neo4j-driver-dotnet-skill

What this skill does

## When to Use
- Writing C# or .NET code connecting to Neo4j
- Setting up `IDriver`, DI registration, or session/transaction lifecycle
- Questions about `ExecutableQuery`, `IResultCursor`, async patterns, result mapping
- Debugging sessions, type mapping, null safety, or error handling in .NET

## When NOT to Use
- **Writing/optimizing Cypher queries** → `neo4j-cypher-skill`
- **Upgrading from older driver version** → `neo4j-migration-skill`

---

## Install

```bash
dotnet add package Neo4j.Driver
```

| Package | Use |
|---|---|
| `Neo4j.Driver` | Async API — **use this** |
| `Neo4j.Driver.Simple` | Synchronous wrapper |
| `Neo4j.Driver.Reactive` | System.Reactive streams |

---

## Driver Lifecycle

`IDriver` — thread-safe, connection-pooled, expensive to create. **Create one per application.**

```csharp
using Neo4j.Driver;

// URI schemes:
//   neo4j+s://xxx.databases.neo4j.io   — TLS + cluster routing (Aura)
//   neo4j://localhost                   — unencrypted + cluster routing
//   bolt+s://localhost:7687             — TLS + single instance
//   bolt://localhost:7687               — unencrypted + single instance

await using var driver = GraphDatabase.Driver(
    "neo4j+s://xxx.databases.neo4j.io",
    AuthTokens.Basic("neo4j", "password"));

await driver.VerifyConnectivityAsync();   // fail fast on startup
```

`IDriver` and `IAsyncSession` implement `IAsyncDisposable` — always `await using`, never plain `using`.

```csharp
// ❌ Wrong — synchronous Dispose() may block thread pool
using var driver = GraphDatabase.Driver(uri, auth);

// ✅ Correct
await using var driver = GraphDatabase.Driver(uri, auth);
```

Auth options: `AuthTokens.Basic(u, p)` / `AuthTokens.Bearer(token)` / `AuthTokens.Kerberos(ticket)` / `AuthTokens.None`

---

## Environment Variables

Load connection config from environment / `appsettings.json` — never hardcode credentials.

```json
// appsettings.json
{
  "Neo4j": {
    "Uri": "neo4j+s://xxx.databases.neo4j.io",
    "User": "neo4j",
    "Password": "secret",
    "Database": "neo4j"
  }
}
```

```csharp
// Access via IConfiguration (injected in Program.cs)
var uri      = builder.Configuration["Neo4j:Uri"];
var user     = builder.Configuration["Neo4j:User"];
var password = builder.Configuration["Neo4j:Password"];
var database = builder.Configuration["Neo4j:Database"] ?? "neo4j";
```

Override with environment variables (standard .NET behavior): `Neo4j__Uri=neo4j+s://...` (double underscore = colon separator). Never commit `appsettings.json` with real credentials — use `appsettings.Development.json` (gitignored) or env vars in CI/production.

---

## DI Registration (ASP.NET Core)

Register `IDriver` as **singleton** — never Scoped or Transient. Never register `IAsyncSession` in DI.

```csharp
// Program.cs
builder.Services.AddSingleton<IDriver>(_ =>
    GraphDatabase.Driver(
        builder.Configuration["Neo4j:Uri"],
        AuthTokens.Basic(
            builder.Configuration["Neo4j:User"],
            builder.Configuration["Neo4j:Password"])));

// Shutdown hook — dispose the singleton cleanly
builder.Services.AddHostedService<Neo4jShutdownService>();

// Neo4jShutdownService.cs
public class Neo4jShutdownService(IDriver driver, IHostApplicationLifetime lifetime)
    : IHostedService
{
    public Task StartAsync(CancellationToken _)
    {
        lifetime.ApplicationStopping.Register(() =>
            driver.DisposeAsync().AsTask().GetAwaiter().GetResult());
        return Task.CompletedTask;
    }
    public Task StopAsync(CancellationToken _) => Task.CompletedTask;
}

// Inject into services — sessions opened per unit of work
public class PersonService(IDriver driver)
{
    public async Task<List<string>> GetNamesAsync(CancellationToken ct = default)
    {
        var (records, _, _) = await driver
            .ExecutableQuery("MATCH (p:Person) RETURN p.name AS name")
            .WithConfig(new QueryConfig(database: "neo4j"))
            .ExecuteAsync(ct);
        return records.Select(r => r.Get<string>("name")).ToList();
    }
}
```

