Delphi TIdTcpClient/TIdTcpServer Simple Application Guide

Jul 31, 2025 by Sebastian Müller 58 views

Creating Simple TCP Client/Server Applications with Delphi and Indy Components

Hey guys! So, you're diving into the world of Delphi and network programming, huh? That's awesome! You've chosen a fantastic language and a super powerful component library, Indy, to build your TCP client and server applications. Delphi, with its rapid application development (RAD) capabilities, combined with Indy's comprehensive set of networking components, makes creating networked applications a breeze. But, like any new adventure, there might be a few bumps along the road. Let's break down how to build a simple TCP client and server application using Delphi and the Indy components, specifically TIdTcpClient and TIdTcpServer. We'll focus on making sure that your FIdTCPServer.OnExecute event handler is working smoothly. We're going to explore the core concepts, the common pitfalls, and the best practices to get your applications up and running.

Understanding the Basics: TCP Client and Server

Before we dive into the code, let's quickly recap what a TCP client and server do. Imagine a restaurant: the server (the application) waits for customers (clients) to come in and place orders (send requests). The server then processes these orders and delivers the food (sends responses) back to the customers. In the networking world, a TCP server listens for incoming connections on a specific port. When a client connects to that port, the server accepts the connection and starts communicating. The client, on the other hand, initiates the connection to the server.

Setting up the Server with TIdTcpServer

The TIdTcpServer component is your main tool for building the server-side application. It handles the nitty-gritty details of listening for connections, accepting them, and managing communication with clients. The most important event you'll be working with is the OnExecute event. This event is triggered every time a client connects to the server and is where you'll write the code to handle incoming requests and send responses.

When setting up your TIdTcpServer component, there are a few key properties you need to configure:

Port: This is the port number the server will listen on. Make sure to choose a port that's not already in use by another application. Ports above 1024 are generally safe to use.
Active: Set this to true to start the server listening for connections, and false to stop it.
DefaultPort: This is the default port the server will listen on if the Port property is not explicitly set. It's good practice to set this as well.
Bindings: This property allows you to specify which IP addresses the server will listen on. By default, it listens on all available IP addresses.

Building the Client with TIdTcpClient

On the client-side, the TIdTcpClient component is your go-to tool. It's responsible for establishing a connection to the server and sending/receiving data. Key properties to configure include:

Host: The IP address or hostname of the server you want to connect to.
Port: The port number the server is listening on.
Connected: This property indicates whether the client is currently connected to the server. You can also use the Connect and Disconnect methods to manage the connection.

Diving into the FIdTCPServer.OnExecute Event

Now, let's tackle the heart of the server-side logic: the FIdTCPServer.OnExecute event. This event is where the magic happens. It's triggered for each connected client, allowing you to handle incoming data, process it, and send responses. This is where a lot of developers find themselves scratching their heads, so let's break it down.

The OnExecute event handler has a crucial parameter: AContext. This parameter is of type TIdContext and represents the connection with a specific client. It's your gateway to communicating with that client. Through AContext, you can access the connection's socket, read incoming data, and send data back.

Common Challenges and Solutions:

Not Receiving Data: A common issue is not receiving data from the client in the OnExecute event. This usually boils down to how you're reading the data. You need to use the AContext.Connection.IOHandler property, which provides methods for reading and writing data to the socket.
Server Not Responding: If the server isn't responding, double-check that your OnExecute event handler is actually sending a response back to the client. Remember, TCP is a connection-oriented protocol, so the client expects a response for every request.
Thread Safety: The OnExecute event runs in a separate thread for each client connection. This is great for performance, but it also means you need to be mindful of thread safety. If you're accessing shared resources (like global variables or components), you need to use synchronization mechanisms (like critical sections or mutexes) to prevent race conditions.

Crafting Your Application: A Step-by-Step Guide

Okay, enough theory! Let's get our hands dirty and build a basic TCP client/server application. We'll create a server that simply echoes back any data it receives from the client.

Server-Side Implementation

Create a new Delphi VCL application.
Drop a TIdTcpServer component onto the form.
Set the Port property to, say, 8080.
Double-click the OnExecute event to create an event handler.

Implement the OnExecute event handler:

procedure TForm1.IdTCPServer1Execute(AContext: TIdContext);
var
  ReceivedData: string;
begin
  // Read data from the client
  ReceivedData := AContext.Connection.IOHandler.ReadLn();

  // Echo the data back to the client
  AContext.Connection.IOHandler.WriteLn('Server Echo: ' + ReceivedData);
end;

Add a TButton to the form and create an OnClick event handler to start and stop the server:

procedure TForm1.Button1Click(Sender: TObject);
begin
  IdTCPServer1.Active := not IdTCPServer1.Active;
  if IdTCPServer1.Active then
    Button1.Caption := 'Stop Server'
  else
    Button1.Caption := 'Start Server';
end;

Client-Side Implementation

Create another Delphi VCL application.
Drop a TIdTcpClient component onto the form.
Add a TEdit control for entering the server's IP address or hostname.
Add another TEdit control for entering the message to send.
Add a TMemo control to display the server's response.

Add a TButton to connect to the server:

procedure TForm1.Button1Click(Sender: TObject);
begin
  IdTCPClient1.Host := Edit1.Text; // Get host from Edit1
  IdTCPClient1.Port := 8080; // Server port
  try
    IdTCPClient1.Connect;
    Memo1.Lines.Add('Connected to server.');
  except
    on E: Exception do
      Memo1.Lines.Add('Error connecting: ' + E.Message); // Show possible connection error
  end;
end;

Add another TButton to send the message:

procedure TForm1.Button2Click(Sender: TObject);
var
  Response: string;
begin
  try
    IdTCPClient1.IOHandler.WriteLn(Edit2.Text); // Send data from Edit2
    Response := IdTCPClient1.IOHandler.ReadLn(); // Read the response
    Memo1.Lines.Add('Server Response: ' + Response); // Show response in Memo1
  except
    on E: Exception do
      Memo1.Lines.Add('Error sending/receiving: ' + E.Message); // Show error if any
  end;
end;

Add a TButton to disconnect from the server:

procedure TForm1.Button3Click(Sender: TObject);
begin
  try
    IdTCPClient1.Disconnect;
    Memo1.Lines.Add('Disconnected from server.');
  except
    on E: Exception do
      Memo1.Lines.Add('Error disconnecting: ' + E.Message);
  end;
end;

Testing Your Application

Run the server application first.
**Click the