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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:
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:
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:
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:
collection.ForEach(name => { collectionForAppend.Add(name); });This diagnostic is classified as:
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