THR018: Field of a Mutable Struct Type in a Thread-Aware Class


Severity	Error
Reported by	PostSharp.Patterns.Threading
Message	`The type of field "<Field>" is not compatible with the [<Aspect>] aspect: mutable struct types are not allowed. Make all fields of the struct read-only, add [ExplicitlySynchronized] to this field or property if access to it is synchronized manually in source code, use [AssumeImmutable], or register a TypeAdapter.`

Cause

A threading model checks the accesses to the fields of its class. A field of a struct type holds the struct itself, not a reference, so code can modify a field of the struct in place, for example this.position.Latitude = 0. PostSharp cannot check such modifications, so a threading model does not accept a field whose type is a mutable struct.

PostSharp considers a struct immutable when all its instance fields are read-only and their types are immutable, as in a readonly struct, or when the struct is marked as immutable with the AssumeImmutableAttribute custom attribute or with a registered type adapter. The primitive types, enumerations and the structs of the System namespace, such as DateTime and decimal, are immutable.

<Aspect> is the name of the class of the threading model aspect, for example SynchronizedAttribute.

In the following example, the Vehicle class has the SynchronizedAttribute threading model, and its position field is a Coordinates struct whose properties have setters.

Full screen

using System;
using PostSharp.Patterns.Threading;

namespace Errors.THR018_Error;

public struct Coordinates
{
    public double Latitude { get; set; }

    public double Longitude { get; set; }
}

[Synchronized]
public class Vehicle
{
    // PostSharp cannot check the modifications made inside a mutable struct.
    private Coordinates position;

    public void MoveTo( double latitude, double longitude )
    {
        this.position = new Coordinates { Latitude = latitude, Longitude = longitude };
    }

    public string Describe()
    {
        var current = this.position;

        return $"Vehicle at {current.Latitude}, {current.Longitude}";
    }
}

public static class Program
{
    public static void Main()
    {
        var vehicle = new Vehicle();
        vehicle.MoveTo( 50.08, 14.42 );
        Console.WriteLine( vehicle.Describe() );
    }
}

Full screen

error THR018: THR018_Error.cs(17,25): The type of field "Errors.THR018_Error.Vehicle.position" is not compatible with the [SynchronizedAttribute] aspect: mutable struct types are not allowed. Make all fields of the struct read-only, add [ExplicitlySynchronized] to this field or property if access to it is synchronized manually in source code, use [AssumeImmutable], or register a TypeAdapter.

Resolution

Make the struct immutable: declare it as a readonly struct, or make all its fields read-only. Properties with an init accessor or with a getter only have read-only backing fields. To change the value, assign a new instance of the struct to the field.
When your code synchronizes the accesses to the field by itself, apply the ExplicitlySynchronizedAttribute custom attribute to the field, or to the automatic property.
When a struct is immutable in practice, apply the AssumeImmutableAttribute custom attribute to it. For a struct of another assembly, apply the attribute to your assembly with the type of the struct as its argument, or register a TypeAdapter for it.

In the following example, Coordinates is a readonly struct.

Full screen

using System;
using PostSharp.Patterns.Threading;

namespace Errors.THR018_Fixed;

// All fields of a read-only struct are read-only, so the struct is immutable.
public readonly struct Coordinates
{
    public double Latitude { get; init; }

    public double Longitude { get; init; }
}

[Synchronized]
public class Vehicle
{
    private Coordinates position;

    public void MoveTo( double latitude, double longitude )
    {
        this.position = new Coordinates { Latitude = latitude, Longitude = longitude };
    }

    public string Describe()
    {
        var current = this.position;

        return $"Vehicle at {current.Latitude}, {current.Longitude}";
    }
}

public static class Program
{
    public static void Main()
    {
        var vehicle = new Vehicle();
        vehicle.MoveTo( 50.08, 14.42 );
        Console.WriteLine( vehicle.Describe() );
    }
}

Full screen

Vehicle at 50.08, 14.42

Cause

Resolution

See Also