Why members cannot be accessed by generic type?

Question

I don't understand what am I doing wrong when trying to access members of class from generic type T. I also specifically used WHERE constraint. I get the following message: 'T' is a type parameter, which is not valid in the current context.

class Test
{
    public static int id = 1;
    public int GetId()
    {
        return id;
    }
}

public class TestUsage
{
    public int IncrementId<T>() where T : Test
    {
        return 1 + T.GetId();
    }
}

Answer 1

Declaring the method as generic means that you can pass an argument of the generic type. So you could do this:

public class TestUsage
{
    public int IncrementId<T>(T test) where T : Test
    {
        return 1 + test.GetId();
    }
}

The generic type argument T doesn't mean anything unless something with that type is used either as an input argument, a return value, a variable of type T within the method, or some combination.

Answer 2

Since in public int IncrementId<T>(T test) the type parameter T could just be any type, C# has no clue which members this type has. You can add a generic type constraint. A way to do this, is to define an interface that types must implement to be used as type argument.

public interface IHasId
{
    int GetId();
}

class Test : IHasId
{
    public static int id = 1;
    public int GetId()
    {
        return id;
    }
}

public int IncrementId<T>(T item) where T : IHasId
{
    return item.GetId() + 1;
}

---

Another option is to pass a property accessor to the method

public int IncrementId<T>(T item, Func<T, int> getId)
{
    return getId(item) + 1;
}

You would call it with

int nextId = IncrementId(test, t => t.GetId());

See also:

• Pass property itself to function as parameter in C#
• My answer to the question How can I find a specific element in a List? explains that you should not use a method GetId but instead should use a property in C#.

Answer 3

You do not provide an instance of T. Try:

public int IncrementId<T>(T t) where T : Test
{
    return 1 + t.GetId();
}