Samhammer.AspNetCore.StringTrimming 1.0.1

Samhammer.AspNetCore.StringTrimming

Trims leading and trailing whitespace of all string inputs of ASP.NET Core MVC, so controllers, validators and repositories don't have to deal with it.

Only reading is affected. Responses are serialized unchanged.

Packages

The two serializer packages don't depend on each other. Install exactly one of them; the common package comes with it.

How to use

System.Text.Json:

builder.Services.AddControllers()
    .AddJsonOptions(...)
    .AddSystemTextJsonStringTrimming();

Newtonsoft.Json:

builder.Services.AddControllers()
    .AddNewtonsoftJson(...)
    .AddNewtonsoftJsonStringTrimming();

Both are applied with PostConfigure, so the order relative to AddJsonOptions() / AddNewtonsoftJson() does not matter. Both register the model binder of the common package.

Without MVC (e.g. your own serializer settings):

var options = new JsonSerializerOptions().AddStringTrimming();          // System.Text.Json
var settings = new JsonSerializerSettings().AddStringTrimming();        // Newtonsoft.Json

Opt-out

Use it where whitespace is meaningful (markdown, templates, regex, secrets, signatures).

public class MessageContract
{
    public string Title { get; set; }       // trimmed

    [NoTrim]                                                // System.Text.Json
    [JsonConverter(typeof(NoTrimStringJsonConverter))]      // Newtonsoft.Json
    public string Markdown { get; set; }    // kept as sent
}

public IActionResult Get([FromQuery][NoTrim] string pattern) { ... }

With System.Text.Json [NoTrim] also works on string collections, dictionaries and complex types (nothing inside is trimmed) and on records ([property: NoTrim]).

Behavior

• string.Trim() is used: all Unicode whitespace incl. line breaks and NBSP; zero-width characters and BOM (U+FEFF) are not removed.
• JSON: null stays null, whitespace-only becomes "".
• Route, query, form: like the default binder, empty or whitespace-only values become null (ConvertEmptyStringToNull).
• Trimmed: string properties, fields, constructor parameters, items of string collections, values of string dictionaries, root strings. Dictionary keys and property names are not trimmed.
• Not trimmed: strings inside object, JToken, JsonElement or dynamic, raw Request.Body reads, IFormFile content, headers.

System.Text.Json

• A global JsonConverter<string> trims every string, including root values ([FromBody] string) and items of any collection.
• [NoTrim] is applied by a type info modifier. If you replace TypeInfoResolver (e.g. source generated contexts) after the trimming was added, the modifier is lost.

Newtonsoft.Json

• A global JsonConverter for string trims every string, including root values ([FromBody] string), collection items and dictionary values (same approach as FUTO).
• Known limitation: with a converter for string, Newtonsoft.Json passes the raw token to the converter. Numbers, booleans and date-like strings (e.g. "2026-10-01T10:00:00", default DateParseHandling.DateTime) sent for a string property are rejected with a JsonSerializationException (400). Without the package they would be accepted as strings.

Things to check when introducing it

• Values that are hashed, signed or compared byte-exact (checksums, signatures, passwords, cache keys).
• Values where whitespace is meaningful (markdown, code, templates, regex, separators).
• Existing data is not migrated: values stored before keep their whitespace.
• Service-to-service calls go through the same pipeline and are trimmed as well.

Build and release

One pipeline (.github/workflows/cicd.yaml) builds and tests all projects. Pushing a tag (e.g. 1.0.0) publishes all three packages to NuGet with the tag as version, so the packages always share the same version.