I am getting weird errors while implementing a generic method of an interface in C#. What exactly is going wrong here?

Question

public interface IGet<T>
{
    T Get<K>(K id);
}

public interface IDemoRepository : IGet<Dto.Message>
{

}

public class DemoRepository : IDemoRepository
{
    public Dto.Message Get<string>(string messageId)
    {
        using (var db = new AppDbContext())
        {
            return db.Messages
                .FirstOrDefault(f => f.MessageId.ToLower() == messageId.ToLower());
        }
    }
}

Below are the errors in the Error List.

Severity    Code    Description
Error       CS1001  Identifier expected
Error       CS1003  Syntax error, '>' expected  
Error       CS1003  Syntax error, '(' expected  
Error       CS1001  Identifier expected 
Error       CS1003  Syntax error, ',' expected  
Error       CS1003  Syntax error, ',' expected  
Error       CS8124  Tuple must contain at least two elements.   
Error       CS1001  Identifier expected 
Error       CS1026  ) expected  
Error       CS0535  'DemoRepository' does not implement interface member 'IGet<Dto.Message>.Get<K>(K)'  
Error       CS0103  The name 'messageId' does not exist in the current context

A .NET fiddle with success and fail scenarios here

It's likely there is just one (or two) error, which leads to the rest. Start typing it over again, making sure to continue only when there are no errors you don't expect to have (e.g. if you didn't yet implement interface, there could be error regarding interface, but no more). You shouldn't be typing `f.MessageId` if intellisense doesn't recognize `f`. — Sinatr, Jul 13 '20 at 07:57
@MindeSwipe Most of the errors are at line : public Mail Get(string messageId) — Rahul, Jul 13 '20 at 08:05
I have created a .net fiddle with both the scenarios failed and sccess. https://dotnetfiddle.net/deSUW0 — Rahul, Jul 13 '20 at 08:18
@Rahul As I mentioned below Guid is NOT System.Guid in your fiddle. Try doing `id.ToByteArray();` for instance. Your Guid is just as good as K, it's just a generic parameter. — Loocid, Jul 13 '20 at 08:22

Sweeper · Accepted Answer · 2020-07-13T08:34:21.943

The reason why the compiler gave out so many errors is because it expected an identifier in the <>, but you put a keyword (string) there. The compiler got confused as a result, and the parsing got very wrong. This is also why there are way fewer errors if you use Guid, because Guid is an identifier. But replacing string with Guid doesn't do what you expect either.

You seem to want DemoRepository to only implement IDemoRepository in a very specific case, i.e. when K is string.

Note that the requirement for implementing IDemoRepository is that it should have a generic Get method, being able to take any type K, and returns a Dto.Message. That is what's meant by:

public interface IGet<T>
{
    T Get<K>(K id);
}

public interface IDemoRepository : IGet<Dto.Message>
{

}

I suppose that's not what you actually want IDemoRepository to require? If you only want it to require a non generic Get method. that takes a specific type K, which can be chosen by the implementer, then you can do something like this:

// I moved the generic parameter K from the method to the interface
public interface IGet<T, K>
{
    T Get(K id);
}

public interface IDemoRepository<K> : IGet<Dto.Message, K>
{

}

// here the implementer chose string as K
public class DemoRepository : IDemoRepository<string>
{
    public Dto.Message Get(string messageId)
    {
        ...
    }
}

Thank you so much. You are right, IGet interface was incorrectly defined. — Rahul, Jul 13 '20 at 08:36

score 0 · Answer 2 · answered Jul 13 '20 at 08:00

0

The interface specifies that Get should take a generic parameter but you're trying to define it using a string parameter.

Either change your DemoRepository to make Get generic (which doesn't look like it'll work in your case since you want to use .ToLower).

Or change your Get to be non-generic.

answered Jul 13 '20 at 08:00

Loocid

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It works totally fine when I provide Guid instead of string. Error is comming when I use string or int. – Rahul Jul 13 '20 at 08:04
1

`Guid` gets redefined as the generic parameter. Its not actually the GUID struct. Try it yourself by using `String` rather than `string`. You get the warning "String hides class String" but it compiles fine. – Loocid Jul 13 '20 at 08:12

I am getting weird errors while implementing a generic method of an interface in C#. What exactly is going wrong here?

2 Answers2