runtime: Blazor webassembly library System.IO.Path.GetDirectoryName and GetFileName broken in 5.0

Describe the bug

Under .NET Core 3.1.9 in a hosted Blazor WebAssembly app, client-side calls to System.IO.Path.GetDirectoryName and GetFileName behave identically to a server app (.NET Framework or Core). But under .NET 5.0 the parsing does not work correctly when using Windows format paths that use a foreslash (\). It works correctly with backslashes (/).

To Reproduce

I created identical projects from the default template (Index, Counter, Weather) in both 3.1.9 and 5.0. The I modified Counter slightly to include an input for a path string and a button to apply both GetDirectoryName and GetFileName.

Code listing in Counter.razor:

@page "/counter"

<h1>Counter</h1>

<p>Current count: @currentCount</p>

<button class="btn btn-primary" @onclick="IncrementCount">Click me</button>

<br />

<input type="text" @bind="FullPath" />

<br />


<div>@Directory</div>
<div>@Filename</div>

<button class="btn btn-primary" @onclick="ParsePath">Click me</button>

@code {
    private int currentCount = 0;

    private string FullPath { get; set; }
    private string Directory { get; set; }
    private string Filename { get; set; }

    private void ParsePath(MouseEventArgs e)
    {
        this.Directory = System.IO.Path.GetDirectoryName(this.FullPath);
        this.Filename = System.IO.Path.GetFileName(this.FullPath);
    }

    private void IncrementCount()
    {
        currentCount++;
    }
}

If you enter a web style path with backslashes the parsing output it correct in both 3.1.9. With Windows style foreslashes it fails (no error, but parsing is incorrect/does not occur) in 5.0.

3.1.9

5.0

Exceptions (if any)

No exception but code does not work as expected.

Further technical details

3.1.9 / 5.0

.NET SDK (reflecting any global.json): Version: 5.0.100 Commit: 5044b93829

Runtime Environment: OS Name: Windows OS Version: 10.0.18362 OS Platform: Windows RID: win10-x64 Base Path: C:\Program Files\dotnet\sdk\5.0.100\

Host (useful for support): Version: 5.0.0 Commit: cf258a14b7

.NET SDKs installed: 2.1.509 [C:\Program Files\dotnet\sdk] 2.1.511 [C:\Program Files\dotnet\sdk] 2.1.512 [C:\Program Files\dotnet\sdk] 2.1.513 [C:\Program Files\dotnet\sdk] 2.1.514 [C:\Program Files\dotnet\sdk] 2.1.515 [C:\Program Files\dotnet\sdk] 2.1.516 [C:\Program Files\dotnet\sdk] 2.1.517 [C:\Program Files\dotnet\sdk] 2.1.519 [C:\Program Files\dotnet\sdk] 3.0.101 [C:\Program Files\dotnet\sdk] 3.1.201 [C:\Program Files\dotnet\sdk] 5.0.100 [C:\Program Files\dotnet\sdk]

.NET runtimes installed: Microsoft.AspNetCore.All 2.1.13 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All] Microsoft.AspNetCore.All 2.1.15 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All] Microsoft.AspNetCore.All 2.1.16 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All] Microsoft.AspNetCore.All 2.1.17 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All] Microsoft.AspNetCore.All 2.1.18 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All] Microsoft.AspNetCore.All 2.1.19 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All] Microsoft.AspNetCore.All 2.1.20 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All] Microsoft.AspNetCore.All 2.1.21 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All] Microsoft.AspNetCore.All 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All] Microsoft.AspNetCore.App 2.1.13 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 2.1.15 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 2.1.16 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 2.1.17 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 2.1.18 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 2.1.19 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 2.1.20 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 2.1.21 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 3.0.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 3.0.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 3.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.NETCore.App 2.1.13 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 2.1.15 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 2.1.16 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 2.1.17 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 2.1.18 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 2.1.19 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 2.1.20 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 2.1.21 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 3.0.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 3.1.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.WindowsDesktop.App 3.0.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App] Microsoft.WindowsDesktop.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App] Microsoft.WindowsDesktop.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App] Microsoft.WindowsDesktop.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET runtimes or SDKs: https://aka.ms/dotnet-download

Visual Studio 2019 16.8.1