Source Generators

CaeriusNet ships two Roslyn incremental source generators that eliminate mapping boilerplate at compile time. Both generators run as part of your build — they produce zero runtime overhead and are fully AOT-compatible.

Overview

Generator	Attribute	Interface generated
DtoSourceGenerator	[GenerateDto]	ISpMapper<T>
TvpSourceGenerator	[GenerateTvp]	ITvpMapper<T>

Both generators target sealed partial records or classes. The partial keyword lets the generator add the interface implementation as a second partial declaration alongside your type.

[GenerateDto] — DTO mapper

Annotate a sealed partial record or class with [GenerateDto]. The generator emits a MapFromDataReader method with ordinal-based column reads, correct nullability guards, and special type conversions.

Requirements

• Type must be sealed
• Type must be partial
• Type must use a primary constructor (the constructor parameters become the mapped columns, in order)

Basic example

using CaeriusNet.Attributes.Dto;

[GenerateDto]
public sealed partial record UserDto(int Id, string Username, byte Age);

Generated code (simplified):

// UserDto.g.cs — auto-generated, do not edit
partial record UserDto : ISpMapper<UserDto>
{
    public static UserDto MapFromDataReader(SqlDataReader reader)
        => new(
            reader.GetInt32(0),
            reader.GetString(1),
            reader.GetByte(2));
}

Nullable fields

The generator respects C# nullable annotations and inserts IsDBNull guards automatically:

[GenerateDto]
public sealed partial record ProductDto(int Id, string? Description, int? Stock);

Generated:

public static ProductDto MapFromDataReader(SqlDataReader reader)
    => new(
        reader.GetInt32(0),
        reader.IsDBNull(1) ? null : reader.GetString(1),
        reader.IsDBNull(2) ? null : reader.GetInt32(2));

Special types

Field type	Generated expression
DateOnly	DateOnly.FromDateTime(reader.GetDateTime(n))
TimeOnly	TimeOnly.FromDateTime(reader.GetDateTime(n))
byte[]	(byte[])reader.GetValue(n)

[GenerateTvp] — TVP mapper

Annotate a sealed partial record or class with [GenerateTvp]. The generator emits:

• TvpTypeName static property (e.g., "dbo.tvp_int")
• _tvpMetaData static SqlMetaData[] field (one entry per property)
• MapAsSqlDataRecords iterator that reuses a single SqlDataRecord instance across all rows (zero-copy streaming)

Requirements

• Type must be sealed
• Type must be partial
• Attribute requires Schema and TvpName named arguments

Basic example

using CaeriusNet.Attributes.Tvp;

[GenerateTvp(Schema = "dbo", TvpName = "tvp_int")]
public sealed partial record UserIdTvp(int Id);

Generated code (simplified):

// UserIdTvp.g.cs — auto-generated, do not edit
partial record UserIdTvp : ITvpMapper<UserIdTvp>
{
    public static string TvpTypeName => "dbo.tvp_int";

    private static readonly SqlMetaData[] _tvpMetaData =
    [
        new SqlMetaData("Id", SqlDbType.Int)
    ];

    public IEnumerable<SqlDataRecord> MapAsSqlDataRecords(IEnumerable<UserIdTvp> items)
    {
        var record = new SqlDataRecord(_tvpMetaData);
        foreach (var item in items)
        {
            record.SetInt32(0, item.Id);
            yield return record;
        }
    }
}

Nullable TVP fields

The generator emits record.SetDBNull(n) for nullable fields when the value is null:

[GenerateTvp(Schema = "dbo", TvpName = "tvp_optional")]
public sealed partial record OptionalTvp(int Id, int? OptionalValue);

Custom schema

Use a schema other than dbo to match your SQL Server type:

[GenerateTvp(Schema = "Types", TvpName = "tvp_UserId")]
public sealed partial record UserIdTvp(int Id);
// TvpTypeName => "Types.tvp_UserId"

Manual vs. generated comparison

Aspect	Manual	Generated
MapFromDataReader	You write it	Compiler emits it
Ordinal indices	You manage them	Auto-assigned from constructor order
Nullable guards	You add IsDBNull	Emitted when field is nullable
SqlDataRecord reuse	You implement it	Always reused (single instance)
Compilation	Compiles as-is	Requires sealed partial
Build-time errors	Runtime	Compile-time

Enabling the generators

The generators ship inside the CaeriusNet NuGet package as an embedded Roslyn analyzer. No additional package or MSBuild configuration is required.

dotnet add package CaeriusNet

Once added, [GenerateDto] and [GenerateTvp] are available in the CaeriusNet.Attributes.Dto and CaeriusNet.Attributes.Tvp namespaces.

Inspecting generated code

Set <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles> in your .csproj to write generated files to obj/Generated/ for inspection.

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Next: Table-Valued Parameters — bulk inputs without DataTable overhead.