using System;
using System.Linq;

public class Test
{
	public static void Main()
	{

var data = new[] {
	new Something {SomeIntProperty=1, SomeStringProperty="A"}
,   new Something {SomeIntProperty=2, SomeStringProperty="A"}
,   new Something {SomeIntProperty=3, SomeStringProperty="A"}
,   new Something {SomeIntProperty=4, SomeStringProperty="A"}
,   new Something {SomeIntProperty=5, SomeStringProperty="A"}
,   new Something {SomeIntProperty=6, SomeStringProperty="B"}
,   new Something {SomeIntProperty=7, SomeStringProperty="B"}
,   new Something {SomeIntProperty=8, SomeStringProperty="C"}
,   new Something {SomeIntProperty=9, SomeStringProperty="D"}
};
var dict = data.GroupBy(s => s.SomeStringProperty)
                     .ToDictionary(g => g.Key);
foreach (var key in dict.Keys) {
	if (data.Any(s => ReferenceEquals(s.SomeStringProperty, key))) {
		Console.WriteLine("Key '{0}' is present.", key);
	} else {
		Console.WriteLine("Key '{0}' is not present.", key);
	}
}

	}
}

public class Something
{
    public int SomeIntProperty { get; set; }
    public string SomeStringProperty { get; set; }  
}