找不到此NullReferenceException的原因

Question

I have the following method:

internal static void removeMethodFromTag(byte p, Method method)
        {
            Console.WriteLine("Remove method " + method.getName() + " from the tag");
            List<Method> outList = new List<Method>();
            methodTaggings.TryGetValue(p, out outList);
            Console.WriteLine(outList.Count);
            outList.Remove(method);
            methodTaggings.Remove(p);
            methodTaggings.Add(p, outList);
        }

This is the dictionarry methodTaggings:

private static Dictionary<byte, List<Method>> methodTaggings = new Dictionary<byte, List<Method>>();

Now I always get a NullReferenceException in the line

Console.WriteLine(outList.Count);

But I can't see why? Even if I don't find a value in the dictionnary, the list shouldn't be null?

Answer 1

Should this not be:

internal static void removeMethodFromTag(byte p, Method method)
        {
            Console.WriteLine("Remove method " + method.getName() + " from the tag");
            List<Method> outList = new List<Method>();
            if (methodTaggings.TryGetValue(p, out outList);
            {
              Console.WriteLine(outList.Count);
              outList.Remove(method);
              methodTaggings.Remove(p);
              methodTaggings.Add(p, outList);
           }
        }

Answer 2

Add an if at the TryGetValue method. I think the TryGetValue is setting the out variable to null when it fails.

internal static void removeMethodFromTag(byte p, Method method)
        {
            Console.WriteLine("Remove method " + method.getName() + " from the tag");
            List<Method> outList = new List<Method>();
            if(methodTaggings.TryGetValue(p, out outList)) {
                Console.WriteLine(outList.Count);
                outList.Remove(method);
                methodTaggings.Remove(p);
                methodTaggings.Add(p, outList);
            }
        }

Answer 3

The only suspect I can see here is this line:

methodTaggings.TryGetValue(p, out outList);

Could it be that methodTaggings.TryGetValue for the parameter p actually set outList to null ?

Place a breakpoint on this line, and I think you will see that after you have stepped over this line, outList will be null .

Answer 4

It can be initialized to null by the algorithm of TryGetValue . out paramters are always initialized insed method that is called. And your code does not need this line:

List<Method> outList = new List<Method>();

If you use some method that would need ref (not out) parameter, than you would need it. But here out. Refer: http://www.c-sharpcorner.com/UploadFile/mahesh/UsingtheoutParameter12232005041002AM/UsingtheoutParameter.aspx for more details.

Answer 5

There is no need to allocate the outList in your code, as you are passing it to TryGetValue() through its 2nd argument, which is declared as an out parameter .

My guess is that TryGetValue() assigns null to outList .

Answer 6

TryGetValue将您的outList替换为其默认值null

Answer 7

The value of the list will no longer there, no matter TryGetValue is successful or not, even you have assigned value to it before calling. A simple example:

int num = 9;
int.TryParse("abc", out num);

The TryParse is failed obviously, while the original value 9 will not be kept.

Answer 8

Your error is when you call TryGetValue. Parameter outList is marked out , so your original empty list is lost there.

Answer 9

From MSDN :

If the key is not found, then the value parameter gets the appropriate default value for the value type TValue; for example, 0 (zero) for integer types, false for Boolean types, and null for reference types.

And List<Method> is a reference type so it'll return null .

Good luck!