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Dereference of a possible null reference C#

I have a C# method which throws the following warning on line await _socketClient.Start();:
CS8602: Dereference of a possibly null reference.
Method:
public async Task Connect()
{
if ((_socketClient == null) || (_socketClient != null && _socketClient.IsRunning))
return;
await _socketClient.Start();
}
I am not able to understand why the compiler is giving that warning even though I have made an explicit null check the line above it and returning back to the caller method?
Interestingly, if I simplify the method to do a plain simple null reference check, the warning goes away.
Modified Method (No warning):
public async Task Connect()
{
if (_socketClient == null)
return;
await _socketClient.Start();
}
However, I need the additional check as well and not sure what is the mistake I am doing over there for the warning to pop up.
Environment:
.Net 6.0 aspnet core application
Visual Studio 2022 IDE

Visual Studio only throws CS8602 when it detects the presence of "maybe-null".
Looks like you have it pretty much narrowed down, does refactoring it like below help at all?
public async Task Connect()
{
if ( _socketClient == null ) {
return;
} else if ( _socketClient.IsRunning ) {
return;
} else {
await _socketClient.Start();
}
}

The problem is that after || you are checking for null again, which makes the compiler "thinks" that _socketClient could be null when first check is false even though it is _socketClient == null
If it is written as follows, it does not warn about the nullability
public async Task Connect()
{
if (_socketClient is null || _socketClient.IsRunning)
return;
await _socketClient.Start();
}

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

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.

'NullReferenceException was unhandled' in C#

I am having a nullreferenceexception error in my code in this line:
public bool BoundingVolumeIsInView(BoundingSphere sphere)
{
**return (Frustum.Contains(sphere) != ContainmentType.Disjoint);**
}
Please tell me what i am doing wrong?
Thanks

Frustum is probably null. Use a debugger and check it. You could do something like this to prevent null pointer exceptions
if(Frustum != null)
return (Frustum.Contains(sphere) != ContainmentType.Disjoint);
return false;