standard, although not mandatory, for client datasets that are saved in a binary format. Client datasets stored in XML format generally have the extension .xml.


    Please make sure that you click the Save button because the file created (C:\EMPLOYEE.CDS) is used in the rest of the example applications in this chapter, as well as some of the examples in the following chapter.

  • Load1Execute loads the data from a file into the client dataset. If LoadFromFile fails (presumably because the file doesn't exist or is not a valid file format), the client dataset is left in a closed state. For this reason, I reopen the client dataset when an exception is raised.

Figure 3.5 shows the CDS application running on my computer. Note the impressive times posted to locate a record. Even when searching through almost the entire dataset to find ID 9763, it only takes approximately 10 ms on my computer.

Figure 3.5 The CDS application at runtime.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020