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Code Analyzer for C#

There is a newer version: 10.2.0.105762

Why is this an issue?
A static field in a generic type is not shared among instances of different closed constructed types, thus
LengthLimitedSingletonCollection<int>.instances and LengthLimitedSingletonCollection<string>.instances will
point to different objects, even though instances is seemingly shared among all LengthLimitedSingletonCollection<>
generic classes.
If you need to have a static field shared among instances with different generic arguments, define a non-generic base class to store your static
members, then set your generic type to inherit from the base class.
Noncompliant code example
public class LengthLimitedSingletonCollection<T> where T : new()
{
  protected const int MaxAllowedLength = 5;
  protected static Dictionary<Type, object> instances = new Dictionary<Type, object>(); // Noncompliant

  public static T GetInstance()
  {
    object instance;

    if (!instances.TryGetValue(typeof(T), out instance))
    {
      if (instances.Count >= MaxAllowedLength)
      {
        throw new Exception();
      }
      instance = new T();
      instances.Add(typeof(T), instance);
    }
    return (T)instance;
  }
}

Compliant solution
public class SingletonCollectionBase
{
  protected static Dictionary<Type, object> instances = new Dictionary<Type, object>();
}

public class LengthLimitedSingletonCollection<T> : SingletonCollectionBase where T : new()
{
  protected const int MaxAllowedLength = 5;

  public static T GetInstance()
  {
    object instance;

    if (!instances.TryGetValue(typeof(T), out instance))
    {
      if (instances.Count >= MaxAllowedLength)
      {
        throw new Exception();
      }
      instance = new T();
      instances.Add(typeof(T), instance);
    }
    return (T)instance;
  }
}

Exceptions
If the static field or property uses a type parameter, then the developer is assumed to understand that the static member is not shared among the
closed constructed types.
public class Cache<T>
{
   private static Dictionary<string, T> CacheDictionary { get; set; } // Compliant
}