Skip to content

Add example of compile-time error #1013

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 2 commits into from
Nov 29, 2023
Merged

Add example of compile-time error #1013

Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
a92c06d
Add example of compile-time error
BillWagner Nov 24, 2023
4647619
respond to feedback
BillWagner Nov 27, 2023
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
22 changes: 22 additions & 0 deletions standard/patterns.md
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -22,6 +22,28 @@ A *declaration_pattern* and a *var_pattern* can result in the declaration of a l

Each pattern form defines the set of types for input values that the pattern may be applied to. A pattern `P` is *applicable to* a type `T` if `T` is among the types whose values the pattern may match. It is a compile-time error if a pattern `P` appears in a program to match a pattern input value ([§11.1](patterns.md#111-general)) of type `T` if `P` is not applicable to `T`.

> *Example*: The following example generates a compile-time error because the compile-time type of `v` is `Stream`. A variable of type `Stream` can never have a value that is reference compatible with `string`:
>
> ```csharp
> Stream v = OpenDataFile(); // compile-time type of 'v' is 'Stream'
> if (v is string) // compile-time error
> {
> // code assuming v is a string
> }
> ```
>
> However, the following doesn't generate a compile-time error because the compile-time type of `v` is `object`. A variable of type `object` could have a value that is reference compatible with `string`:
>
> ```csharp
> object v = OpenDataFile();
> if (v is string s)
> {
> // code assuming v is a string
> }
> ```
>
> *end example*

Each pattern form defines the set of values for which the pattern *matches* the value at runtime.

### 11.2.2 Declaration pattern
Expand Down