Gem in C# - Span

System.Span<T> is a value type representing a continuous memory region for a collection of value type objects. It works like an array of value type <T>. Span<T> is a ref struct. Ref struct can only exist in stack and can't be the fields. Span can be used to access heap, stack, and unmanaged memory.

• Supports type check by compiler
• Boundary check at runtime
• Indexer and Enumerator

Why should developers care?

• Avoid overhead of allocation
• Avoid overhead of memory copy
• Avoid overhead of garbage collection 

What real-world problems can Span<T> solve?

• Iterating a collection of value types (simple types, struct, enum), such as byte[], involves memory copy because value type by default is pass-by-value.
• String.Substring could be used to carve out just the piece that’s interesting to them, but that’s a relatively expensive operation, involving a string allocation,  memory copy, and garbage collection. 
• Coding with raw pointer is dangerous because it has no strong-type and boundary-check in runtime.  
• Same logic has to be implemented for different memories, such as heap, stack, and native code (unmanaged memory, interop).

Final words

Microsoft Search Engine Bing is written in .NET Core. The performance of Bing has improved by 34% after using Span<T>.

Span<T> is one of the gems in C#.

Enjoy!