C# Generic interface and inheritance

Question

I have a generic interface which inherits from another generic interface from a third party library (which makes it impossible to use covariance and contravariance) .

Here is my code

public interface IFABaseAppService<TDto, TKey> 
         : ICrudAppService<TDto, TKey> where TDto : FABaseDto
{
}

I have another interface IUserAppService

public interface IUserAppService : IFABaseAppService<UserDto, Guid>
{
}

Then I have a base class which accepts IFABaseAppService<FABaseDto, Guid> in constructor as a parameter

public class FABaseStandardFormViewModel : FABaseViewModel
{
    public FABaseStandardFormViewModel(IFABaseAppService<FABaseDto, Guid> appService) : base(appService)
    {
    }
}

Then I have created a class which accepts IUserAppService in the constructor and passes it to the base class, but it is giving me the following error:

cannot convert from 'FinancialsAce.ApplicationServices.Common.IUserAppService' to 'FinancialsAce.ApplicationServices.IFABaseAppService<FinancialsAce.DataTransferObjects.FABaseDto, System.Guid>

Yevhen Cherkes · Accepted Answer · 2021-12-25 12:37:43Z

1

This error message means that IFABaseAppService<UserDto, Guid> can not be converted to IFABaseAppService<FABaseDto, Guid> because IFABaseAppService is not declared as covariant relatively to TDto.

See more about covariance: Covariance and Contravariance (C#).

You can fix it by adding out keyword as shown below:

public interface IFABaseAppService<out TDto, TKey> ...

edited Dec 25, 2021 at 12:37

answered Dec 25, 2021 at 12:07

Yevhen Cherkes

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1 Comment

Ammad Islam Over a year ago

Sir as i mentioned in my question i can't use covariance and contravariance because my interface is inherited from another generic interface from a third party library. when i add out it doesn't let me pass it to other interface,

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