System
Nullable<T> : (struct) enables value types to be null → int?, bool?
System.Threading.Tasks
Task<T> : (class) async operation returning T
ValueTask<T> : (struct) lightweight async return → reduces allocation
TaskCompletionSource<T> : (class) manually completes Task<T>
System.Lazy
Lazy<T> : (class) lazy-initialized value container
System.Tuple
Tuple<T1, T2...> : (class) legacy tuple type
ValueTuple<T1, T2...> : (struct) modern tuple type behind (a, b, c)
System.Memory
Memory<T> : (struct) sliceable, safe memory buffer
ReadOnlyMemory<T> : (struct) read-only memory slice
ReadOnlySpan<T> : (ref struct) read-only span
System.Buffers
MemoryPool<T> : (class) pooled Memory<T> blocks
IMemoryOwner<T> : (interface) owner of pooled Memory<T>
System.IO / Array utilities
ArraySegment<T> : (struct) slice of an array without copying