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How to use source generators

Conjecture includes a Roslyn incremental source generator (bundled in Conjecture.Core) that derives IStrategyProvider<T> implementations from your types at compile time. No additional packages needed.

Mark a type with [Arbitrary]

using Conjecture.Core;

[Arbitrary]
public partial record Person(string Name, int Age);

The generator emits a PersonArbitrary class that implements IStrategyProvider<Person>:

// Auto-generated
public sealed class PersonArbitrary : IStrategyProvider<Person>
{
    public Strategy<Person> Create() =>
        Generate.Compose<Person>(ctx =>
        {
            var name = ctx.Generate(/* resolved strategy for string */);
            var age = ctx.Generate(/* resolved strategy for int */);
            return new Person(name, age);
        });
}

Use it in tests

[Property]
public bool People_have_names([From<PersonArbitrary>] Person person)
{
    return person.Name is not null;
}

Requirements

  • The type must be partial (the generator emits companion code alongside it)
  • Must have at least one accessible constructor
  • All constructor parameter types must have auto-resolvable strategies: primitives, strings, collections, enums, or other [Arbitrary] types

Nested types

[Arbitrary] types can reference each other — the generator resolves them automatically:

[Arbitrary]
public partial record Address(string Street, string City, int ZipCode);

[Arbitrary]
public partial record Customer(string Name, Address ShippingAddress);

When generating Customer, the generator uses AddressArbitrary to produce the ShippingAddress parameter.

Classes and structs

Works with records, classes, and structs — any type with a constructor:

[Arbitrary]
public partial class Order
{
    public Order(int id, string product, int quantity) { /* ... */ }
}

[Arbitrary]
public partial struct Point
{
    public Point(double x, double y) { /* ... */ }
}

Supported parameter types

Category Examples
Integer types int, long, byte, short, uint, ulong, etc.
Floating point double, float
Other primitives bool, string, char
Nullable value types int?, double?, etc.
Enums Any enum type
Collections List<T>, IReadOnlyList<T> (with supported element type)
Nested [Arbitrary] Other types marked with [Arbitrary]

Generator diagnostics

ID Severity Description
CON200 Error No accessible constructor found
CON201 Error Type is not partial
CON202 Warning Parameter type has no resolvable strategy

See also

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