This method uses the default comparer returned by EqualityComparer<T>.Default, which uses an ordinal comparison for strings. If you passed a specified comparer when you constructed the dictionary, ContainsValue could return different results than from ContainsKey.

For example, the following code shows this behavior:

[C#]
 
using System;
using System.Collections.Generic;
 
namespace Samples
{
    class Program
    {
        static void Main(string[] args)
        {
            Dictionary<string, string> dictionary = new Dictionary<string, string>(StringComparer.CurrentCulture);
            dictionary.Add("Encyclopædia", "Encyclopædia");
            
            Console.WriteLine(dictionary.ContainsKey("Encyclopaedia"));
            Console.WriteLine(dictionary.ContainsValue("Encyclopaedia"));
        }
    }
}

The above outputs the following:

True
False

As you can see, ContainsKey and ContainsValue are returning different results for the same search string. This is because ContainsKey is using a culture-sensitive comparison, whereas, ContainsValue is using an ordinal comparison.

As a workaround to get consistent results, you could define an extension method on IDictionary<TKey, TValue> that allows you to specify a comparer:

[C#]
 
using System;
using System.Collections.Generic; 
 
namespace Samples
{
    public static class DictionaryExtensions
    {
        public static bool ContainsValue<TKey, TValue>(this IDictionary<TKey, TValue> dictionary, TValue value, IEqualityComparer<TValue> comparer)
        {
            if (comparer == null)
                comparer = EqualityComparer<TValue>.Default;
 
            foreach (var pair in dictionary)
            {
                if (comparer.Equals(pair.Value, value))
                    return true;
            }
 
            return false;
        }
    }
}

The above extension method, then allows you to do the following:

[C#]
 
using System;
using System.Collections.Generic;
 
namespace Samples
{
    class Program
    {
        static void Main(string[] args)
        {
            Dictionary<string, string> dictionary = new Dictionary<string, string>(StringComparer.CurrentCulture);
            dictionary.Add("Encyclopædia", "Encyclopædia");
             
            Console.WriteLine(dictionary.ContainsKey("Encyclopaedia"));
            Console.WriteLine(dictionary.ContainsValue("Encyclopaedia", StringComparer.CurrentCulture));
        }
    }
}

The above outputs the following:

True
True