Dotnet API changes

Upgrade to .NET Standard 2.1

In our solution there are a few .NET Core 3.1 applications and a bunch of libraries shared between them targeting .NET Standard in different versions. There is no point in keeping them in such state so I decided to upgrade all the libraries to highest possible version which is .NET Standard 2.1. After doing that compiler showed me some errors like this:

  error [CS0121] The call is ambiguous between the following methods or properties:
  'System.Collections.Generic.CollectionExtensions.GetValueOrDefault<TKey, TValue>(
      System.Collections.Generic.IReadOnlyDictionary<TKey, TValue>, TKey)'
  and 'HelperClasses.BasicExtensions.GetValueOrDefault<K, V>(
      System.Collections.Generic.IDictionary<K, V>, K)'

It turned out that a new CollectionExtensions appeared in the API since dotnet Standard 2.1. It seems that our custom extension method can be removed

public static class BasicExtensions
{
    public static V GetValueOrDefault<K, V>(this IDictionary<K, V> dictionary, K key)
    {
        return dictionary.TryGetValue(key, out var dictVal) ? dictVal : default(V);
    }
    /// ...
}

But in some cases we call GetValueOrDefault on IDictionary. IDictionary and IReadOnlyDictionary are separate interfaces, none of them derive from the other. However Dictionary implements both of them.

Method dispatch

To eliminate this issue we can cast Dictionary objects to one of interfaces IDictionary or IReadOnlyDictionary, but it is not convenient solution since there are multiple usages of Dictionary where we call GetValueOrDefault. To address this problem in a more systematic way we can use the fact, that in C# methods on more derived class is applicable and add another implementation for Dictionary:

public static V GetValueOrDefault<K, V>(this Dictionary<K, V> dictionary, K key)
{
    return ((IReadOnlyDictionary<K, V>) dictionary).GetValueOrDefault(key);
}

After that, when we call Dictionary.GetValueOrDefault this implementation above is used, because it has higher priority than methods implemented for interfaces implemented by Dictionary. And also everything works correctly for cases when we call GetValueOrDefault on IDictionary and ReadOnlyDictionary, because they already have their own extensions.