The type or namespace name 'T' could not be found

Question

I have following code that I am compiling in a .NET 4.0 project

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;  
                }  
            }  
        }  
    }  
}  

but getting following errors. I have System.dll already included as default in references. What I may be doing wrong?

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

You have to put the type argument on the function itself.

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)  
    {  

If you dont care if its an extension method or not, you can add a generic constrain to the class. My guess is you want the extension method.

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

You need to declare T, which occurs after the method name or class name. Change your method declaration to :

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

Answer 4

I had the same error, but solution required was slightly different. I needed to change this:

public static void AllItemsSatisy(this CollectionAssert collectionAssert, ICollection<T> collection, Predicate<T> predicate) 
{ ... }

To this:

public static void AllItemsSatisy<T>(this CollectionAssert collectionAssert, ICollection<T> collection, Predicate<T> predicate) 
{ ... }