What's wrong with C#?

• null everywhere.
• const nowhere.
• APIs are inconsistent, e.g. mutating an array returns void but appending to a StringBufferreturns the same mutable StringBuffer.
• Collection interfaces are incompatible with immutable data structures, e.g. Add in System.Collections.Generic.IList<_> cannot return a result.
• No structural typing so you write System.Windows.Media.Effects.SamplingMode.Bilinear instead of just Bilinear.
• Mutable IEnumerator interface implemented by classes when it should be an immutable struct.
• Equality and comparison are a mess: you've got System.IComparable and Equals but then you've also got:
  System.IComparable<_> System.IEquatableSystem.Collections.IComparer System.Collections.IStructuralComparable System.Collections.IStructuralEquatable System.Collections.Generic.IComparer System.Collections.Generic.IEqualityComparer
• Tuples should be structs but structs unnecessarily inhibit tail call elimination so one of the most common and fundamental data types will allocate unnecessarily and destroy scalable parallelism.