Since C# 1.0, any type can have an indexer property defined that can accept any parameter types and can can return anything - though it wasn't until C# 7.0 that

Since C# 1.0, any type can have an indexer property defined that can accept any parameter types and can can return anything - though it wasn't until C# 7.0 that we could return a first-class reference to a value, including array members (`public ref int this[int idx] => ref this.someArray[idx]`).