C＃使用IF / ELSE语句中的方法返回的值

Question

Right now I am working on a simple program, and this is a problem I've been thinking over many times. Many times I run my methods twice because of checking on the return value before running them, and I would like to know if there is a way I can prevent this, like using the returned value from the method I am checking against. It's quite hard to explain so here is a real life example from my program.

public class SFDBRepository
{
    public static Domain.SF.SFObject GetSFOrder(string WorkOrd)
    {
        //As you can see here i'm checking on the output of this method, before trying to return it.
        if (Domain.SF.SF.GetOrder(WorkOrd) != null)
        {
            //If the value is not null (My method returns null if no result), return the object
            return Domain.SF.SF.GetOrder(WorkOrd);
        }
        //Same thing happens here. My method runs twice every time almost. 
        else if(Domain.Building_DeliveryPerformance.Building_DeliveryPerformance.GetObject(WorkOrd) != null)
        {
            return Domain.Building_DeliveryPerformance.Building_DeliveryPerformance.GetObject(WorkOrd);
        }
        else
        {
            return null;
        }
    }

}

Answer 1

You can simplify this down to the following code, which will only call those methods once and make the code much more readable:

public class ShopFloorDBRepository
{
    public static Domain.ShopFloor.ShopFloorObject GetShopFloorOrder(string workOrd)
    {
        return Domain.ShopFloor.Shopfloor.GetOrder(workOrd) ??
               Domain.DG9_DeliveryPerformance.DG9_DeliveryPerformance.GetObject(workOrd);
    }
}

To explain why this works - the ?? operator (the null-coalescing operator!) basically says "if the returned value on the left hand side of the ?? is null, then return the value of the expression on the right hand side".

This way you only need to call your functions once.

Answer 2

public static Domain.ShopFloor.ShopFloorObject GetShopFloorOrder(string WorkOrd)
{
    //As you can see here i'm checking on the output of this method, before trying to return it.
    Domain.ShopFloor.ShopFloorObject wo = Domain.ShopFloor.Shopfloor.GetOrder(WorkOrd);
    if (wo != null)
    {
        //If the value is not null (My method returns null if no result), return the object
        return wo;
    }
    //Same thing happens here. My method runs twice every time almost. 
    Domain.ShopFloor.ShopFloorObject yowo = Domain.DG9_DeliveryPerformance.DG9_DeliveryPerformance.GetObject(WorkOrd);
    if(yowo != null)
    {
        return yowo;
    }

    /* default return */
    return null;

}

PS

You're kinda doing the "Factory Pattern"

See

http://www.dofactory.com/net/factory-method-design-pattern

Answer 3

Looks to me like you could be using a temporary variable to hold the result, which you can test and return.

public class ShopFloorDBRepository
{
  public static Domain.ShopFloor.ShopFloorObject GetShopFloorOrder(string WorkOrd)
  {
    var result = Domain.ShopFloor.GetOrder(WorkOrd);

    if (result != null) return result;
    ...

This is a common paradigm, especially when the method being called is expensive and/or has side effects you don't wish to incur twice.

Here, the "var" declaration sets the type of "result" to the type returned by the method being called; you could also use the name of the actual type.

If you wish to make two different kinds of tests like this, you'll need two different variables unless they have the same type (which in this case it appears they do).

An alternate mechanism that does require the full type, that you'll also see:

public static ShopFloorObject GetShopFloorOrder(string WorkOrd)
{
    ShopFloorObject result;

    if ( (result = Domain.ShopFloor.GetOrder(WorkOrd)) != null )
        return result;
    if ( (result = Domain.DG9_DeliveryPerformance.DG9_DeliveryPerformance.GetObject(WorkOrd)) != null)
        return result;
    return null;

Here you're explicitly declaring the type of the return value, then making the two calls you've indicated, testing the results against null, and returning the first non-null value.