Null Reference exception in C#

问题

I'm experiencing "null reference exception" when I'm attempting to return a value from a structure.

here is the code:

AssetItem item = new AssetItem(); 

        item = initModified();

        bool found = false;
        int index = getIndex(barcode);
        string modifiedFile = filepath + "Modified\\" + dir + "\\" + index + ".asdt";

        if(File.Exists(modifiedFile))
        {   
            using(StreamReader reader = new StreamReader(modifiedFile))
            {
                string line = reader.ReadLine();
                while(line.Trim()!="")
                {
                    string[] split = line.Split(',');
                    if(split[1]==barcode)
                    {
                        found = true;
                        break;
                    }
                    line = reader.ReadLine();
                }
                reader.Close();
            }
        }

        if(found)
        {
            item.modified = true; 
        }
        else
        {
            item.modified = false;
        }


        return item;

I initialize item by calling a sub containing that item.modified = false. After checking that the file exist, I used a streamreader to read the lines of the file one by one until it finds a specific line and stops. The problem is when it checked if the file exist and doesn't find the specific line. It returns null even tough I initialize item to false and set it to false when it doesn't find the line. Note: this occurs seldom and works fine when I access other files to read and even in the same file that it returns null.

Note: Another problem I encountered is that it skips a line that it reads.

What could be the cause of this?

回答1:

And the end of the file, ReadLine() returns null - and you then call .Trim() on it without checking (in the scenario where the item isn't there and you read the file all the way through) - hence you need to add a null-check (note also I've moved the ReadLine so it happens consistently):

using(StreamReader reader = new StreamReader(modifiedFile))
{
    string line;
    while((line = reader.ReadLine()) != null && line.Trim() != "") {
        ...
    }
}

Note that the above code (based on yours) will end on the first empty line; personally I'd probably skip empty lines:

using(StreamReader reader = new StreamReader(modifiedFile))
{
    string line;
    while((line = reader.ReadLine()) != null) {
        if(line.Trim() == "") continue;
        ...
    }
}

回答2:

One problem that I can find in your code is that you don't need the following line:

reader.Close();

using automatically does this for you.

Furthermore, your looping condition should check EndOfStream instead of trimming the line.

i.e., Modifying your code to something like this:

using(StreamReader reader = new StreamReader(modifiedFile))
{

    while(!reader.EndOfStream)
    {
        string line = reader.ReadLine();
        string[] split = line.Split(',');
        if(split[1]==barcode)
        {
            found = true;
            break;
        }
    }
}

回答3:

On a side note, why create a new instance then re-assign to it without using it for any purpose..

AssetItem item = new AssetItem();  
item = initModified();

Could become

AssetItem item =  initModified();

来源：https://stackoverflow.com/questions/4122514/null-reference-exception-in-c-sharp

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streaming

nullreferenceexception

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