c# - Inline variable declaration not compiling

Question

I've been getting a message in Visual Studio 2017, specifically, IDE0018 Variable declaration can be inlined.

So I try using an inline variable declaration the way it's mentioned in the visual studio 2017 release notes, but I can't get my project to compile.

It show no error messages, but the output shows "Rebuild All failed..... error CS1525: Invalid expression term 'int'"

The error only shows up in the output, not as an actual error in the error list.

Here is an actual example of the code I'm using that is failing.

if (int.TryParse(ExpYear, out int IExpYear))
  {
    if (IExpYear < DateTime.Now.Year || IExpYear > DateTime.Now.AddYears(10).Year)
    {
      e += "Expiration Year is invalid.
";
    }
  }
  else
  {
    e += "Expiration Year is not a number.
";
  }

If I revert the change, it compiles as expected. Is it possible that I'm not using the c#7 compiler somehow?

Thank you.

Update: I found the language setting in Build > Advanced and set it to C# 7.0. Building the project now gives me this error:

CSC : error CS1617: Invalid option '7' for /langversion; must be ISO-1, ISO-2, Default or an integer in range 1 to 6.

Answer 1

I was able to resolve this by installing the Microsoft.Net.Compilers nuget package for v2.0.0-rc3, the only version installed prior was 1.3.2.

I still don't understand why the intellisense and compiler errors would show up if the installed compiler didn't support this.