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C#: null warning even though it's not possible

Ok so here I have this function that can return a null value in some cases. However, even when testing if it is before, it still says it might be null.
My code:
if (Account.FindAccount(usernameInput) == null) { return; }
if (Account.FindAccount(usernameInput).password == "1234") // warning CS8602: Dereference of a possibly null reference
{
//Do stuff
}

Is that FindAccount operation guaranteed to be idempotent? The compiler has no such guarantee. What it returns in one call it might not return in another.
More to the point... If you believe it will always return the same result for the same input, why are you invoking it twice? Just invoke it once and store the result:
var account = Account.FindAccount(usernameInput);
if (account == null) { return; }
if (account.password == "1234")
{
//Do stuff
}

Multiple component references in an if/then logic statements in Unity?

How do you make multiple component references in a single if/then logic statements in Unity? I thought it would look something like this:
if(other.GetComponent<Component1>() != null || (GetComponent2>() != null)
{
//Run code
}
Such code returns this error:
error CS1026: ) expected

You are missing a get for the second component and a close )
if(other.GetComponent<Component1>() != null || other.GetComponent<Component2>() != null)
{
//Run code
}
With out knowing more, this is a start

C# doesn't create Unreachable code detected warning

C# seems to allow to compare between primatives (bool, int) to null.
bool a = true;
if (a != null)
{
return null;
}
Console.Write("a");
Furthermore it doesn't generate
Warning:
Unreachable code detected
That you will get when changing a != null to true.
Changing a != null to true != null won't compile.
Is there an explanation for this behavior? Or is it a compiler bug?

Getting NullReferenceException in impossible situation (when checking for null)

I have a very simple check right at the beginning of one of my methods as follows:
public void MyMethod(MyClass thing)
{
if(thing == null)
throw new ArgumentNullException("thing");
//Do other stufff....
}
But I'm getting stacktraces (from Elmah in a production environment) which appears to indicate that the "if(thing == null)" line is throwing a NullReferenceException. The first 2 lines of the stack trace are something like:
System.Web.HttpUnhandledException: Exception of type 'System.Web.HttpUnhandledException' was thrown. ---> System.NullReferenceException: Object reference not set to an instance of an object.
at MyLibrary.BL.AnotherClass.MyMethod(MyClass thing) in C:\Development\MyProject\trunk\MyLibrary.BL\AnotherClass.cs:line 100
MyClass is a fairly simple class with no operator overloads or anything like that, so I'm a bit stumped as to what is throwing the NullReferenceException!
Can anybody suggest scenarios that might cause this?
EDIT: I suspect "thing" might be null, but I really would expect an ArgumentNullException not a NullReferenceException - this is basically what this question is about. Is there maybe something that the framework or Elmah that is changing or mis-reporting the exception - or is the only explanation that the binaries are somehow out of date?

It is impossible for if (thing == null) to throw a NullReferenceException.
This means that something else is going on. It's time to start using your imagination in conjunction with a firm resolve to ignore the possibility that the if caused a problem.

The if statement can throw a NullReferenceException if MyClass defines the == operator incorrectly e.g.
class MyClass
{
int A {get;set;}
public static bool operator ==(MyClass a, MyClass b)
{
return a.A == b.A;
}
public static bool operator !=(MyClass a, MyClass b)
{
return !(a == b);
}
}

Looks like the exception is coming from something up the chain that calls MyMethod. MyMethod() is throwing the Exception and nothing above is handling it, so whatever web framework you're in is throwing the HttpUnhandledException.

I also encountered this impossible situation. It turned out to be due to the use of the as keyword, I have no idea why. I was using the SharpPdf library and had a line of code like this:
var destElement = annotDict.Elements["/Dest"] as PdfName;
if (destElement == null)
{
continue;
}
If I remove the as PdfName portion, it works. So I now have two levels of checking in my code:
var destElement = annotDict.Elements["/Dest"];
if (destElement == null)
{
continue;
}
var destElementName = destElement as PdfName;
if (destElementName == null)
{
continue;
}

thing is null.
That would cause it.
[EDIT]: Here's the code I tested with:
protected void Button3_Click(object sender, EventArgs e)
{
MyMethod(null);
}
public void MyMethod(String thing)
{
if (thing == null) // This caused the exception to be thrown.
throw new Exception("test");
//Do other stufff....
}

Compiling error Nullreference

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.

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();
}

Try using the following:
if (DataBuffer != null && DataBuffer.Contains(ok))
{
okFound = true;
}

You should set the value of DataBuffer to something other than null in your constructor. If you can't do that then you may set it to string.Empty instead of null to avoid null exception. But it always better to check for null before initiating an instance method on object.