Custom Transports

The IObdTransport interface abstracts the communication channel between your app and the OBD adapter. The library ships with BLE, but you can implement WiFi, USB, or any other transport.

For transports that support discovery (BLE, WiFi), implement IObdDeviceScanner as well:

public interface IObdDeviceScanner
{
    Task Scan(Action<ObdDiscoveredDevice> onDeviceFound, CancellationToken ct = default);
}

IObdTransport Interface

public interface IObdTransport : IAsyncDisposable
{
    bool IsConnected { get; }
    Task Connect(CancellationToken ct = default);
    Task Disconnect();
    Task<string> Send(string command, CancellationToken ct = default);
}

Send Contract

The Send method must:

1. Write the command string to the adapter (the string already includes the trailing \r added by ObdConnection)
2. Read the response until the ELM327 > prompt character
3. Return the response text without the > prompt

WiFi Transport Example

Many ELM327 adapters expose a TCP socket (typically 192.168.0.10:35000):

using System.Net.Sockets;
using System.Text;

public class WifiObdTransport : IObdTransport
{
    readonly string host;
    readonly int port;
    TcpClient? client;
    NetworkStream? stream;

    public WifiObdTransport(string host = "192.168.0.10", int port = 35000)
    {
        this.host = host;
        this.port = port;
    }

    public bool IsConnected => this.client?.Connected ?? false;

    public async Task Connect(CancellationToken ct = default)
    {
        this.client = new TcpClient();
        await this.client.ConnectAsync(this.host, this.port, ct);
        this.stream = this.client.GetStream();
    }

    public Task Disconnect()
    {
        this.stream?.Dispose();
        this.client?.Dispose();
        this.stream = null;
        this.client = null;
        return Task.CompletedTask;
    }

    public async Task<string> Send(string command, CancellationToken ct = default)
    {
        if (this.stream == null)
            throw new ObdException("Not connected");

        var bytes = Encoding.ASCII.GetBytes(command);
        await this.stream.WriteAsync(bytes, ct);

        var buffer = new StringBuilder();
        var readBuffer = new byte[1024];

        while (true)
        {
            var count = await this.stream.ReadAsync(readBuffer, ct);
            var text = Encoding.ASCII.GetString(readBuffer, 0, count);
            buffer.Append(text);

            if (text.Contains('>'))
            {
                return buffer.ToString().Replace(">", "").Trim();
            }
        }
    }

    public ValueTask DisposeAsync()
    {
        this.stream?.Dispose();
        this.client?.Dispose();
        return default;
    }
}

Usage

var transport = new WifiObdTransport("192.168.0.10", 35000);
var connection = new ObdConnection(transport);
await connection.Connect();

var speed = await connection.Execute(StandardCommands.VehicleSpeed);

Testing with a Fake Transport

Use a fake transport for unit testing your OBD code without a real adapter:

public class FakeObdTransport : IObdTransport
{
    readonly Dictionary<string, string> responses = new();
    public bool IsConnected { get; private set; }

    public FakeObdTransport AddResponse(string command, string response)
    {
        this.responses[command.TrimEnd('\r') + "\r"] = response;
        return this;
    }

    public Task Connect(CancellationToken ct = default)
    {
        this.IsConnected = true;
        return Task.CompletedTask;
    }

    public Task Disconnect()
    {
        this.IsConnected = false;
        return Task.CompletedTask;
    }

    public Task<string> Send(string command, CancellationToken ct = default)
    {
        if (this.responses.TryGetValue(command, out var response))
            return Task.FromResult(response);

        return Task.FromResult("OK");
    }

    public ValueTask DisposeAsync() => default;
}

Usage in Tests

var transport = new FakeObdTransport()
    .AddResponse("010D", "41 0D 50")       // 80 km/h
    .AddResponse("010C", "41 0C 0B B8")    // 750 RPM
    .AddResponse("ATI", "ELM327 v1.5");

var connection = new ObdConnection(transport);
await connection.Connect();

var speed = await connection.Execute(StandardCommands.VehicleSpeed);
Assert.Equal(80, speed);