UriKind.RelativeOrAbsolute workaround

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In .NET new Uri ("/foo", UriKind.RelativeOrAbsolute) returns a relative Uri whereas Mono assumes it is an absolute file path.

Mono diverges from .NET because UNIX file paths starting with ‘/’ are absolute file paths.

Changing the behavior to match the .NET one is not possible without breaking existing Mono applications that rely on UriKind.RelativeOrAbsolute to parse absolute file paths. Thus we propose some workarounds.

Manual check for rooted path

The current workaround that can be used consists on using UriKind.Relative instead of UriKind.RelativeOrAbsolute when the uri starts with ‘/’.

var uri = new Uri (str, str?.StartsWith ("/") == true ? UriKind.Relative : UriKind.RelativeOrAbsolute);

Using Mono specific extension value (available since Mono 4.2)

The workaround consists in using Mono specific 300 value instead of UriKind.RelativeOrAbsolute.

This approach will work on Mono only and will throw System.ArgumentException on .NET due to undefined value. Therefore we recommend to define DotNetRelativeOrAbsolute as following

static readonly UriKind DotNetRelativeOrAbsolute = Type.GetType ("Mono.Runtime") == null ? UriKind.RelativeOrAbsolute : (UriKind) 300;

Problematic usages of UriKind.RelativeOrAbsolute which should always resolve as relative path

var uri = new Uri (str, UriKind.RelativeOrAbsolute);

can then be fixed by using DotNetRelativeOrAbsolute value instead

var uri = new Uri (str, DotNetRelativeOrAbsolute);

Global workarounds

It is possible to change all uri = new Uri (str, UriKind.RelativeOrAbsolute) references in your app and libraries to match .NET behaviour.

This approach should be used with care as it changes all code like new Uri ("/foo", UriKind.RelativeOrAbsolute) to be relative even in 3rd party libraries which could depend on this behaviour.

Using Environment variable (available since Mono 4.2)

MONO_URI_DOTNETRELATIVEORABSOLUTE=true mono app.exe

Using Reflection (available since Mono 4.2)

if (Type.GetType ("Mono.Runtime") != null) {
    var field = typeof (Uri).GetField ("useDotNetRelativeOrAbsolute",
        BindingFlags.Static | BindingFlags.GetField | BindingFlags.NonPublic);
    field?.SetValue (null, true);
}