C# 7 local function not working as expected and no errors being displayed

Question

I have an Asp.Net MVC App running with framework version .NET 4.5 and I'm using VS2017 pro version. Users can upload attachments including but not limited to:

• Excel
• Word
• PowerPoint
• pdf
• jpeg
• png

So I have a private function which is as follows:

private string ImageExtension(string path) {   string noImagePath = HttpContext.Current.Server.MapPath("~/Content/images/Template/No-Image-Available-100x100.jpg");   string fileExtension = System.IO.Path.GetExtension(path);   switch (fileExtension)   {     case ".jpg":     case ".jpeg":     case ".gif":     case ".png":        return path;     default:        return noImagePath;    } } 

As you can see it's very simple and does not do anything fancy. As I'm only using this in one place I thought about making the use of new C# 7 feature of local function. So I've gone ahead and created my main function as follows and added ImageExtension(string path) inside it.

public void BugInfo(HttpPostedFileBase file) {   if(file != null && file.ContentLength > 0)   {     string fileName = file.FileName;     string directoryPath = "directory path";       //save path of       string savePath = System.IO.Path.Combine(directoryPath, fileName);      string testString = ImageExtension(savePath);       string ImageExtension(string path)      {         string noImagePath = HttpContext.Current.Server.MapPath("~/Content/images/Template/No-Image-Available-100x100.jpg");         string fileExtension = System.IO.Path.GetExtension(path);         switch (fileExtension)         {           case ".jpg":           case ".jpeg":           case ".gif":           case ".png":              return path;           default:              return noImagePath;          }       }     }   //save values to db here  } 

With the above code my project builds without any errors. As soon as I hit F5 or Ctrl + F5 I get the following error screen

If I check in the ErrorList to see if there are any errors I get none at all as you can see below.

Can someone tell me where I'm going wrong please. Do I have to change any settings or need to include any additional DLLs to make use of C# 7 features.

Looking at this answer it seems like all of the C# 7 features should work on .NET 4.5

Answer 1

You need to update nuget package named "Microsoft.Net.Compilers" to the latest version. Most likely you have version 1.3.2 installed in your project but you need 2.0.1 to use C# 7 features. Alternatively - you can remove this package at all (together with packages that depend on it) - then it will also work, because it will use your installed compiler, but I don't recommend doing that.

As this package description says:

.Net Compilers package. Referencing this package will cause the project to be built using the specific version of the C# and Visual Basic compilers contained in the package, as opposed to any system installed version.

So that is why it uses C# 6 compiler for you now.