Following is the starting code structure

public abstract class ChildBase;
public class Parent1Child : ChildBase;
public class Parent2Child : ChildBase;

public class ParentBase<T1>
    where T1 : ChildBase
{
    public List<ChildBase> Children = new List<ChildBase>();
}

public class Parent1 : ParentBase<Parent1Child>;

I would like to create an extension method for ef core’s EntityTypeBuilder so I am able to extend the entity in a generic way

Following are the attempts:

public static class ParentExtensions
{
    public static void Test1<T>(this ParentBase<T> parent)
        where T : ChildBase
    {
        var children = parent.Children;
    }
}

public static class EfExtensions
{
    public static void Test2<TEntity>(this EntityTypeBuilder<TEntity> builder)
        where TEntity : ParentBase<ChildBase>
    {
        builder.HasMany(x => x.Children).WithOne();
    }

    public static void Test3<TEntity, TChild>(this EntityTypeBuilder<TEntity> builder)
        where TEntity : ParentBase<TChild>
        where TChild : ChildBase
    {
        builder.HasMany(x => x.Children).WithOne();
    }
}

for the scenario when I want to invoke the extension method directly on the object – the generics are implied (omitted) and compiler does not complain:

var parent = new Parent1();
parent.Test1(); // works

but when I try to invoke the extension method on a builder – the compiler starts to complain unless I explicitly provide the generic types!

EntityTypeBuilder<Parent1> builder;
builder.Test2(); // wrong
builder.Test3(); // wrong
builder.Test3<Parent1, Parent1Child>(); // works!

Why is it required to provide the generic explicitly when invoking the builder extension method? I think that passing the child class type is redundant and therefore should be omitted.. Any way to fix it?