How do I use Yield in C#?

public class fader : MonoBehaviour {

 public GameObject gamobjBac;
 Color thecolor;

 // Use this for initialization
 void Start ()
 {
     corfunAlp (gamobjBac, 0, 1, 1);
     // Turn opaque 
     corfunAlp (gamobjBac, 4, 1, 0);
     // Turn transparent     
 }

 // Update is called once per frame
 void Update ()
 {
 }

 //function that fades alpha for a game object up or down with a start delay and duration of fade
 public IEnumerator corfunAlp (GameObject gamobjGamObj, float floWai, float floDur, int intAlpGoa)
 {

     float floAlp = (1 - intAlpGoa);
     intAlpGoa = ((2 * intAlpGoa) - 1);

     yield return new WaitForSeconds (floWai);

     while (floAlp >= 0 && floAlp <= 1) 
     {
         thecolor = gamobjGamObj.renderer.material.GetColor ("_Color");
         thecolor.a = floAlp; 
         gamobjGamObj.renderer.material.SetColor ("_Color", thecolor);

         floAlp = floAlp + ((Time.deltaTime/floDur)*intAlpGoa);    
         yield return true; 
     }
 }

}