找不到类型或命名空间名称"T"

Question

我有以下代码,我正在.NET 4.0项目中编译

namespace ConsoleApplication1  
{  
    class Program  
    {  
        static void Main(string[] args)  
        {  

        }  
    }  

    public static class Utility  
    {  
        public static IEnumerable<T> Filter1(this IEnumerable<T> input, Func<T, bool> predicate)  
        {  
            foreach (var item in input)  
            {  
                if (predicate(item))  
                {  
                    yield return item;  
                }  
            }  
        }  
    }  
}  

但得到以下错误.我已将System.dll作为默认值包含在引用中.我可能做错了什么？

Error   1   The type or namespace name 'T' could not be found (are you missing a using directive or an assembly reference?) 

Error   2   The type or namespace name 'T' could not be found (are you missing a using directive or an assembly reference?) 

Error   3   The type or namespace name 'T' could not be found (are you missing a using directive or an assembly reference?) 

Answer 1

您必须将type参数放在函数本身上.

public static IEnumerable<T> Filter1<T>(...)

Answer 2

public static class Utility 
{  
    public static IEnumerable<T> Filter1<T>( // Type argument on the function
       this IEnumerable<T> input, Func<T, bool> predicate)  
    {  

如果你不关心它是否是一个扩展方法,你可以在类中添加一个通用约束.我的猜测是你想要扩展方法.

public static class Utility<T> // Type argument on class
{  
    public static IEnumerable<T> Filter1( // No longer an extension method
       IEnumerable<T> input, Func<T, bool> predicate)  
    {  

Answer 3

您需要声明T,这发生在方法名称或类名称之后.将您的方法声明更改为:

public static IEnumerable<T> 
    Filter1<T>(this IEnumerable<T> input, Func<T, bool> predicate)