NDLRGEN021: Positional record must be partial for [Options]¶

Cause¶

A positional record (a record with primary constructor parameters) has the [Options] attribute but is not declared as partial.

Rule Description¶

Positional records like record Foo(string Bar, int Baz) do not have parameterless constructors. Microsoft's configuration binder uses Activator.CreateInstance() to instantiate options types, which requires a parameterless constructor.

When a positional record is declared as partial, Needlr's source generator emits a parameterless constructor that chains to the primary constructor with default values. This enables configuration binding to work.

Non-partial records cannot be extended by source generators, so configuration binding will fail at runtime with a MissingMethodException.

How to Fix¶

Add the partial modifier to the positional record:

// ❌ Before: Will fail at runtime
[Options("Database")]
public record DatabaseConfig(string Host, int Port);

// ✅ After: Generator creates parameterless constructor
[Options("Database")]
public partial record DatabaseConfig(string Host, int Port);

Alternatively, use a record with init-only properties instead of positional parameters:

// ✅ Alternative: Init-only properties have parameterless constructor
[Options("Database")]
public record DatabaseConfig
{
    public string Host { get; init; } = "";
    public int Port { get; init; } = 5432;
}

Generated Code¶

For a partial positional record, Needlr generates:

// User code:
[Options("Redis")]
public partial record RedisConfig(string Host, int Port);

// Generated code (OptionsConstructors.g.cs):
public partial record RedisConfig
{
    public RedisConfig() : this(string.Empty, default) { }
}

The generated constructor uses:

string.Empty for string parameters
default for value types (0 for int, false for bool, etc.)
default! for other reference types

When to Suppress¶

Suppress this warning only if you are manually providing a parameterless constructor or if you are not using the record with configuration binding:

#pragma warning disable NDLRGEN021
[Options("Custom")]
public record CustomConfig(string Value)
{
    public CustomConfig() : this("default-value") { }
}
#pragma warning restore NDLRGEN021