understand generic class

Question

3.00/5 (2 votes)

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I am not strong on generic class. Say we have a class.

C#

class Appointment
    {
        public Guid UniqueId { get; set; }
    }

And an extensiton class:

C#

public static class ExtendReceive<T> where T:Appointment
    {
        public static void TryReceive<T>(this BufferBlock<T> bufferBlock, int count)
        {
            try
            {
                for (var i = 0; i < count; i++)
                {
                    T item;
                    if (bufferBlock.TryReceive(out item))
                    {
                        Call d = new Call();
                        d.RunScript<T>((T)item);
                    }
                    else
                    {
                        break;
                    }
                }
            }

And finally

C#

class Call
    {
       
        internal void RunScript<T>(T t)
        {
            var x = t.UniqueId;
        }

However this line is wrong:

C#

var x = t.UniqueId; //error

Also we call this extension method also got error:

C#

public static BufferBlock<Appointment> m_Queue = new BufferBlock<Appointment>();
m_Queue.TryReceive(4); // error as well

The best overload method match ...has some invalid arguments.

Thanks for advice.

Posted 8-Oct-14 16:34pm

zhshqzyc

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Comments

Sergey Alexandrovich Kryukov 8-Oct-14 22:58pm

Where is the declaration of BufferBlock? And BufferBlock.TryReceive?
—SA

BillWoodruff 9-Oct-14 6:06am

For a question like this about "best practices" it's a good thing to state what you are trying to achieve in your question. What is your goal here ? Why did you choose to write an Extension Method that then creates a new Class every time it needs to execute 'RunScript ?

2 solutions

Solution 1

You started correctly, and then made a mess. How a compiler can know that x has the member UniqueId?

You need to do it as in the generic class ExtendReceive. In this class, the compiler knows that the parameter is Appointment (this exact type or derived from this class), due to the constraint where T: Appointment. This is the whole idea behind those constraints.

In case of second error, I the declaration of System.Threading.Tasks.Dataflow.BufferBlock.TryReceive (thanks for clarification) shows this signature (http://msdn.microsoft.com/en-us/library/hh194808%28v=vs.110%29.aspx[^]):

C#

public static bool TryReceive<TOutput>(
	this IReceivableSourceBlock<TOutput> source,
	out TOutput item
)

I have no idea why did you decide that there is a method with the same name expecting an integer argument. Moreover, this method is static, you cannot possibly call it on m_Queue, the call syntax would be BufferBlock.TryReceive(/* ... */). Here, you have a problem with programming basics, not even specifically related to .NET generics.

—SA

Posted 8-Oct-14 16:56pm

Sergey Alexandrovich Kryukov

Updated 9-Oct-14 4:48am

v5

Comments

[no name] 9-Oct-14 8:46am

@SA,
BufferBlock is .net 4.5 Tasks Parallel Library. see http://msdn.microsoft.com/en-us/library/hh160414(v=vs.110).aspx

public sealed class BufferBlock<t> : IPropagatorBlock<t, t="">, ITargetBlock<t>, IReceivableSourceBlock<t>, ISourceBlock<t>, IDataflowBlock

Sergey Alexandrovich Kryukov 9-Oct-14 10:50am

Thank you for clarification, but I already answered, so one problem was apparent even without seeing the declaration. Additional issue is that the method is generic. In all cases, you just tried to call a method which simply does not exist, hence the second error.
—SA

Maciej Los 9-Oct-14 10:49am

+5!

Sergey Alexandrovich Kryukov 9-Oct-14 10:53am

Thank you, Maciej.
—SA

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RIPSolution · Accepted Answer · 2014-10-08T17:51:00

You declared a static class with Generic type. Static classes cannot be instantiated and you will end up using the class name which will diminish the purpose of declaring an extension method.

Instead of declaring generic on static class put the constraint on the extension method itself.

C#

public static class ExtendReceive 
    {
        public static void TryReceive<t>(this BufferBlock<t> bufferBlock, int count)                 where T:Appointment

        {
            try
            {
                for (var i = 0; i < count; i++)
                {
                    T item;
                    if (bufferBlock.TryReceive(out item))
                    {
                        Call d = new Call();
                        d.RunScript<t>((T)item);
                    }
                    else
                    {
                        break;
                    }
                }
            }
</t></t></t>

In case of class Call, you need to decide whether you need the generic type reference throughout the call or only for the method RunScript and accordingly add constraint

C#

class Call
    {
       
        internal void RunScript<t>(T t) where T:Appointment
        {
            var x = t.UniqueId;
        }
</t>