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Describe how the SpanT and MemoryT types improve performance.

ICSM Computer 185 16-Jun-2025

Describe how the Span<T> and Memory<T> types improve performance.

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Updated on 19-Jun-2025
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Anubhav Kumar

Anubhav Kumar

19-Jun-2025

The Span<T> and Memory<T> types in .NET are high-performance, memory-safe abstractions introduced to work with slices of data without allocations or copying. They improve performance especially in low-level, high-throughput, and memory-sensitive scenarios, such as parsing, serialization, or buffer manipulation.

What Are Span<T> and Memory<T>?

Type Description Stack/Heap
Span<T> A stack-only, non-allocating view over contiguous memory (arrays, strings, etc.) Stack
Memory<T> A heap-safe, sliceable memory abstraction that can be stored and used asynchronously Heap

Performance Benefits

1. Avoids Memory Allocations

  • Span<T> lets you work with slices of arrays or buffers without copying the data.
  • Example: Instead of creating a new array or string slice, you just point to part of the original memory.
  • Reduces GC pressure and heap allocations.

2. Improved Cache Locality

Because Span<T> works on contiguous memory, operations on it tend to have better CPU cache performance than working with scattered objects.

3. Safe, Zero-Cost Slicing

You can slice Span<T> like this:

Span<int> slice = originalSpan.Slice(2, 5);

This is zero-cost: no new array is created — just a new view.

4. Safe Alternative to Unsafe Code

  • Span<T> gives C-like pointer performance, but with type safety and bounds checking.
  • You avoid using unsafe pointers or fixed blocks for performance-sensitive code.

5. Async Support with Memory<T>

  • Span<T> can’t be stored or returned from async methods (stack-only).
  • Memory<T> solves this: it can be stored, passed around, and awaited.
    • It still supports slicing and efficient access via .Span.

Real-World Scenarios

Scenario Benefit
String parsing Avoid string allocations (e.g., use Span<char>)
File/network I/O Use Memory<byte> buffers without copying
High-speed JSON/XML parsing Work on byte arrays directly
Buffer pools Manage memory efficiently in reusable chunks

Limitations

  • Span<T> can’t be:
    • Stored in fields of reference types
    • Captured in async/iterator methods
  • Memory<T> is more flexible but slightly less performant

Example

Span<byte> buffer = stackalloc byte[256];
var slice = buffer.Slice(0, 128);  // no allocation
ReadOnlyMemory<char> memory = "Hello World".AsMemory();
DoAsyncStuff(memory);  // safe to pass into async methods

Summary

Feature Span<T> Memory<T>
Alloc-free slicing Yes Yes
Stack-only (high perf) Yes NO
Safe in async NO Yes
GC pressure Low Low
Replacement for arrays/strings Yes No

Span<T> and Memory<T> give you the power of unsafe pointers with the safety and performance of modern .NET.

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