# V3221\. Modifying a collection during its enumeration will lead to an exception\.

The analyzer has detected that a collection is being modified during iteration\. This may result in an InvalidOperationException\.

The example: 

```cpp
public void AppendMissingNames(List<string> currentNames, 
                               List<string> otherNames)
{
  foreach(string name in currentNames)
  {
    if (!otherNames.Contains(name))
    {
      currentNames.Add(name);
    }
  }
}
```

In the `foreach` loop body, the `Add` method is called on the `currentNames` collection while iterating over the same collection\. Modifying `currentNames` during iteration will cause the `InvalidOperationException`\.

The fixed code might look as follows:

```cpp
public void AppendMissingNames(List<string> currentNames, 
                               List<string> otherNames)
{
  foreach(string name in currentNames)
  {
    if (!otherNames.Contains(name))
    {
      otherNames.Add(name);
    }
  }
}
```

Missing names should be added to `otherNames` instead of `currentNames`\. This ensures that the collection being iterated over remains unchanged\.

Take a look at the following example of iterating over a collection using the `ForEach` method:

```cpp
collection.ForEach(name => { collection.Add(name); });
```

The error is similar to the previous one: the collection being iterated over is modified\. To fix this, change the collection where new elements are added:

```cpp
collection.ForEach(name => { collectionForAppend.Add(name); });
```