Generic type in generic constraint

Question

I want to be able to specify something like this:

public abstract class ColumnFilter<TCell, TFilterControl> : ColumnFilter
    where TFilterControl : FilterControl<>, new()
    where TCell : IView, new()
{
}

Class FilterControl<> is a base class. I can not know what will be the generic argument for FilterControl<>.

Answer 1

You can't use unbound generic types in type constraints. You'd have to add a third type parameter, like this:

public abstract class ColumnFilter<TCell, TFilterControl, TFilterControlType> : ColumnFilter
    where TFilterControl : FilterControl<TFilterControlType>, new()
    where TCell : IView, new()
{
}

Or create a non-generic base type of FilterControl:

public FilterControl { }
public FilterControl<T> : FilterControl { }

public abstract class ColumnFilter<TCell, TFilterControl> : ColumnFilter
    where TFilterControl : FilterControl, new()
    where TCell : IView, new()
{
}

You can also make the base type abstract with internal constructors if you want to force consumers to use your generic derived type.

Answer 2

ColumnFilter will have to be told what that type will be.

Add a third generic type parameter, like so:

public abstract class ColumnFilter<TCell, TFilterControl, TFilter> : ColumnFilter
    where TFilterControl : FilterControl<TFilter>, new()
    where TCell : IView, new()
{
}