dotnet-trim-strings

A simple scaffold .NET project that demonstrates how to use JSON converters as filters to automatically trim strings on every incoming API request — no manual .Trim() calls needed throughout your codebase.

How It Works

.NET's System.Text.Json allows you to register custom JsonConverter<T> implementations globally. This project registers a TrimStringConverter that intercepts every string value during JSON deserialization and:

• Trims leading and trailing whitespace from the value.
• Returns null if the value is empty or whitespace-only.

The converter is registered once in Program.cs:

builder.Services
    .AddControllers()
    .AddJsonOptions(option =>
    {
        option.JsonSerializerOptions.Converters.Add(new TrimStringConverter());
    });

The Converter

public class TrimStringConverter : JsonConverter<string>
{
    public override string? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        var value = reader.GetString();
        if (string.IsNullOrWhiteSpace(value))
        {
            return null;
        }

        return value.Trim();
    }

    public override void Write(Utf8JsonWriter writer, string value, JsonSerializerOptions options)
    {
        writer.WriteStringValue(value);
    }
}

Behaviour

Input JSON value	Deserialized result
" John "	"John"
"John "	"John"
""	null
" "	null
null	null

Project Structure

API/
  Program.cs                  # Registers TrimStringConverter globally
  TrimStringConverter.cs      # Custom JsonConverter<string> implementation
  Controllers/
    WeatherForecastController.cs
Tests/
  TrimStringConverterTests.cs # Unit tests for the converter

Running the Project

dotnet run --project API

The API will be available with Swagger UI at https://localhost:{port}/swagger.

Running the Tests

dotnet test