---

## Choose the Right API

| API | When | Auto-retry | Streaming |
|---|---|---|---|
| `driver.ExecutableQuery()` | Most queries — simple default | ✅ | ❌ eager |
| `session.ExecuteReadAsync/WriteAsync()` | Large results, multi-query tx | ✅ | ✅ |
| `session.RunAsync()` | `LOAD CSV`, `CALL {} IN TRANSACTIONS` | ❌ | ✅ |
| `session.BeginTransactionAsync()` | Multi-function, external coordination | ❌ | ✅ |

---

## ExecutableQuery — Recommended Default

Fluent builder; manages session, transaction, retries, and bookmarks automatically.

```csharp
// Read
var (records, summary, keys) = await driver
    .ExecutableQuery("MATCH (p:Person {name: $name})-[:KNOWS]->(f) RETURN f.name AS name")
    .WithParameters(new { name = "Alice" })
    .WithConfig(new QueryConfig(
        database: "neo4j",
        routing: RoutingControl.Readers))    // route reads to replicas
    .ExecuteAsync(cancellationToken);

foreach (var r in records)
    Console.WriteLine(r.Get<string>("name"));

// Use ResultConsumedAfter for wall-clock timing (ResultAvailableAfter = time-to-first-byte only)
Console.WriteLine($"{summary.ResultConsumedAfter.TotalMilliseconds} ms");

// Write
var (_, writeSummary, _) = await driver
    .ExecutableQuery("CREATE (p:Person {name: $name, age: $age})")
    .WithParameters(new { name = "Bob", age = 30 })
    .WithConfig(new QueryConfig(database: "neo4j"))
    .ExecuteAsync();
Console.WriteLine($"Created {writeSummary.Counters.NodesCreated} nodes");

// WithMap — project inline
var names = await driver
    .ExecutableQuery("MATCH (p:Person) RETURN p.name AS name")
    .WithConfig(new QueryConfig(database: "neo4j"))
    .WithMap(r => r["name"].As<string>())
    .ExecuteAsync();   // names.Result is IReadOnlyList<string>
```

Never `await` omitted: `ExecuteAsync()` returns `Task` — missing `await` compiles silently but query never runs.

Never string-interpolate Cypher. Always `WithParameters()` — prevents injection, enables plan caching.

---

## Managed Transactions

Use for large result sets (lazy streaming) or multiple queries per transaction. Callback auto-retried on transient failure — keep it idempotent, no side effects inside.

```csharp
await using var session = driver.AsyncSession(conf => conf.WithDatabase("neo4j"));

// Read — routes to replicas
var names = await session.ExecuteReadAsync(async tx =>
{
    var cursor = await tx.RunAsync(
        "MATCH (p:Person) WHERE p.name STARTS WITH $prefix RETURN p.name AS name",
        new { prefix = "Al" });
    return await cursor.ToListAsync(r => r.Get<string>("name"));
    // Consume cursor INSIDE callback — invalid after callback returns
});

// Write — void, no async needed
await session.ExecuteWriteAsync(tx =>
    tx.RunAsync("MERGE (p:Person {name: $name})", new { name = "Carol" }));

// Write — async when needing counters
var summary = await session.ExecuteWriteAsync(async tx =>
{
    var cursor = await tx.RunAsync(
        "CREATE (p:Person {name: $name})", new { name = "Alice" });
    return await cursor.ConsumeAsync();   // drains cursor, returns IResultSummary
});
Console.WriteLine($"Created {summary.Counters.NodesCreated} nodes");
```

Cursor rules:
- Consume with `ToListAsync()` or `FetchAsync()` loop **inside** the callback
- Returning a cursor from the callback → transaction closes → cursor invalid → exception

```csharp
// ❌ Returns cursor — tx closes immediately after lambda returns
var cursor = await session.ExecuteReadAsync(async tx =>
    await tx.RunAsync("MATCH (p:Person) RETURN p.name AS name"));
await cursor.FetchAsync();   // throws

// ✅ Consume inside
var names = await session.ExecuteReadAsync(async tx =>
{
    var cursor = await tx.RunAsync("MATCH (p:Person) RETURN p.name AS name");
    return await cursor.ToListAsync(r => r.Get<string>("name"));
});
```

Async void trap:
```csharp
// ❌ CS1998 warning — async with no await; RunAsync Task discarded
await session.ExecuteWriteAsync(async tx =>
    tx.RunAsync(