Question

I have been creating object for a project and there are some instances that I have to create a deep copy for this objects I have come up with the use of a built in function for C# which is MemberwiseClone(). The problem that bothers me is whenever there is a new class that i created , I would have to write a function like the code below for a shallow copy..Can someone please help me improve this part and give me a shallow copy that is better than the second line of code. thanks 🙂

SHALLOW COPY:

public static RoomType CreateTwin(RoomType roomType)
{
    return (roomType.MemberwiseClone() as RoomType);
}

DEEP COPY:

public static T CreateDeepClone<T>(T source)
{
    if (!typeof(T).IsSerializable)
    {
        throw new ArgumentException("The type must be serializable.", "source");
    }

    if (Object.ReferenceEquals(source, null))
    {
        return default(T);
    }

    IFormatter formatter = new BinaryFormatter();
    Stream stream = new MemoryStream();
    using (stream)
    {
        formatter.Serialize(stream, source);
        stream.Seek(0, SeekOrigin.Begin);
        return (T)formatter.Deserialize(stream);
    }
}

Answer 1

MemberwiseClone is not a good choice to do a Deep Copy (MSDN):

The MemberwiseClone method creates a shallow copy by creating a new
object, and then copying the nonstatic fields of the current object to
the new object. If a field is a value type, a bit-by-bit copy of the
field is performed. If a field is a reference type, the reference is
copied but the referred object is not; therefore, the original
object and its clone refer to the same object.

This mean if cloned object has reference type public fields or properties they would reffer to the same memory location as the original object’s fields/properties, so each change in the cloned object will be reflected in the initial object. This is not a true deep copy.

You can use BinarySerialization to create a completely independent instance of the object, see MSDN Page of the BinaryFormatter class for an serialization example.

---

Example and Test Harness:

Extension method to create a deep copy of a given object:

public static class MemoryUtils
{
    /// <summary>
    /// Creates a deep copy of a given object instance
    /// </summary>
    /// <typeparam name="TObject">Type of a given object</typeparam>
    /// <param name="instance">Object to be cloned</param>
    /// <param name="throwInCaseOfError">
    /// A value which indicating whether exception should be thrown in case of
    /// error whils clonin</param>
    /// <returns>Returns a deep copy of a given object</returns>
    /// <remarks>Uses BInarySerialization to create a true deep copy</remarks>
    public static TObject DeepCopy<TObject>(this TObject instance, bool throwInCaseOfError)
        where TObject : class
    {
        if (instance == null)
        {
            throw new ArgumentNullException("instance");
        }

        TObject clonedInstance = default(TObject);

        try
        {
            using (var stream = new MemoryStream())
            {
                BinaryFormatter binaryFormatter = new BinaryFormatter();
                binaryFormatter.Serialize(stream, instance);

                // reset position to the beginning of the stream so
                // deserialize would be able to deserialize an object instance
                stream.Position = 0;

                clonedInstance = (TObject)binaryFormatter.Deserialize(stream);
            }
        }
        catch (Exception exception)
        {
            string errorMessage = String.Format(CultureInfo.CurrentCulture,
                            "Exception Type: {0}, Message: {1}{2}",
                            exception.GetType(),
                            exception.Message,
                            exception.InnerException == null ? String.Empty :
                            String.Format(CultureInfo.CurrentCulture,
                                        " InnerException Type: {0}, Message: {1}",
                                        exception.InnerException.GetType(),
                                        exception.InnerException.Message));
            Debug.WriteLine(errorMessage);

            if (throwInCaseOfError)
            {
                throw;
            }
        }

        return clonedInstance;
    }
}

NUnit tests:

public class MemoryUtilsFixture
{
    [Test]
    public void DeepCopyThrowWhenCopyInstanceOfNonSerializableType()
    {
        var nonSerializableInstance = new CustomNonSerializableType();
        Assert.Throws<SerializationException>(() => nonSerializableInstance.DeepCopy(true));
    }

    [Test]
    public void DeepCopyThrowWhenPassedInNull()
    {
        object instance = null;
        Assert.Throws<ArgumentNullException>(() => instance.DeepCopy(true));
    }

    [Test]
    public void DeepCopyThrowWhenCopyInstanceOfNonSerializableTypeAndErrorsDisabled()
    {
        var nonSerializableInstance = new CustomNonSerializableType();            
        object result = null;

        Assert.DoesNotThrow(() => result = nonSerializableInstance.DeepCopy(false));
        Assert.IsNull(result);
    }

    [Test]
    public void DeepCopyShouldCreateExactAndIndependentCopyOfAnObject()
    {
        var instance = new CustomSerializableType
                        {
                            DateTimeValueType =
                                DateTime.Now.AddDays(1).AddMilliseconds(123).AddTicks(123),
                            NumericValueType = 777,
                            StringValueType = Guid.NewGuid().ToString(),
                            ReferenceType =
                                new CustomSerializableType
                                    {
                                        DateTimeValueType = DateTime.Now,
                                        StringValueType = Guid.NewGuid().ToString()
                                    }
                        };

        var deepCopy = instance.DeepCopy(true);

        Assert.IsNotNull(deepCopy);
        Assert.IsFalse(ReferenceEquals(instance, deepCopy));
        Assert.That(instance.NumericValueType == deepCopy.NumericValueType);
        Assert.That(instance.DateTimeValueType == deepCopy.DateTimeValueType);
        Assert.That(instance.StringValueType == deepCopy.StringValueType);
        Assert.IsNotNull(deepCopy.ReferenceType);
        Assert.IsFalse(ReferenceEquals(instance.ReferenceType, deepCopy.ReferenceType));
        Assert.That(instance.ReferenceType.DateTimeValueType == deepCopy.ReferenceType.DateTimeValueType);
        Assert.That(instance.ReferenceType.StringValueType == deepCopy.ReferenceType.StringValueType);
    }

    [Serializable]
    internal sealed class CustomSerializableType
    {            
        public int NumericValueType { get; set; }
        public string StringValueType { get; set; }
        public DateTime DateTimeValueType { get; set; }

        public CustomSerializableType ReferenceType { get; set; }
    }

    public sealed class CustomNonSerializableType
    {            
    }
}