Update 16.7 error message when trying to use net5.0

src/Tasks/GetReferenceAssemblyPaths.cs

@@ -246,7 +246,16 @@ private IList<String> GetPaths(string rootPath, string targetFrameworkFallbackSe
                 // 1/26/16: Note this was changed from a warning to an error (see GitHub #173).
                 if (pathsToReturn.Count == 0)
                 {
-                    Log.LogErrorWithCodeFromResources("GetReferenceAssemblyPaths.NoReferenceAssemblyDirectoryFound", frameworkmoniker.ToString());
+                    // Fixes bad error message when an old SDK assumes "net50" means ".NETFramework 5.0" instead of "netcoreapp 5.0"
+                    // https://github.com/dotnet/msbuild/issues/5820
+                    if (frameworkmoniker.Identifier == ".NETFramework" && frameworkmoniker.Version.Major >= 5)
+                    {
+                        Log.LogErrorWithCodeFromResources("GetReferenceAssemblyPaths.OutOfDateSDK", frameworkmoniker.ToString());
+                    }
+                    else
+                    {
+                        Log.LogErrorWithCodeFromResources("GetReferenceAssemblyPaths.NoReferenceAssemblyDirectoryFound", frameworkmoniker.ToString());
+                    }
                 }
             }
 

src/Tasks/Resources/Strings.resx

@@ -2160,7 +2160,10 @@
     <value>MSB3645: .NET Framework v3.5 Service Pack 1 was not found. In order to target "{0}", .NET Framework v3.5 Service Pack 1 or later must be installed.</value>
     <comment>{StrBegin="MSB3645: "}</comment>
   </data>
-
+  <data name="GetReferenceAssemblyPaths.OutOfDateSDK">
+    <value>MSB3971: The reference assemblies for "{0}" were not found. You might be using an older SDK to target .NET 5.0 or higher. Update Visual Studio and/or your .NET SDK.</value>
+    <comment>{StrBegin="MSB3971: "}</comment>
+  </data>
 
   <!--
         The WinMDExp task has the error buckets of MSB3762 - MSB3772
@@ -2832,6 +2835,7 @@
     <value>MSB3954: Failed to compute hash for file '{0}' because it does not exist or is inaccessible.</value>
     <comment>{StrBegin="MSB3954: "}</comment>
   </data>
+
   <!--
         The tasks message bucket is: MSB3001 - MSB3999
 
@@ -2915,6 +2919,7 @@
             MSB3931 - MSB3940   Task: Unzip
             MSB3941 - MSB3950   Task: ZipDirectory
             MSB3951 - MSB3960   Task: VerifyFileHash
+            MSB3971 - MSB3980   Task: GetReferenceAssemblyPaths overflow
 
             MSB4000 - MSB4200   Portable targets & tasks (vsproject\flavors\portable\msbuild)
             MSB9000 - MSB9900   MSBuild targets files (C++)