C# Determine Duplicate in List [duplicate]

Answer 1

In order to allow short circuiting if the duplicate exists early in the list, you can add a HashSet and check the return value of its .Add method.

By using .Any you can short circuit the enumeration as soon as you find a duplicate.

Here's a LINQ extension method in both C# and VB:

CSharp:

public static bool ContainsDuplicates(this IEnumerable enumerable)
{
    var knownKeys = new HashSet();
    return enumerable.Any(item => !knownKeys.Add(item));
}

Visual Basic:

Public Function ContainsDuplicates(Of T)(ByVal enumerable As IEnumerable(Of T)) As Boolean
    Dim knownKeys As New HashSet(Of T)
    Return enumerable.Any(Function(item) Not knownKeys.Add(item))
End Function

Note: to check if there are no duplicates, just change Any to All