c# - LINQ：为什么这个查询在查询语法中有效，但在方法语法中无效？

Question

我有这个查询

string[] names = { "Tom", "Dick", "Harry", "Mary", "Jay" };

var query =
    from     n in names
    where    n.Contains ("a")   // Filter elements
    orderby  n.Length           // Sort elements
    select   n.ToUpper();       // Translate each element (project)

foreach(var item in query)
    Console.WriteLine(item);

完美运行，JAY MARY HARRY按预期给出结果。

但是当我运行这个

string[] names = { "Tom", "Dick", "Harry", "Mary", "Jay" };

var query2 = names.Where(n=> n.Contains("a")).OrderBy(n=>n.Length).Select(n=>n.ToUpper);

foreach(var item in query2)
    Console.WriteLine(item);

我收到这条消息The type arguments for method 'System.Linq.Enumerable.Select<TSource,TResult>(System.Collections.Generic.IEnumerable<TSource>, System.Func<TSource,TResult>)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

我也试过运行这个

var query2 = names.Where(n=> n.Contains("a")).OrderBy(n=>n.Length).Select<string, string>(n=>n.ToUpper);

它说Cannot convert method group 'ToUpper' to non-delegate type 'string'. Did you intend to invoke the method? Cannot convert lambda expression to delegate type 'System.Func<string,string>' because some of the return types in the block are not implicitly convertible to the delegate return type.

我不知道发生了什么？谁能告诉我为什么查询语法工作正常方法语法不起作用？

Answer 1

您在 之后缺少一对括号ToUpper：

var query2 = names
    .Where(n=> n.Contains("a"))
    .OrderBy(n=>n.Length)
    .Select(n=>n.ToUpper() /* <<== HERE */);

Answer 2

您需要将 ToUpper 作为方法调用（带括号）：

var query2 = names.Where(n=> n.Contains("a")).OrderBy(n=>n.Length).Select(n=>n.ToUpper());