Update CS0518 documentation with actionable guidance for modern .NET projects (#47685)

* Initial plan

* Update CS0518 documentation with modern .NET project guidance

Co-authored-by: BillWagner <[email protected]>

* Update docs/csharp/language-reference/compiler-messages/cs0518.md

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: BillWagner <[email protected]>
Co-authored-by: Genevieve Warren <[email protected]>

Author: 3 people

docs/csharp/language-reference/compiler-messages/cs0518.md

@@ -11,8 +11,11 @@ ms.assetid: b0b61cbb-c9a7-48c9-9e60-7cdd5ecb3e6c
 # Compiler Error CS0518
 
 Predefined type 'type' is not defined or imported  
-  
-The main cause for this problem is that the project is not importing mscorlib.dll, which defines the entire System namespace. This can be caused by one of the following:  
+
+> [!NOTE]
+> The resolution for this error depends on whether you're using a modern SDK-style project (`.csproj` files that start with `<Project Sdk="Microsoft.NET.Sdk">`) or legacy project formats. SDK-style projects manage runtime references automatically through the `<TargetFramework>` property.
+
+The main cause for this problem is that the project cannot access the predefined types from the .NET runtime library. In modern SDK-style projects, this is typically due to an incorrect or missing `<TargetFramework>` specification. In legacy projects, this issue is caused by not importing mscorlib.dll, which defines the entire <xref:System> namespace. This can be caused by one of the following:  
 
 [!INCLUDE[csharp-build-only-diagnostic-note](~/includes/csharp-build-only-diagnostic-note.md)]
 
@@ -25,13 +28,27 @@ The main cause for this problem is that the project is not importing mscorlib.dl
 - Residual components from an earlier installation that are incompatible with the latest installation remain.  
 
  To resolve this problem, take one of the following actions:  
-  
+
 - Do not specify the /nostdlib option from the command line compiler.  
-  
-- Make sure that the project refers to the correct mscorlib.dll.  
-  
-- Reinstall the .NET Framework common language runtime (if the previous solutions do not solve the problem).
 
-Optionally
+- For modern SDK-style projects, ensure the project targets the correct .NET runtime. In your `.csproj` file, verify the `<TargetFramework>` property specifies the intended runtime:
+
+  ```xml
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+  </PropertyGroup>
+  ```
+
+  For multi-targeting projects, use `<TargetFrameworks>` (plural):
+
+  ```xml
+  <PropertyGroup>
+    <TargetFrameworks>net8.0;net48</TargetFrameworks>
+  </PropertyGroup>
+  ```
+
+- For legacy project formats, make sure that the project refers to the correct mscorlib.dll.  
+
+- Reinstall the .NET Framework common language runtime (if the previous solutions do not solve the problem).
 
-- Reload the project in the Visual Studio.
+- Reload the project in Visual Studio.