Question

I’m compiling a program which was originally build in Visual C# 2005. I’am using visual C# 2010. And I keep getting “NullReference Execption was unhandled” errors when running the program on the following functions:
The error occurs on the line with DataBuffer. DataBuffer is an private string set to null on initialisation.

if (DataBuffer.Contains(ok)) 
            {
                okFound = true;
            }

and

    string temp = getLine(DataBuffer.Substring(mylocation));
    if (!checkTypeFound())
    {
        if (temp != null)
        {
            parseDeviceType(temp);
        }
        checkTypeFound();
    }

When I check what the value of DataBuffer is in the code above (when I get the error) this is not null. It actually contains the data I expect.
DataBuffer information is loaded in this function:

private void ser1_DataReceived(object sender, SerialDataReceivedEventArgs e)
{
    while (ser1.BytesToRead > 0)
    {
        string data = ser1.ReadExisting();
        DataBuffer += data;
    }
}

The serial port is opened somewhere else in the code. There have been no changes to the code only the compiler is different. What line should I add, and where to solve this error? Note, I can prevent this error from happening using an if and try-catch statement. But this is not what I’m looking for, I need this code to work.
This application has not been changed in any way other than the compiler.

Answer 1

You should check if DataBuffer is null before you call its methods.

if (DataBuffer != null && DataBuffer.Contains(ok)) 
{
    okFound = true;
}

// or simpler:
okFound = (DataBuffer != null && DataBuffer.Contains(ok));

and your second code snipped should check for null as well.

string temp = String.Empty;

if (DataBuffer != null)
    temp = getLine(DataBuffer.Substring(mylocation));

if (!checkTypeFound())
{
    if (!String.IsNullOrEmpty(temp))
        parseDeviceType(temp);

    checkTypeFound();
}