CODEWITHSUNDEEP
c#genericsc#-4.0
Okinawa Terminal
Okinawa Terminal

Reputation: 155

Cast concrete type to nested generic base type

Let's say I have nested generic data classes similar to the following:

public class BaseRecordList<TRecord, TUserInfo>
    where TRecord : BaseRecord<TUserInfo>
    where TUserInfo : BaseUserInfo
{
    public virtual IList<TRecord> Records { get; set; }
    public virtual int Limit { get; set; }
}

public class BaseRecord<TUserInfo>
    where TUserInfo : BaseUserInfo
{
    public virtual DateTime CreationTime { get; set; }
    public virtual TUserInfo UserInfo { get; set; }
}

public class BaseUserInfo
{
    public virtual string Name { get; set; }
    public virtual int Age { get; set; }
}

With 2 concrete versions like so:

// Project 1: Requires some extra properties
public class Project1RecordList : BaseRecordList<Project1Record, Project1UserInfo> {}

public class Project1Record : BaseRecord<Project1UserInfo>
{
    public Guid Version { get; set; }
}

public class Project1UserInfo : BaseUserInfo
{
    public string FavouriteFood { get; set; }
}

and 

// Project 2: Some properties need alternate names for JSON serialization
public class Project2RecordList : BaseRecordList<Project2Record, Project2UserInfo>
{
    [JsonProperty("allRecords")]
    public override IList<Project2Record> Records { get; set; }
}

public class Project2Record : BaseRecord<Project2UserInfo> {}

public class Project2UserInfo : BaseUserInfo
{
    [JsonProperty("username")]
    public override string Name { get; set; }
}

I'm then happy to have 2 repositories that return Project1RecordList and Project2RecordList respectively, but at some point in my code I find myself needing to be able to handle both of these in one place. I figure that at this point I need to be able to treat both of these types as

BaseRecordList<BaseRecord<BaseUserInfo>, BaseUserInfo>

as this is the minimum required to meet the generic constraints, but trying to cast or use "as" throws up errors about not being able to convert.

Is there any way to do this, or even a more sane way to handle this situation without massive amounts of code duplication? If it makes any difference this is for a web app and there are already a large number of data classes, many of which use these nested generics.

Upvotes: 2

Views: 822

Answers (1)

Nabren
Nabren

Reputation: 582

What you are talking about is called covariance and MSDN has a great article on this here: https://msdn.microsoft.com/en-us/library/dd799517(v=vs.110).aspx

First, create a new interface:

interface IBaseRecord<out TUserInfo>
  where TUserInfo : BaseUserInfo
{
}

Have BaseRecord inherit from the new interface:

public class BaseRecord<TUserInfo> : IBaseRecord<TUserInfo>
    where TUserInfo : BaseUserInfo
{
    public virtual DateTime CreationTime { get; set; }
    public virtual TUserInfo UserInfo { get; set; }
}

If done right, this should compile:

IBaseRecord<BaseUserInfo> project1 = new Project1Record();
IBaseRecord<BaseUserInfo> project2 = new Project2Record();

To expand this to the BaseRecordList, create IBaseRecordList:

interface IBaseRecordList<out TRecord, out TUserInfo>
  where TRecord : IBaseRecord<TUserInfo>
  where TUserInfo : BaseUserInfo
{
}

Have BaseRecordList inherit from that:

public class BaseRecordList<TRecord, TUserInfo> : IBaseRecordList<TRecord, TUserInfo>

And then use as such:

IBaseRecordList<IBaseRecord<BaseUserInfo>, BaseUserInfo> project1 = new Project1RecordList();
IBaseRecordList<IBaseRecord<BaseUserInfo>, BaseUserInfo> project2 = new Project2RecordList();

Once you have that setup, just add whatever properties or functions you need to use generically to the interfaces.

Upvotes: 3

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