What conditions do custom types need to meet in order to use [1.. ^ 1]

[zms9110750]

I found that both string and Memory can use this syntax. But I couldn't find it in the API documentation

string this[Range] { get; }

[KennethHoff]

It's the syntax for the System.Range type.

[333fred]

See https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-8.0/ranges#implicit-range-support.

[ParsaMehdipour]

@zms9110750
The code you mentioned is an indexer that uses the Range type. This feature was introduced in C# 8.0 as part of the Index and Range feature set. It allows for more convenient slicing of strings and collections.
You're correct that this syntax isn't explicitly documented in the API for string or Memory. This is because it's a language feature rather than a specific method on these types. The C# compiler translates this syntax into appropriate method calls behind the scenes.
For strings, this indexer is translated into calls to the Substring method. For Memory, it's translated into calls to the Slice method.

[zms9110750]

Does this mean that if I customize a type, I can only define it through an indexer?

[ParsaMehdipour]

You can check the link to the documentation itself:
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-8.0/ranges#implicit-range-support