C# 强制包含可选参数？

Question

我有以下来自微软的方法：

public static string GetPathByAction(this LinkGenerator generator, 
   HttpContext httpContext, 
   string action = null, 
   string controller = null, 
   object values = null, 
   PathString? pathBase = null, 
   FragmentString fragment = default(FragmentString), 
   LinkOptions options = null);

当我在没有最后一个参数的情况下调用此方法时，我得到了错误：

  An expression tree may not contain a call or invocation that uses optional arguments

如果参数是选项，为什么会出现这样的错误？

LinkGenerator 是这个类：
https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.routing.linkgenerator?view=aspnetcore-2.2

更新

我在一个类中注入 LinkGenerator，如下所示（这段代码没有出错，因为我传递了所有参数）：

public class RequestHandler : IRequestHandler<Request, Response>> {

  private LinkGenerator _linkGenerator;

  public RequestHandler(LinkGenerator linkGenerator) {

    _linkGenerator = linkGenerator;

  } 

  public async Task<Response> Handle(Request request, CancellationToken cancellationToken) {

    List<File> files = getFiles();    

    // WORKS
    var url = _linkGenerator.GetUriByAction(action: "GetByUserId", controller: "FileController", null, "", new HostString());

    // DOES NOT WORK
    var = await files
      .Select(x => new Response {
        Id = x.File.Id,
        Url = _linkGenerator.GetUriByAction(action: "GetByUserId", controller: "FileController", null, "", new HostString())
      }).ToListAsync();

    // Remaining code 

  }

}

public class File { 
  public Int32 Id { get; set; }
  public String Url { get; set; }
}

Answer 1

参考： https://github.com/Dresel/RouteLocalization/issues/6 和 An expression tree may not contain a call or invocation that uses optional arguments

您不能将其与默认参数一起使用

Answer 2

另请参阅：An expression tree may not contain a call or invocation that uses optional arguments

简单的答案不是调用没有导致错误的可选参数的方法，而是调用的方式。如果您在 lambda 中调用该方法，则它仅在您指定可选参数时才有效。