Working around lack of partial generic type inference with constraints

Question

I have an interface (which is used by repositories) that has this member:

T FindById(TId id)
    where T : class, IEntity
    where          


        

Answer 1

That can't work the usual way around because you can't convince the compiler of the TId constraint after the fact. You can, however, reverse the sequence, i.e.

var obj = ById(id).Find<SomeType>();

Not as elegant, but it works. Implementation:

public Finder<TId> ById<TId>(TId id) where TId : IEquatable<TId>
{
    return new Finder<TId>(this, id);
}
public struct Finder<TId> where TId : IEquatable<TId>
{
    private readonly YourParent parent;
    private readonly TId id;
    internal Finder(YourParent parent, TId id)
    {
        this.id = id;
        this.parent = parent;
    }
    public T Find<T>() where T : class, IEntity<TId>
    {
        return parent.FindById<T, TId>(id);
    }
}

caveat: it is probably easier just to tell it both the parameter types explicitly.