C#: What is the difference between Span<T> and ReadOnlySpan<T>?

Span<T> — mutable. ReadOnlySpan<T> — immutable.

Span<T>:

• Mutable.
• Read/write.
• Ref struct.

ReadOnlySpan<T>:

• Immutable.
• Read-only.
• Safer.

1// Span
2Span<int> span = stackalloc int[5];
3span[0] = 1;  // OK
4
5// ReadOnlySpan
6ReadOnlySpan<char> text = "Hello".AsSpan();
7// text[0] = 'h';  // Error

Key: Span for mutable, ReadOnlySpan for immutable.