How is this field null?

Question

I'm getting a null exception, but the field was initialized as an empty list. So how could it be null?

The error occurs on the second line in this method (on _hydratedProperties):

protected virtual void NotifyPropertyChanged<T>(Expression<Func<T>> expression)
{
    string propertyName = GetPropertyName(expression);

    if (!this._hydratedProperties.Contains(propertyName)) { this._hydratedProperties.Add(propertyName); }
}

And this is how the field is declared:

public abstract class EntityBase<TSubclass> : INotifyPropertyChanged where TSubclass : class
{
    private List<string> _hydratedProperties = new List<string>();

This is how it's set:

    public Eta Eta
    {
        get { return this._eta; }

        set
        {
            this._eta = value;
            NotifyPropertyChanged(() => this.Eta);
        }
    }

This is the full class (with the comments and non-relevant parts removed):

[DataContract]
public abstract class EntityBase<TSubclass> : INotifyPropertyChanged where TSubclass : class
{
    private List<string> _hydratedProperties = new List<string>();

    public bool IsPropertyHydrated(string propertyName)
    {
        return this._hydratedProperties.Contains(propertyName);
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void NotifyPropertyChanged<T>(Expression<Func<T>> expression)
    {
        string propertyName = GetPropertyName(expression);

        if (!this._hydratedProperties.Contains(propertyName)) { this._hydratedProperties.Add(propertyName); }

        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    public string GetPropertyName<T>(Expression<Func<T>> expression)
    {
        MemberExpression memberExpression = (MemberExpression)expression.Body;
        return memberExpression.Member.Name;
    }
}

Derived class:

[DataContract]
public class Bin : EntityBase<Bin>
{
    private Eta _eta;

    [DataMember]
    public Eta Eta
    {
        get { return this._eta; }

        set
        {
            this._eta = value;
            NotifyPropertyChanged(() => this.Eta);
        }
    }
}

Answer 1

Here's the clue:

[DataContract]

Yup. DataContractSerializer does not call any constructor. Instead, it uses FormatterServices.GetUninitializedObject to create the object that will be deserialized. This bypasses the constructor call.

Your initializer:

private List<string> _hydratedProperties = new List<string>();

is translated to an implicit default constructor by the compiler.

As a workaround, you can use a deserialization callback with OnDeserializingAttribute:

[DataContract]
public abstract class EntityBase<TSubclass> : INotifyPropertyChanged
    where TSubclass : class
{
    private List<string> _hydratedProperties;

    protected EntityBase()
    {
        Init();
    }

    private void Init()
    {
        _hydratedProperties = new List<string>()
    }

    [OnDeserializing]
    private void OnDeserializing(StreamingContext context)
    {
        Init();
    }

    // ... rest of code here
}

Answer 2

I found a simpler answer than what Lucas provided. I'm not sure if this one is actually better, but it's simple and it worked. All I did was add the DataMember attribute to the field. Since that specifies that the field is part of the data contract, it is included with serialization/deserialization and no longer causes a null reference error.

[DataContract]
public abstract class EntityBase<TSubclass> : INotifyPropertyChanged where TSubclass : class
{
    [DataMember]
    private List<string> _hydratedProperties = new List<string>();

    // More code here
}