How to get type of lambda in C# using Roslyn?

Question

I'm writing my codefixer with roslyn. I want it to work in such way:
Before:

IEnumerable<int> list = new List<int> {1, 2, 3, 4};
list = list.Where(x => x%2 == 0);

After:

...
IEnumerable<int> list = new List<int> {1, 2, 3, 4};
list = list.Where(lambdaMethod);
...
private static bool lamdaMethod(int x)
{
    return x % 2 == 0;
}

The problem is that I can't find the proper way to get return type for the method and type of argument.
My method from analyzer template:

public sealed override async Task RegisterCodeFixesAsync(CodeFixContext context)
        {
            var root = await context.Document.GetSyntaxRootAsync(context.CancellationToken).ConfigureAwait(false);

            // TODO: Replace the following code with your own analysis, generating a CodeAction for each fix to suggest
            var diagnostic = context.Diagnostics.FirstOrDefault();
            var diagnosticSpan = diagnostic.Location.SourceSpan;

            // Find the type declaration identified by the diagnostic.
            var declaration = root.FindToken(diagnosticSpan.Start).Parent.AncestorsAndSelf()
                .OfType<LambdaExpressionSyntax>().FirstOrDefault();
            if (declaration == null)
                return;
            // Register a code action that will invoke the fix.

            context.RegisterCodeFix(
                CodeAction.Create(
                    title: $"{"Just Title"}",
                    createChangedDocument: c => MakeLambdaOptimization(context.Document, declaration, c),
                    equivalenceKey: nameof(CodeFixResources.CodeFixTitle)),
                diagnostic);
        }

As you see, I'm working with LambdaExpressionSyntax. There are also SimpleLambdaExpressionSyntax and ParenthesizedLambdaExpressionSyntax which help me to get ParameterSyntax. In the ParameterSyntax I can find Type, but it's null and I get NullReferenceException.
I tried to get the whole type of lambda (e.g. Func<int, bool>), I guess it would help me. How can I do it? I've heard about SemanticModel, but I don't know how to use it

Answer 1

You can use SemanticModel.GetSymbolInfo(ExpressionSyntax) to get the lambda symbol. From the lambda symbol you can find the information you need (return type and parameter types). Here is a code snippet (written in the context of a Console Application instead of a codefix for simplicity - you'll be able to get it working for codefixes):

using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;

// You don't need to create or get a compilation yourself. I'm creating it manually since I'm in a console app.
CSharpCompilation comp = CSharpCompilation.Create(null, new[] { SyntaxFactory.ParseSyntaxTree("System.Func<int, bool> f = x => x % 2 == 0;") }, new[] { MetadataReference.CreateFromFile(typeof(Func<>).Assembly.Location) });

// You already have the LambdaExpressionSyntax, ignore the following two lines, which again because I'm in a console app.
var root = comp.SyntaxTrees[0].GetRoot();
var lambda = root.DescendantNodes().OfType<LambdaExpressionSyntax>().Single();

// Here you'll need to get a semantic model via 'await context.Document.GetSemanticModelAsync(context.CancellationToken).ConfigureAwait(false);'
var model = comp.GetSemanticModel(lambda.SyntaxTree);

// That's the core of what you want to do.
if (model.GetSymbolInfo(lambda).Symbol is IMethodSymbol lambdaSymbol)
{
    // prints 'bool'
    Console.WriteLine(lambdaSymbol.ReturnType.ToDisplayString());
    foreach (var parameter in lambdaSymbol.Parameters)
    {
        // prints 'int' (loop is entered once since my lambda has a single parameter).
        Console.WriteLine(parameter.Type.ToDisplayString());
    }
}