# Source Generators

How automatic gRPC service implementation generation works.

## Overview

`Svrnty.CQRS.Grpc.Generators` uses Roslyn source generators to automatically create gRPC service implementations at compile time. This eliminates boilerplate code and ensures type safety between .proto definitions and C# handlers.

**Benefits:**
- ✅ **Zero boilerplate** - No manual service implementation
- ✅ **Compile-time safety** - Errors caught during build
- ✅ **Type checking** - Ensures proto and C# types match
- ✅ **Automatic updates** - Regenerates when proto changes
- ✅ **IDE support** - IntelliSense for generated code

## How It Works

```
┌──────────────────────────────────┐
│  Build Process                   │
├──────────────────────────────────┤
│  1. Compile .proto files         │
│  2. Generate C# types (Grpc.Tools)│
│  3. Source generator runs         │
│     - Reads proto definitions     │
│     - Discovers CQRS handlers     │
│     - Generates service impls     │
│  4. Compile generated code        │
│  5. Build completes               │
└──────────────────────────────────┘
```

## Generated Code

### CommandServiceImpl

**From .proto:**
```protobuf
service CommandService {
  rpc CreateUser (CreateUserCommand) returns (CreateUserResponse);
  rpc DeleteUser (DeleteUserCommand) returns (google.protobuf.Empty);
}
```

**Generated C# (simplified):**
```csharp
public class CommandServiceImpl : CommandService.CommandServiceBase
{
    private readonly IServiceProvider _serviceProvider;

    public CommandServiceImpl(IServiceProvider serviceProvider)
    {
        _serviceProvider = serviceProvider;
    }

    public override async Task<CreateUserResponse> CreateUser(
        CreateUserCommand request,
        ServerCallContext context)
    {
        using var scope = _serviceProvider.CreateScope();

        // Get validator
        var validator = scope.ServiceProvider
            .GetService<IValidator<CreateUserCommand>>();

        // Validate
        if (validator != null)
        {
            var validationResult = await validator.ValidateAsync(
                request,
                context.CancellationToken);

            if (!validationResult.IsValid)
            {
                throw CreateValidationException(validationResult);
            }
        }

        // Get handler
        var handler = scope.ServiceProvider
            .GetRequiredService<ICommandHandler<CreateUserCommand, int>>();

        // Execute
        var userId = await handler.HandleAsync(request, context.CancellationToken);

        // Return response
        return new CreateUserResponse { UserId = userId };
    }

    public override async Task<Empty> DeleteUser(
        DeleteUserCommand request,
        ServerCallContext context)
    {
        using var scope = _serviceProvider.CreateScope();

        var handler = scope.ServiceProvider
            .GetRequiredService<ICommandHandler<DeleteUserCommand>>();

        await handler.HandleAsync(request, context.CancellationToken);

        return new Empty();
    }

    private RpcException CreateValidationException(ValidationResult validationResult)
    {
        var badRequest = new BadRequest();

        foreach (var error in validationResult.Errors)
        {
            badRequest.FieldViolations.Add(new FieldViolation
            {
                Field = ToCamelCase(error.PropertyName),
                Description = error.ErrorMessage
            });
        }

        var status = new Google.Rpc.Status
        {
            Code = (int)Code.InvalidArgument,
            Message = "Validation failed",
            Details = { Any.Pack(badRequest) }
        };

        return status.ToRpcException();
    }
}
```

### QueryServiceImpl

**From .proto:**
```protobuf
service QueryService {
  rpc GetUser (GetUserQuery) returns (UserDto);
}
```

**Generated C# (simplified):**
```csharp
public class QueryServiceImpl : QueryService.QueryServiceBase
{
    private readonly IServiceProvider _serviceProvider;

    public QueryServiceImpl(IServiceProvider serviceProvider)
    {
        _serviceProvider = serviceProvider;
    }

    public override async Task<UserDto> GetUser(
        GetUserQuery request,
        ServerCallContext context)
    {
        using var scope = _serviceProvider.CreateScope();

        var handler = scope.ServiceProvider
            .GetRequiredService<IQueryHandler<GetUserQuery, UserDto>>();

        var result = await handler.HandleAsync(request, context.CancellationToken);

        return result;
    }
}
```

## Type Mapping

### Proto to C# Mapping

The source generator maps .proto types to C# CQRS types:

| Proto Message | C# Type | Handler Type |
|---------------|---------|--------------|
| `CreateUserCommand` | `CreateUserCommand` | `ICommandHandler<CreateUserCommand, TResult>` |
| `DeleteUserCommand` | `DeleteUserCommand` | `ICommandHandler<DeleteUserCommand>` |
| `GetUserQuery` | `GetUserQuery` | `IQueryHandler<GetUserQuery, UserDto>` |
| `UserDto` | `UserDto` | Return type |

### Response Type Detection

**Command with result:**
```protobuf
rpc CreateUser (CreateUserCommand) returns (CreateUserResponse);
```
Generator looks for: `ICommandHandler<CreateUserCommand, TResult>`

**Command without result:**
```protobuf
rpc DeleteUser (DeleteUserCommand) returns (google.protobuf.Empty);
```
Generator looks for: `ICommandHandler<DeleteUserCommand>`

## Build Integration

### Project Configuration

**.csproj:**
```xml
<Project Sdk="Microsoft.NET.Sdk.Web">
  <PropertyGroup>
    <TargetFramework>net10.0</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <!-- Include .proto files -->
    <Protobuf Include="Protos\**\*.proto" GrpcServices="Server" />
  </ItemGroup>

  <ItemGroup>
    <!-- Source generator package -->
    <PackageReference Include="Svrnty.CQRS.Grpc.Generators" Version="1.0.0" />

    <!-- gRPC packages -->
    <PackageReference Include="Grpc.AspNetCore" Version="2.68.0" />
    <PackageReference Include="Grpc.Tools" Version="2.76.0">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
    </PackageReference>
  </ItemGroup>
</Project>
```

