Is there a way to enable "Implicit Usings" feature when compiling code with .NET Compiler SDK?

Question

The "ImplicitUsings" feature allows code to omit usings statements for standard namespaces. Is there a way to tell a C# compilation to enable the feature and include such namespaces?

using System.Diagnostics;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;

var sourceText = "class C{" +
    "public void M(){" +
    " Console.Write(123);" +
    "}}";
var syntaxTree = CSharpSyntaxTree.ParseText(sourceText);

var coreReferences =
    ((string)AppContext.GetData("TRUSTED_PLATFORM_ASSEMBLIES"))
    .Split(Path.PathSeparator)
    .Select(p => MetadataReference.CreateFromFile(p));
var compilation = CSharpCompilation.Create("MyAnalysis")
    .WithOptions(new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary))
    .AddReferences(coreReferences)
    .AddSyntaxTrees(syntaxTree);

var diagnostics = compilation.GetDiagnostics();
Debug.Assert(diagnostics.Length == 0);
// diagnostics = "error CS0103: The name 'Console' does not exist in the current context"

In the above example, diagnostics contains the error that Console does not exist. If I add "using System" in the sourceText above, the code compiles without the error. Is there a way to enable the Implicit Usings feature so that compilation resolves symbols from standard namespaces?

Answer 1

implicit using is not part of the compiler, but actually the build system. when enabled, a c# file with global using statements is added to the project based the selected sdk.

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/using-directive

Answer 2

Hi @Mark M , welcome to Microsoft Q&A. For implicit references, only the files that are integrated in them are referenced automatically.

You could check out this document: Implicit using directives. For libraries that are not included, you can manually add them in csproj. This way you don't need to add it again when you use it.

<ItemGroup>
  <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="4.5.0" />
  <Using Include="Microsoft.CodeAnalysis" />
  <Using Include="Microsoft.CodeAnalysis.CSharp" />
  <Using Include= "System.Diagnostics" />
</ItemGroup>

Best Regards,

Jiale

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Answer 3

Based on all comments and using the document Jiale referenced, a possible solution is to create a global usings file(s) corresponding to specific known SDK(s). For example, for Microsoft.NET.Sdk I created the following file (Microsoft.NET.Sdk.usings.cs):

global using System;
global using System.Collections.Generic;
global using System.IO;
global using System.Linq;
global using System.Net.Http;
global using System.Threading;
global using System.Threading.Tasks;

Then I can load it and parse as an additional syntax tree getting rid of the compilation error. There are warnings about unnecessary using directives. The complete code is this:

using System.Diagnostics;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;

var sourceText = "class C{" +
    "public void M(){" +
    " Console.Write(123);" +
    "}}";
var syntaxTree = CSharpSyntaxTree.ParseText(sourceText);

var globalUsings = File.ReadAllText("Microsoft.NET.Sdk.usings.cs");
var usingsTree = CSharpSyntaxTree.ParseText(globalUsings);

var coreReferences =
    ((string)AppContext.GetData("TRUSTED_PLATFORM_ASSEMBLIES"))
    .Split(Path.PathSeparator)
    .Select(p => MetadataReference.CreateFromFile(p));
var compilation = CSharpCompilation.Create("MyAnalysis")
    .WithOptions(new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary))
    .AddReferences(coreReferences)
    .AddSyntaxTrees(usingsTree, syntaxTree);

var diagnostics = compilation.GetDiagnostics();
Debug.Assert(!diagnostics.Any(d => d.Severity == DiagnosticSeverity.Error));