Out parameter might not be initialized before accessing

Question

Why is the code below ``` private static List MergeDatasetsListBranch(out List datasetsList) { if(datasetsList == null) datasetsList=new List(); datasetsList=new List(); return datasetsList; } ``` generating an error at the first `if` statement: ``` Out parameter 'datasetsList' might not be initialized before accessing. ``` I know it should be uninitialized at this point, but the word `might` suggest that the error lies in possible uninitialized object accessing (when it's not even accessed, it's the reference, that is checked). Ofc that doesn't happen with `ref` keyword, but I'm curious how is the reference checking violating out-parameters policy. EDIT I've edited the question and the example: the out object will be initialized inside the method anyway. The question is: WHY uninitialized object cannot be null compared? How is that different from: ``` object o; if(o==null) ... ```

Accepted Answer

Compiler Error CS0269

Use of unassigned out parameter 'parameter' The compiler could not verify that the `out` parameter was assigned a value before it was used; its value may be undefined when assigned. Be sure to assign a value to out parameters in the called method before accessing the value. If you need to use the value of the variable passed in, use a `ref` parameter instead.

So treat an `out`-parameter as unassigned. You are the one who is responsible.

So just remove the `if`:

```
datasetsList = new List<WorkflowVariableDataSet>();
```

If you want to process a list that is passed to this method use `ref` intead (as suggested above):

Out parameter might not be initialized before accessing

Solution

Problem