## System.Uri parsing adheres to RFC 3987

### Scope
Major

### Version Introduced
4.5

### Source Analyzer Status
Available

### Change Description

URI parsing has changed in several ways in .NET 4.5. Note, however, that these
changes only affect code targeting .NET 4.5. If a binary targets .NET 4.0, the
old behavior will be observed. Changes to URI parsing in .NET 4.5 include:

- URI parsing will perform normalization and character checking according to the latest IRI rules in RFC 3987.
- Unicode normalization form C will only be performed on the host portion of the URI.
- Invalid mailto: URIs will now cause an exception.
- Trailing dots at the end of a path segment are now preserved.
- `file://` URIs do not escape the `?` character.
- Unicode control characters `U+0080` through `U+009F` are not supported.
- Comma characters `,` or `%2c` are not automatically unescaped.

- [x] Quirked
- [ ] Build-time break

### Recommended Action

If the old .NET 4.0 URI parsing semantics are necessary (they often aren't),
they can be used by targeting .NET 4.0. This can be accomplished by using a
<xref:System.Runtime.Versioning.TargetFrameworkAttribute?displayProperty=name>
on the assembly, or through Visual Studio's project system UI in the 'project
properties' page.

### Affected APIs
* `M:System.Uri.#ctor(System.String)`
* `M:System.Uri.#ctor(System.String,System.Boolean)`
* `M:System.Uri.#ctor(System.String,System.UriKind)`
* `M:System.Uri.#ctor(System.Uri,System.String)`
* `M:System.Uri.TryCreate(System.String,System.UriKind,System.Uri@)`
* `M:System.Uri.TryCreate(System.Uri,System.String,System.Uri@)`
* `M:System.Uri.TryCreate(System.Uri,System.Uri,System.Uri@)`

### Category
Core

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