title: CSharpCodeProvider use C# language version 7.1 or greater
body: <p>I have a small console project where I'm trying to compile some C# files into a .dll.</p>
  
  <p>The code looks like:</p>
  
  <pre><code>    public Result CreateDll(string[] files, string assemblies, string toPath, string dllName)
      {
          if (!Directory.Exists(toPath))
              Directory.CreateDirectory(toPath);
  
          using (var provider = new Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider(new CompilerSettings()))
          {
              var parameters = new CompilerParameters(assemblies)
              {
                  GenerateInMemory = false,
                  GenerateExecutable = false,
                  OutputAssembly = $@"{toPath}\{dllName}.dll",
                  IncludeDebugInformation = false,
                  TreatWarningsAsErrors = true,
                  CompilerOptions = "/unsafe /optimize"
              };
  
              CompilerResults results = provider.CompileAssemblyFromFile(parameters, files);
          }
  
          return Result.Success;
      }
  </code></pre>
  
  <p>One of the files I'm trying to compile looks like:</p>
  
  <pre><code>    public abstract class BaseClass
  {
      private string backendString;
      private string property;
      public string TestString =&gt; backendString;
  
      public string Property
      {
          get =&gt; property;
          set
          {
              if (value == default)
                  return;
              property = value;
          }
      }
  }
  </code></pre>
  
  <p>But I get this error:</p>
  
  <blockquote>
    <p>Feature 'default literal' is not available in C# 7.0. Please use language version 7.1 or greater.</p>
  </blockquote>
  
  <p>If I remove the <code>*default*</code> from <code>BaseClass</code>, then it does not complain.
  How do I change this?</p>