Implementing a multi parameter generic method in C#

Question

I am trying to learn generics, but there is some things I just cant wrap my head around.

Here are the interfaces and classes in play:

public interface IQueryExecutor
{
    T ExecuteQuery<T>(IQuery<T> query);

    T ExecuteQuery<T, T1>(IQuery<T, T1> query);        
}

public interface IQuery<out T>
{
    T Execute();        
}

public class StringQuery : IQuery<string>
{
    public string Execute()
    {
        return "String query executed";
    }
}

public interface IQuery<out T, in T1>
{
    T Execute(T1 input);
}

public class ListQuery : IQuery<List<string>, string>
{
    public List<string> Execute(string input)
    {
        return new List<string>() { input };
    }
}

The problem occurs when I try to implement the second method in the queryExecutor:

public class QueryExecutor : IQueryExecutor
{
    public T ExecuteQuery<T>(IQuery<T> query)
    {
        return query.Execute();
    }

    public T ExecuteQuery<T, T1>(IQuery<T, T1> query)
    {
        return query.Execute(T1 input);
    }        
}

I want to pass T1 as parameter to the method implementation, but that is not possible (could be done by using an IoC container like Autofac, but that's not what I want). Help! There must be some very basics of generics I don't understand.

So the question is, how do I implement:

public T ExecuteQuery<T, T1>(IQuery<T, T1> query)
{
        return ????
}  

Answer 1

You need to pass input parameter somewhere. The best option would be to extend method signature to accept input parameter like this:

public interface IQueryExecutor
{
    T ExecuteQuery<T>(IQuery<T> query);

    T ExecuteQuery<T, T1>(IQuery<T, T1> query, T1 input);        
}

The second option is to remove a second version of your IQuery interface pass input parameter in constructor.

public class ListQuery : IQuery<List<string>>
{
    private string _input;

    public ListQuery(string input)
    {
       _input= input;
    }
    public List<string> Execute()
    {
        return new List<string>() { _input };
    }
}