Merge pull request #3463 from dotnet/known-issue-user-secrets

Add known issue for UserSecrets build failure

release-notes/3.0/3.0-known-issues.md

@@ -14,6 +14,19 @@ This document lists known issues for **.NET Core 3.0 GA and beyond releases** wh
 
     To update the path for every new terminal session, the `export` entry will need to be added to your terminal resource file. This file will be different depending upon your default terminal (eg bash, zsh, ...) and configuration (.bash_profile, .bashrc, .zshrc, ...).
 
+- **Projects fail to build if the `UserSecretsId` property is set but the project doesn't reference Microsoft.Extensions.Configuration.UserSecrets ([core-sdk/3290](https://github.com/dotnet/core/issues/3290))**
+
+    The .NET SDK now generates a `Microsoft.Extensions.Configuration.UserSecrets.UserSecretsIdAttribute` if the `UserSecretsId` property is set.  If the assembly which defines this attribute (Microsoft.Extensions.Configuration.UserSecrets.dll) isn't referenced, then the project will fail to build, with an error similar to the following:
+    
+    > Error CS0234 The type or namespace name 'Extensions' does not exist in the namespace 'Microsoft' (are you missing an assembly reference?)
+    
+    To work around the issue, you can do any of the following:
+    
+    - Set the `GenerateUserSecretsAttribute` MSBuild property to false.
+    - Unset the `UserSecretsId` property
+    - If targeting .NET Core 3.0, add a `FrameworkReference` item to `Microsoft.AspNetCore.App`
+    - Add a `PackageReference` to the `Microsoft.Extensions.Configuration.UserSecrets`
+
 ### Preview 9
 
 - **Misnamed or colliding non-resx resources ([microsoft/msbuild#4740](https://github.com/microsoft/msbuild/issues/4740))**