How to deal with a sealed class when I wanted to inherit and add properties

Question

In a recent question on Stack Overflow, I asked how I might parse through a file name to extra meta info about a file.

After I worked through that problem, I decided that I might want to create new type of object to hold the meta data and the original file. I thought I might do something like this:

class BackupFileInfo : FileInfo, IEquatable<BackupFileInfo>
{
    //Properties and Methods here
}

The idea would be that I would retain the original FileInfo object while adding meta information in the properties of the object that implements FileInfo, such as IsMainBackup.

However, FileInfo is sealed, which means other classes cannot inherit from it.

Instead, I ended up with the following:

class BackupFileInfo : IEquatable<BackupFileInfo>
{
    public bool IsMainBackup { get; set; }
    public int ImageNumber { get; set; }
    public int IncrementNumber { get; set; }
    public FileInfo FileInfo { get; set; }

    //public BackupFileInfo() //constructor here

    public bool Equals(BackupFileInfo other)
    {
        return (this.FileInfo.Name == other.FileInfo.Name
             && this.FileInfo.Length == other.FileInfo.Length);
    }

}

I'm not terribly excited about this solution because instead of being able to use BackupFileInfo.Length, I'm going to have to use BackupFileInfo.FileInfo.Length. Perhaps this is the best practice already, but something doesn't feel right.

Is there a better way to deal with this problem?

Answer 1

You could just expose the properties on FileInfo you care about. Something like this:

public long Length { get { return FileInfo.Length; } }

This obviously becomes less practical if you want to delegate a lot of properties to FileInfo.

Answer 2

This is one of the classic composition instead of inheritance examples and you went in the right direction.

To solve your property problem just create a property called Length that delegates to the encapsulated FileInfo object.

Answer 3

Pass-thru?

class BackupFileInfo : IEquatable<BackupFileInfo>
{
    public long Length {get {return FileInfo.Length;}}
    //.... [snip]
}

Also, a prop called FileInfo is asking for trouble... it may need disambiguation against the FileInfo class in a few places.

Answer 4

You can easily wrap the file info properties in your own properties if you like.

public long Length
{
    get
    {
       return this.FileInfo.Length;
    }
}

Answer 5

This doesn't really solve your larger problem, but of course you can just make the properties you want to use act as proxies to the real properties underneath. E.g.

public long Length
{
    get {return FileInfo.Length;}
}

(With approriate null-checking of course.)

Answer 6

You could add an implicit operator to your class.

Eg:

class BackupFileInfo .... {
  /* your exiting code */

  public static implicit FileInfo( BackupFileInfo self ){
     return self.FileInfo;
  }
}

You could then treat your BackupFileInfo object like a FileInfo object like so

BackupFileInfo bf = new BackupFileInfo();
...
int mylen = ((FileInfo)bf).Length;

Answer 7

It would be nice if extension methods allowed you to also add properties to sealed classes.