V3124. Appending an element and checking for key uniqueness is performed on two different variables.

The analyzer detected a suspicious code fragment where a key is tested for being present in one dictionary, while the new element is appended to another. This situation may indicate a typo or a logic error.

Consider the following example:

Dictionary<string, string> dict = new Dictionary<string, string>();
Dictionary<string, string> _dict = new Dictionary<string, string>();
....
void Add(string key, string val)
{
  if (!dict.ContainsKey(key))
    _dict.Add(key, val);
}

There may be two programming mistakes at once here. The first mistake has to do with appending an element to a wrong dictionary, which may distort the program's logic. The second deals with checking if the 'key' key is present in the 'dict' dictionary instead of '_dict'. If '_dict' already contains a value associated with the 'key' key, an 'ArgumentException' will be thrown when executing the '_dict.Add(key, val)' statement. There are two ways to fix this construct (both imply that the key is tested for the same dictionary the new element is appended to):

Dictionary<string, string> dict = new Dictionary<string, string>();
Dictionary<string, string> _dict = new Dictionary<string, string>();
....
void Add1(string key, string val)
{
  if (!_dict.ContainsKey(key))
    _dict.Add(key, val);
}
...
void Add2(string key, string val)
{
  if (!dict.ContainsKey(key))
    dict.Add(key, val);
}