### Build Output

During build, you'll see:
```
Restoring NuGet packages...
Generating C# from .proto files...
Running source generators...
  - Svrnty.CQRS.Grpc.Generators
    Generated: CommandServiceImpl.g.cs
    Generated: QueryServiceImpl.g.cs
Compiling...
Build succeeded.
```

## Viewing Generated Code

### In Visual Studio

1. Expand project in Solution Explorer
2. Expand "Dependencies" → "Analyzers" → "Svrnty.CQRS.Grpc.Generators"
3. View generated files

### In Rider

1. Navigate to a service usage
2. Right-click → "Go to Declaration"
3. View generated implementation

### Output Directory

Generated files are written to:
```
obj/Debug/net10.0/generated/Svrnty.CQRS.Grpc.Generators/
```

### Manual Inspection

```bash
# View generated files
cat obj/Debug/net10.0/generated/Svrnty.CQRS.Grpc.Generators/*.cs
```

## Customization

### Disabling Generation

Temporarily disable the generator:

**.csproj:**
```xml
<ItemGroup>
  <PackageReference Include="Svrnty.CQRS.Grpc.Generators" Version="1.0.0">
    <!-- Disable analyzer/generator -->
    <PrivateAssets>all</PrivateAssets>
    <ExcludeAssets>analyzers</ExcludeAssets>
  </PackageReference>
</ItemGroup>
```

### Excluding Specific Services

Use `[GrpcIgnore]` attribute:

```csharp
[GrpcIgnore]
public record InternalCommand
{
    public int Id { get; init; }
}
```

No gRPC RPC will be generated for this command.

## Error Handling

### Common Build Errors

#### Error: Handler not found

**Message:**
```
Could not find ICommandHandler<CreateUserCommand, int>
```

**Cause:** Handler not registered in DI

**Solution:**
```csharp
builder.Services.AddCommand<CreateUserCommand, int, CreateUserCommandHandler>();
```

#### Error: Type mismatch

**Message:**
```
CreateUserCommand in .proto does not match C# type
```

**Cause:** Property names don't match

**Proto:**
```protobuf
message CreateUserCommand {
  string user_name = 1;  // snake_case
}
```

**C#:**
```csharp
public record CreateUserCommand
{
    public string UserName { get; init; }  // PascalCase - OK
}
```

Properties match (case-insensitive) - this should work.

**Proto:**
```protobuf
message CreateUserCommand {
  string name = 1;
}
```

**C#:**
```csharp
public record CreateUserCommand
{
    public string FullName { get; init; }  // Different name - ERROR
}
```

**Solution:** Ensure property names match (case-insensitive).

#### Error: Circular dependency

**Message:**
```
Circular reference detected in message definitions
```

**Cause:** Proto messages reference each other

**Solution:** Break circular reference using separate DTOs.

## Performance

### Compile-Time Generation

- **Zero runtime overhead** - Code generated at build time
- **No reflection** - Direct method calls
- **Optimized** - IL identical to hand-written code

### Incremental Build

Source generators support incremental builds:
- Only regenerate when .proto files change
- Fast rebuilds when only C# code changes

## Troubleshooting

### Generator Not Running

**Symptoms:** No service implementations generated

**Checks:**
1. Verify package installed:
   ```xml
   <PackageReference Include="Svrnty.CQRS.Grpc.Generators" Version="1.0.0" />
   ```

2. Clean and rebuild:
   ```bash
   dotnet clean
   dotnet build
   ```

3. Check build output for generator messages

### Generated Code Not Visible

**Symptoms:** Can't find generated classes in IDE

**Solutions:**
1. Close and reopen solution
2. Rebuild project
3. Restart IDE
4. Check obj/Debug/net10.0/generated/ directory

### Build Warnings

**Warning:** "Generator produced no output"

**Cause:** No matching handlers found

**Solution:** Ensure handlers are registered before build.

## Best Practices

### ✅ DO

- Keep .proto files in Protos/ directory
- Use GrpcServices="Server" in .csproj
- Register all handlers in DI
- Clean build after .proto changes
- Review generated code occasionally

### ❌ DON'T

- Don't modify generated files (they'll be overwritten)
- Don't commit generated files to source control
- Don't disable generators without reason
- Don't ignore build warnings

## Advanced Scenarios

### Multiple Proto Files

```xml
<ItemGroup>
  <Protobuf Include="Protos\commands.proto" GrpcServices="Server" />
  <Protobuf Include="Protos\queries.proto" GrpcServices="Server" />
  <Protobuf Include="Protos\common.proto" GrpcServices="None" />
</ItemGroup>
```

### Shared Proto Files

```xml
<ItemGroup>
  <!-- Reference shared proto from another project -->
  <Protobuf Include="..\Shared.Contracts\Protos\*.proto"
            GrpcServices="Client"
            Link="Protos\Shared\%(Filename)%(Extension)" />
</ItemGroup>
```

## See Also

- [gRPC Integration Overview](README.md)
- [Getting Started](getting-started-grpc.md)
- [Proto File Setup](proto-file-setup.md)
- [Service Implementation](service-implementation.md)
- [gRPC Troubleshooting](grpc-troubleshooting.md)
- [Roslyn Source Generators](https://learn.microsoft.com/en-us/dotnet/csharp/roslyn-sdk/source-generators-overview)