Cannot open solution with code analyzer class

Question

I'm trying to build a simple program to static code analysis with provided package from Microsoft. So I wan to open solution then open projects via code then get all the document and analyze with my methods.

This is my method, I'm calling somewhere else, and I double checked solution path is correct. You can see my namespaces that I used.

using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.Formatting;
using Microsoft.CodeAnalysis.MSBuild;

 public class Fixer
{
 public Fixer(string solutionPath)
        {
           using (var workspace = MSBuildWorkspace.Create())
            {
                _solution = workspace.OpenSolutionAsync(_solutionPath).Result;
            }
        }
}

After that I want to

public IEnumerable<Document> GetDocuments(Solution solution)
        {
            foreach (var projectId in solution.ProjectIds)
            {
                var project = solution.GetProject(projectId);
                foreach (Document document in project.Documents)
                {
                    if (document.SupportsSyntaxTree)
                        yield return document;
                }
            }
        }

public IEnumerable<MethodDeclarationSyntax> GetMethods(IEnumerable<Document> documents)
            {
                return documents.SelectMany(p => p.GetSyntaxRootAsync().Result.DescendantNodes().OfType<MethodDeclarationSyntax>());
            }

Main problem is its not open solution at all. When I checked workspace object. There is diagnostic property and says

{[Failure] Cannot open project '.\RunningDiagnostics.csproj' because the language 'C#' is not supported.}

For all project in that solution.

I tried to update nuget packages that I used. Nothing happened. What is the main problem ? And guidance will be appreciated.

Thank you.