Amplified.ValueObjects

Provides types and reflection information for implementing and working with single-value ValueObjects.

Installation

The library is available on NuGet.

Install-Package Amplified.ValueObjects

See also

• Amplified.ValueObjects.Newtonsoft.Json

Usage

ValueObject Type definition

Value objects implement IValueObject<T> to signal that the specified object is a ValueObject.

public struct UserId : IValueObject<Guid>
{
    private readonly Guid _value;
    
    public UserId(Guid value)
    {
        _value = value;
    }
    
    Guid IValueObject<Guid>.Value => _value;
}

A value object should also implement IEquatable<TValueObject> to adhere to correct value object semantics.

public struct UserId : IValueObject<Guid>, IEquatable<UserId> 
{
    private readonly Guid _value;
    
    public UserId(Guid value);

    Guid IValueObject<Guid>.Value { get; }
    
    public bool Equals(object other);
    public bool Equals(UserId other);
    
    public int GetHashCode();
    
    public bool operator ==(UserId left, UserId right);
    public bool operator !=(UserId left, UserId right);
}

Value objects may implement IComparable<TValueObject> when it makes sense. An example of this is the value object PageNumber.

public struct PageNumber : IValueObject<int>, IEquatable<PageNumber>, IComparable<PageNumber>
{
    private readonly int _value;
    
    public PageNumber(int value);
    
    int IValueObject<int>.Value { get; }
    
    public bool Equals(object other);
    public bool Equals(UserId other);
    
    public int GetHashCode();
    
    public bool operator ==(PageNumber left, PageNumber right);
    public bool operator !=(PageNumber left, PageNumber right);
    
    public int CompareTo(PageNumber other);
    
    public int operator >(PageNumber left, PageNumber right);
    public int operator <(PageNumber left, PageNumber right);
}

We recommend hiding the value behind the explicit implementation of IValueObject<T>, and exposing it using explicit type conversion operators.

public struct UserId : IValueObject<Guid>
{
    private readonly Guid _value;
    
    public UserId(Guid value) => _value = value;

    Guid IValueObject<Guid>.Value => _value;
    
    public explicit operator Guid(UserId value) => value.Value;
    public explicit operator Guid?(UserId? value) => value?.Value;
}

Reflection

Value object types supports additional type information through the use of extension methods:

public static bool IsValueObject(this Type type);
public static ValueObjectTypeInfo GetValueObjectTypeInfo(this Type type);

Calling IsValueObject on any type is allowed, while calling GetValueObjectTypeInfo on non-ValueObject types, will result in an exception being thrown. You should always call IsValueObject before GetValueObjectTypeInfo, if you do not for sure that the type represents a ValueObject. The type information returned by either of these is cached, so the overhead is minimal.

ValueObjectTypeInfo

This object provides type information and useful methods associated with value objects.

Example: Creating a new instance of a value object type

public TValueObject CreateValueObject<TValueObject, T>(T argument) where TValueObject : IValueObject<T>
{
    var typeInfo = typeof(TValueObject).GetValueObjectTypeInfo();
    var valueObject = typeInfo.Create(argument);
    return (TValueObject) valueObject;
}

Example: Accessing the value of value object

public object GetValueObjectValue(object instance)
{
    var typeInfo = instance.GetType().GetValueObjectTypeInfo();
    var value = typeInfo.GetValue(instance);
    return (T) value;
}

License

MIT License

Copyright (c) 2017 Nicklas Jensen

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.