Does List <String> .Contains (mystring) perform a reference comparison or comparison of values?

Question

Does List.Contains (mystring) contain a reference comparison or a comparison of values? For example, I have this code:

/// <summary>
/// If the value isn't null or an empty string, 
/// and doesn't exist in the list, it adds it to the list
/// </summary>
static void AddToListIfNotEmpty(List<string> thelist, SqlString val)
{
  string value = val.ToString().Trim();
  if (!string.IsNullOrEmpty(value))
  {
    bool found = false;
    foreach (string x in thelist) if (x == value) found = true;
    if (!found) thelist.Add(value);
  }
}

Can I simplify the foreach line and the following line:

if (!thelist.Contains(value)) thelist.Add(value);

thank

Answer 1

IList<T>uses Comparator<T>.Defaultto perform comparisons and, in turn, compares values ​​for objects String.

If you want, you can transfer your elements to IDictionary<T, bool>or something similar, where you can specify your own IComparator- by specifying the one that checks the link. (although in this case you are better off with a foreach loop)

LINQ, .

Answer 2

. , , SqlString, ToString() ? .

Answer 3

MSDN List<T>.Contains

, EqualityComparer<T>.Default T, .

... , IEquatable<T> EqualityComparer<T> . EqualityComparer<T>, Object.Equals Object.GetHashCode T.

( ), String Equality - , . Equals(Object) IEquatable.Equals(T) String::EqualsHelper

Answer 4

, ( , complexity *), foreach, .

, :

static void AddToListIfNotEmpty(List<string> thelist, SqlString val)
{
    string value = val.ToString().Trim();
    if (value != string.Empty && !thelist.Contains(value))
        thelist.Add(value);
}

, string.IsNullOrEmpty() null . Edit: , , SqlString.ToString() NULL SqlString "Null", , . , :

if (val.IsNull)
    return;

Finally, returning to my comment on complexity: if you are dealing with a large number of elements, you can look HashSetinstead of using List(same namespace). It will not support the order of adding strings, but it Containsworks O (1) , while your current is O (n) .