Can I have a variable number of generic parameters?

Question

In my project I have the following three interfaces, which are implemented by classes that manage merging of a variety of business objects that have different structures.

public interface IMerger<TSource, TDestination> {     TDestination Merge(TSource source, TDestination destination); }  public interface ITwoWayMerger<TSource1, TSource2, TDestination> {     TDestination Merge(TSource1 source1, TSource2 source2, TDestination destination); }  public interface IThreeWayMerger<TSource1, TSource2, TSource3, TDestination> {     TDestination Merge(TSource1 source1, TSource2 source2, TSource3 source3, TDestination destination); } 

This works well, but I would rather have one IMerger interface which specifies a variable number of TSource parameters, something like this (example below uses params; I know this is not valid C#):

public interface IMerger<params TSources, TDestination> {     TDestination Merge(params TSource sources, TDestination destination); } 

It there any way to achieve this, or something functionally equivalent?

Answer 1

You can't. That is a key part of the API. You could, however, do something around the side, such as accepting a Type[] argument. You might also think up some exotic "fluent API / extension method" way of doing it, but to be honest it probably won't be worth it; but something like:

obj.Merge<FirstType>(firstData).Merge<SecondType>(secondData)      .Merge<ThirdType>(thirdData).Execute<TDestination>(dest); 

or with generic type inference:

obj.Merge(firstData).Merge(secondData).Merge(thirdData).Execute(dest); 

Each merge step would simple store away the work to do, only accessed by Execute.

Answer 2

It depends on whether you want your objects to be able to merge objects of different types or not.

For a homogeneous merge, all you need is this:

public interface IMerger<TSource, TDestination> {     TDestination Merge(IEnumerable<TSource> sources, TDestination destination); } 

For a heterogeneous merge, consider requiring all source types to derive from a common base type:

public interface IMerger<TSourceBase, TDestination> {     TDestination Merge(IEnumerable<TSourceBase> sources, TDestination destination); } 

I don't see any need for a param array, just pass in the collection of objects.