Is generic constructor in non-generic class supported?

Question

Is it not supported, is it supported but I have to do some tricks?

Example:

class Foo
{
  public Foo(Func f1,Func

        

Answer 1

Here is an practical example about how you would like to have extra constructor type parameter, and the workaround.

I am going to introduce a simple RefCounted wrapper for IDisposable:

public class RefCounted where T : IDisposable
{
    public RefCounted(T value)
    {
        innerValue = value;
        refCount = 1;
    }

    public void AddRef()
    {
        Interlocked.Increment(ref refCount);
    }

    public void Dispose()
    {
        if(InterlockedDecrement(ref refCount)<=0)
            innerValue.Dispose();
    }

    private int refCount;
    private readonly innerValue;
}

This seems to be fine. But sooner or later you would like to cast a RefCounted to RefCounted whilst keep both object reference counting, i.e. only when both instances being disposed to dispose the underlying object.

The best way is if you could write (like C++ people can do)

public RefCounted(RefCounted other)
{
    ...whatever...
}

But C# does not allow this. So the solution is use some indirection.

private readonly Func valueProvider;
private readonly Action disposer;

private RefCounted(Func value_provider, Action disposer)
{
    this.valueProvider = value_provider;
    this.disposer = disposer;
}

public RefCounted(T value) : this(() => value, value.Dispose)
{
}

public RefCounted Cast() where U : T 
{
    AddRef();
    return new RefCounted(() => (U)(valueProvider()),this.Dispose);
}

public void Dispose(){
    if(InterlockedDecrement(ref refCount)<=0)
        disposer();
}

If your class have any fields that are of generic type, you have no choice but to put all those types to the class. However, if you just wanted to hide some type from the constructor, you will need to use the above trick - having a hidden constructor to put everything together, and define a normal generic function to call that constructor.