CODEWITHSUNDEEP
c#generics
Wesley
Wesley

Reputation: 5621

Updating this in Class that inherits from List<T>

I have an abstract class used for caching that implements as follows (simplified)

public abstract class DataCacheMember<T> : List<T>
{
    private List<T> _data;
    public List<T> Data
    {
        get
        {
            if (_data == null || _data.Count() < 1)
                _data = GetData();

            return _data;
        }
    }

    private string ApiEndPoint {get; set;}
    private Timer timer;

    private List<T> GetData()
    {
        //call api and get data
    }

    private void RefreshData()
    {
        _data = GetData();
    }

    protected DataCacheMember(string apiEndPoint)
    {
        ApiEndPoint = apiEndPoint;

        timer = new System.Threading.Timer(
            e => RefreshData(),
            null,
            TimeSpan.Zero,
            TimeSpan.FromMinutes(10));
    }
}

It allows for rapid creation of cached objects with a simple string for the api endpoint:

public class StateMap<Properties> : DataCacheMember<Properties>
{
    public StateMap(string apiEndPoint = "Property/GetProperties")
        : base(apiEndPoint)
    {}
}

The whole reason for inheriting from List<T> was to remove the need for the fields.

However, if I try to modify the constructor and refresh to:

private void RefreshData()
{
    this = GetData() as DataCacheMember<T>;
}

protected DataCacheMember(string apiEndPoint)
{
    this = GetData() as DataCacheMember<T>;
}

I get an error of Cannot assign to <this> because it is Read Only.

What's the proper way to resolve this? Do I just need to use Clear() and AddRange() to manage the object?

If I do that, I see that the first call to the object will return empty, because the object can return before the constructor finishes it's call.

Upvotes: 0

Views: 64

Answers (2)

Shar1er80
Shar1er80

Reputation: 9041

According to Using this() in C# Constructors 

private void RefreshData()
{
    this = GetData() as DataCacheMember<T>;
}

protected DataCacheMember(string apiEndPoint)
{
    this = GetData() as DataCacheMember<T>;
}

These would only work in a struct and doesn't really do anything useful and is bad design.

Upvotes: 0

D Stanley
D Stanley

Reputation: 152511

To answer the question you cannot assign this in a constructor or any other method. You could add the items returned from GetData():

private void RefreshData()
{
    this.Clear();
    this.AddRange(GetData());
}

protected DataCacheMember(string apiEndPoint)
{
    this.Clear();
    this.AddRange(GetData());
}

But inheriting form List<T> probably isn't the right design here.

Upvotes: 3

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