Sep for .NET separated values

Trigger On

• delimited data needs are performance-sensitive and allocation-aware
• project needs explicit control over separator inference, escaping, trimming, and header behavior
• reading/writing large or long-lived file pipelines in ML, ETL, or analytics workloads
• startup/perf tests require AOT/trimming-friendly CSV/TSV processing

Install

• NuGet:

  • dotnet add package Sep

  • dotnet add package Sep --version <version>

• XML package reference:

  • <PackageReference Include="Sep" Version="x.y.z" />

• Verify baseline support by checking the package page:
  • NuGet: Sep
• Source:
  • GitHub: nietras/Sep

Workflow

flowchart LR
  A[Input source: file/text/stream] --> B[Sep.Reader or Sep.New(...).Reader]
  B --> C[SepReaderOptions]
  C --> D[Rows -> Cols -> Span/Parse]
  D --> E[Transform and validate]
  E --> F[SepWriter via SepWriterOptions]
  F --> G[To file/text output]

1. Decide schema shape
   • header present or no header
   • separator known (

     ;

     ,

     ,

     , tab, custom) or infer from first row
   • row/column quoting rules
2. Build reader with

   Sep.Reader(...)

   and explicit options only where needed:

   • Sep.Reader()

     for inferred separator from header-like first row

   • Sep.New(',').Reader(...)

     for explicit separator mode

   • Sep.Reader(o => o with { HasHeader = false })

     if header is absent
3. Read rows and map columns as

   ReadOnlySpan<char>

   first, convert only when needed.
4. For output, use

   reader.Spec.Writer()

   when you need the same separator/culture as input.
5. Control writer behavior with

   Sep.Writer(...)

   and

   SepWriterOptions

   (

   WriteHeader

   ,

   Escape

   ,

   DisableColCountCheck

   ).
6. Add async only where it brings value and your runtime is C# 13 / .NET 9+ for

   await foreach

   over async reader rows.
7. Use

   ParallelEnumerate

   for CPU-heavy transformations only after benchmarking single-threaded baseline.

Install and read patterns

using var reader = Sep.Reader(o => o with
{
    HasHeader = true,
    Unescape = true,
    Trim = SepTrim.Both
}).FromText(data);

foreach (var row in reader)
{
    var id = row["Id"].Parse<int>();
    var name = row[1].ToString();
    // process row
}

Write patterns

using var reader = Sep.Reader().FromFile("input.csv");
using var writer = reader.Spec.Writer().ToFile("output.csv");

foreach (var row in reader)
{
    using var writeRow = writer.NewRow(row);
    writeRow["Amount"].Format(row["Amount"].Parse<double>() * 1.2);
}

Async reading and writing

var text = "A;B\n1;hello\n";

using var reader = await Sep.Reader().FromTextAsync(text);
await using var writer = reader.Spec.Writer().ToText();

await foreach (var row in reader)
{
    await using var writeRow = writer.NewRow(row);
    var normalized = row["B"].ToString().ToUpperInvariant();
    writeRow["B"].Set(normalized);
}

Common configuration patterns

• Header-driven read
  • default

    HasHeader = true

  • query by name:

    row["ColName"]

• Headerless pipelines

  • HasHeader = false

  • use index-based access:

    row[0]

    ,

    row[1]

• Round-trip output
  • start writer with

    reader.Spec.Writer()

    to preserve inference and formatting contract
• Speed-first processing
  • keep default buffer + culture unless profiling proves a need to tune

Best practices

• Parse to primitive types with

  Parse<T>

  in hot paths to avoid extra allocations.
• Keep

  ToString

  /format conversions at the edge (presentational layers), not in inner loops.
• Prefer

  Unescape

  ,

  Trim

  , and

  DisableQuotesParsing

  settings deliberately and test with realistic samples.
• For large transforms, isolate heavy CPU work after enumeration and then apply

  ParallelEnumerate

  where appropriate.

Limitations to check before production

• SepReader.Row

  and

  SepWriter.Row

  are

  ref struct

  s:
  • avoid patterns that store rows beyond immediate scope
  • materialize if you truly need random async/LINQ-style buffering

• SepReader

  row iteration is row-by-row by design; it is intentionally not the same as a classic collection model.

Deliver

• installation and usage guide that is ready to copy into a .NET repo
• practical reader/writer configuration patterns
• clear notes on defaults, tradeoffs, and constraints

Validate

• dotnet add package Sep

  installs correctly and project compiles
• one file-read sample and one file-write sample execute successfully
• header/no-header and explicit-separator cases are covered
• at least one validation sample for quoting/unescaping or async path exists if required by task

Load References

• references/overview.md - official links and practical decision notes.