環境

• Mac
• 3.0.100-preview6-012264

エラー

こんな感じのエラー

$ dotnet run
Application startup exception: System.InvalidOperationException: Cannot identify unique bin directory for Blazor app. Found both '/BlazorProject/bin/Debug' and '/BlazorProject/bin/Release'. Ensure that only one is present on disk. This is a temporary limitation (see https://github.com/aspnet/Blazor/issues/261).
   at Microsoft.AspNetCore.Blazor.DevServer.Server.Startup.FindClientBinDir(String clientAppSourceRoot)
   at Microsoft.AspNetCore.Blazor.DevServer.Server.Startup.FindClientAssemblyPath(IWebHostEnvironment environment)
   at Microsoft.AspNetCore.Blazor.DevServer.Server.Startup.Configure(IApplicationBuilder app, IWebHostEnvironment environment, IConfiguration configuration)
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor, Boolean wrapExceptions)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at Microsoft.AspNetCore.Hosting.Internal.ConfigureBuilder.Invoke(Object instance, IApplicationBuilder builder)
   at Microsoft.AspNetCore.Hosting.Internal.ConfigureBuilder.<>c__DisplayClass4_0.<Build>b__0(IApplicationBuilder builder)
   at Microsoft.AspNetCore.Hosting.Internal.ConventionBasedStartup.Configure(IApplicationBuilder app)
   at Microsoft.AspNetCore.HostFilteringStartupFilter.<>c__DisplayClass0_0.<Configure>b__0(IApplicationBuilder app)
   at Microsoft.AspNetCore.Hosting.Internal.WebHost.BuildApplication()
crit: Microsoft.AspNetCore.Hosting.Internal.WebHost[6]
      Application startup exception

対策

binフォルダにあるDebugかReleaseのどちらかを削除する