如何处理可能返回null并生成System.NullReferenceException的方法

Question

I have a method with the return type Fruit , that does the following:

Search for the right apple, if it matches return it; else
Search for the right banana, if it matches return it; else
Search for the right orange, if it matches return it; else
return null

Fruit is an interface that has the following:

bool Rotten { get; set; }

The problem is that when I try to use it:

store.GeTAFruit("magic apple").Rotten;

If it does not find the fruit it will return null, and that will give a NullReferenceException .

Of course I can surround it with a try catch but that means that every time I use this function I will have to surround it with try catch, that doesn't seem like a good idea at all.

I'm looking either for a solution to this problem, or rather what would be the best approach for this.

Answer 1

If GetAFruit can return null, then (and here's the technical bit): check for null :

var fruit = store.GetAFruit(...);
if(fruit != null) {
    //... Do stuff
}

Answer 2

this may help

if (store.GeTAFruit("magic apple")!=null) {
store.GeTAFruit("magic apple").Rotten;
} 

edit to make it a tiny bit more efficient:

var fruit = store.GeTAFruit("magic apple");
if (fruit!=null)) {
    fruit.Rotten;
} 

Answer 3

Simply check that store.GeTAFruit("magic apple") is not null:

   if (store.GeTAFruit("magic apple") != null) {

   }

Answer 4

There are two approaches if you do not want to use exception handling. But the essence of them is the same. You must evaluate the lookup result to test it for null before using it.

The first option is to assign the result of your lookup to a variable and then test if before you use it.

Fruit fruit = store.GeTAFruit("magic apple");
if(fruit != null)
{
    //safely use your Rotten property
    bool lFlag = fruit.Rotten;
}

An alternative is to test it like so ...

if(store.GeTAFruit("magic apple") != null)
{
    store.GetTAFruit("magic apple").Rotten;
}

The benefits of the first approach is that you only perform the lookup once.

Answer 5

Define a NullFruit : IFruit. Return a instance of it if nothing is found.