Analyzers

Mutty includes Roslyn analyzers to help you use the library correctly and avoid common mistakes.

MUTTY001: MutableGeneration attribute can only be applied to records

Description

The [MutableGeneration] attribute is designed to work only with record types. Applying it to classes, structs, interfaces, enums, or other non-record types will not generate the expected mutable wrapper and will produce this analyzer error.

Severity

Error

Example

❌ Incorrect Usage

using Mutty;

// Error MUTTY001: The [MutableGeneration] attribute 
// can only be applied to record types
[MutableGeneration]
public class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
}

// Error MUTTY001: The [MutableGeneration] attribute 
// can only be applied to record types
[MutableGeneration]
public struct Point
{
    public int X { get; set; }
    public int Y { get; set; }
}

// Error MUTTY001: The [MutableGeneration] attribute 
// can only be applied to record types
[MutableGeneration]
public interface IPerson
{
    string Name { get; }
    int Age { get; }
}

// Error MUTTY001: The [MutableGeneration] attribute
// can only be applied to record types
[MutableGeneration]
public enum Status
{
    Active,
    Inactive
}

✅ Correct Usage

using Mutty;

// Valid: Applied to a record
[MutableGeneration]
public record Person(string Name, int Age);

// Valid: Applied to a record class (explicit syntax)
[MutableGeneration]
public record class Employee(string Name, string Department);

// Valid: Applied to a record struct
[MutableGeneration]
public record struct Point(int X, int Y);

// Valid: Applied to a record with additional members
[MutableGeneration]
public record Product(string Name, decimal Price)
{
    public string Category { get; init; } = "General";
}

How to Fix

To fix this error, you have several options:

Convert to a record: If the type is meant to be immutable, convert it to a record type:
// Before [MutableGeneration] public class Person { public string Name { get; set; } public int Age { get; set; } } // After [MutableGeneration] public record Person(string Name, int Age);
Remove the attribute: If the type should remain a class/struct/interface, remove the [MutableGeneration] attribute:
// Before [MutableGeneration] public class PersonService { } // After public class PersonService { }
Create a separate record: If you need both a mutable class and an immutable record, create them separately:
// Immutable record with mutable wrapper generation [MutableGeneration] public record PersonData(string Name, int Age); // Separate mutable class for other purposes public class PersonEntity { public string Name { get; set; } public int Age { get; set; } }

Why This Matters

The Mutty library is specifically designed to generate mutable wrappers for immutable record types. Records in C# are designed to be immutable by default, which makes them perfect candidates for scenarios where you need both immutable and mutable representations of the same data structure.

Applying the [MutableGeneration] attribute to non-record types would not work because:

The source generator expects specific record syntax and semantics
The generated code relies on record features like positional parameters and init-only properties
The library's design patterns are optimized for record types

Suppression

This analyzer should not be suppressed as it indicates a fundamental misuse of the library. If you believe you have a valid use case that triggers this analyzer incorrectly, please file an issue on the Mutty GitHub repository.

Last modified: 17 June 2025