Understanding C# Covariance And Contravariance (3) Variances in .NET

Understanding C# Covariance And Conreavariance:

Not many generic types in .NET have variant type parameter(s). LINQ can be uses to query these generic types from .NET libraries.

The following method queries a specified directory, and retrieve all .NET assemblies:

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public static partial class ReflectionHelper
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{
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    public static IEnumerable<Assembly> GetAssemblies(string directory)
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    {
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        return Directory.EnumerateFiles(directory, "*.dll")
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            .Select(file =>
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                {
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                    try
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                    {
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                        return Assembly.LoadFrom(file);
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                    }
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                    catch (BadImageFormatException)
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                    {
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                        return null;
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                    }
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                })
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            .Where(assembly => assembly != null);
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    }
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}

The following method queries one specified assembly, and filter generic types with any variant type parameter:

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public static partial class ReflectionHelper
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{
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    public static IEnumerable<Type> GetTypesWithVariance(Assembly assembly)
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    {
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        try
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        {
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            return assembly.ExportedTypes.Where(type =>
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                type.IsGenericTypeDefinition && type.GetGenericArguments().Any(argument =>
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                    (argument.GenericParameterAttributes & GenericParameterAttributes.Covariant)
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                    == GenericParameterAttributes.Covariant
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                    ||
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                    (argument.GenericParameterAttributes & GenericParameterAttributes.Contravariant)
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                    == GenericParameterAttributes.Contravariant));
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        }
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        catch (TypeLoadException)
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        {
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            return Enumerable.Empty<Type>();
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        }
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    }
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}

The last method queries the assemblies in the same directory of mscorlib.dll, and retrieves the wanted types, and orders them by name:

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public static partial class ReflectionHelper
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{
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    public static IEnumerable<Type> GetTypesWithVariance()
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    {
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        string mscorlibPath = typeof(object).Assembly.GetName().CodeBase;
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        string directory = Path.GetDirectoryName(new Uri(mscorlibPath).AbsolutePath);
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        return GetAssemblies(directory)
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            .SelectMany(GetTypesWithVariance)
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            .OrderBy(type => type.Name);
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    }
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}

Here is the result of executing the last method:

System namespace:
- Action`1 to Action`16, Func`1 to Func`17
- Comparison
- Converter`2
- IComparable,
- IObservable, IObserver
- IProgress
- Predicate
System.Collections.Generic namespace:
- IComparer, IEqualityComparer
- IEnumerable, IEnumerator
- IReadOnlyCollection, IReadOnlyList
System.Linq namespace:
- IGrouping`2
- IOrderedQueryable, IQueryable

MSDN has a List of Variant Generic Interface and Delegate Types, but it is inaccurate. For example, it says TElement is covariant for IOrderedEnumerable, but actually not:

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namespace System.Linq
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{
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    public interface IOrderedEnumerable<TElement> : IEnumerable<TElement>, IEnumerable
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    {
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        IOrderedEnumerable<TElement> CreateOrderedEnumerable<TKey>(Func<TElement, TKey> keySelector, IComparer<TKey> comparer, bool descending);
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    }
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}