Deducing instance to handle an object in C#

Question

I am extending my C# J2K library to dynamically register codecs from other assemblies. That part is working, but I am having trouble understanding IsAssignableFrom(TypeInfo) and how I can deduce which class instance to handle a given type (SKBitmap, Image, etc).

So far this is what I have come up with. I have an interface named IImageCreator and all codecs are instantiated into

static readonly List<IImageCreator> _creators

I then have an abstract base class that implements IImageCreator like so:

abstract class ImageCreator<TBase> : IImageCreator

The codecs are then implemented into sealed classes like so:

sealed class SKBitmapImageCreator : ImageCreator<SKBitmap> {...}
sealed class WindowsBitmapImageCreator : ImageCreator<Image> {...}

etc.

I have a static method for encoding an image:

static BlkImgDataSrc ToPortableImageSource(object imageObject)
{
    _creators.Single(c => c.GetType().GetTypeInfo().IsAssignableFrom(imageObject.GetType().GetTypeInfo()));

This is where I am getting hung up as it throws an exception Sequence contains no matching element

Note that imageObject will be of a handled type such as SKBitmap

I was under the impression that this would deduce the proper instance to handle the imageObject based on the documentation:

Returns true if any of the following conditions is true:
(...)
c is a generic type parameter, and the current instance represents one of the constraints of c.

Where have I gone astray? How can I deduce the proper instance to handle the object?

Charlieface · Accepted Answer · 2025-10-13 01:29:50Z

3

The imageObject is not of type IImageCreator, it's of some TBase image type.

So you need to construct a dynamic ImageCreator<TBase> TypeInfo to compare to.

var type = typeof(ImageCreator<>).MakeGenericType(imageObject.GetType());
var creator = _creators.SingleOrDefault(c =>
    c.GetType().IsAssignableFrom(type))
    ?? throw new Exception($"Image type {imageObject.GetType()} not supported");

However, a much better way of doing it would be to have a ImageType property on your base interface:

public interface IImageCreator
{
    Type ImageType { get; }
}

Then the abstract generic class implements it:

abstract class ImageCreator<TBase> : IImageCreator
{
    public Type ImageType => typeof(TBase);
}

And then you can simply do:

var creator = _creators.SingleOrDefault(c =>
    c.ImageType.IsAssignableFrom(imageObject.GetType()))
    ?? throw new Exception($"Image type {imageObject.GetType()} not supported");

edited Oct 13 at 1:29

answered Oct 12 at 16:39

Charlieface

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3 Comments

Jeremy Lakeman Oct 13 at 2:21

Or just build a Dictionary<Type, IImageCreator> once, so you don't have to search the list at runtime...

Charlieface Oct 13 at 3:02

Doesn't work for IsAssignableFrom, only for exact match. You'd need to loop through all base classes and interfaces to find the first match.

Jeremy Lakeman Oct 13 at 3:14

Which is more efficient would depend on how many IImageCreator types you have. And if the image types are sealed. Once you find a match, you can then add it to the dictionary for next time.

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