Inject dependencies in methods or in the constructor?

//Inject the dependency into the methods that require ImportantClass
Class Something {

    public Something()
    {
         //empty
    }

    public void A()
    {
         //do something without x
    }

    public void B(ImportantClass x)
    {
         //do something with x
    }

    public void C(ImportantClass x)
    {
         //do something with x
    }
}

//Inject the dependency into the constructor once
Class Something {
    private ImportantClass _x
    public Something(ImportantClass x)
    {
         this._x = x;
    }

    public void A()
    {
         //do something without x
    }

    public void B()
    {
         //do something with this._x
    }

    public void C()
    {
         //do something with this._x
    }

}

class Foo {
    private final Bar _bar;

    Foo(Bar bar) {
        _bar=bar;
    }
}

public interface IFoo
{
   void Do();
}

public class IFoo DefaultFoo : IFoo
{
   public void Do()
   {
   }
}

public class UsesFoo
{
   private IFoo foo;
   public IFoo Foo
   {
       set { this.foo = value; }
   }

   public UsesFoo()
   {
      this.Foo = new DefaultFoo();
   }

   public UsesFoo( IFoo foo )
   {
      this.Foo = foo;
   }

   public void DoFoo()
   {
      this.Foo.Do();
   }
}