## 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 Framework 4.5. Note, however, that these
changes only affect code targeting .NET Framework 4.5. If a binary targets .NET Framework 4.0, the
old behavior will be observed. Changes to URI parsing in .NET Framework 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 Framework 4.0 URI parsing semantics are necessary (they often aren't),
they can be used by targeting .NET Framework 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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