Dictionary.FirstOrDefault() how to determine if a result was found

Do it this way:

if ( entry.Key != null )

The thing is that the FirstOrDefault method returns a KeyValuePair<string, int> which is a value type, so it cannot ever be null. You have to determine if a value was found by checking if at least one of its Key, Value properties has its default value. Key is of type string, so checking that for null makes sense considering that the dictionary could not have an item with a null key.

Other approaches you could use:

var entry = dict.Where(e => e.Value == 1)
                .Select(p => (int?)p.Value)
                .FirstOrDefault();

This projects the results into a collection of nullable ints, and if that is empty (no results) you get a null -- there's no way you can mistake that for the int that a successful search would yield.

Regardless of the types of Key and Value, you could do something like this:

    static void Main(string[] args)
    {
        var dict = new Dictionary<int, string>
        {
            {3, "ABC"},
            {7, "HHDHHGKD"}
        };

        bool found = false;
        var entry = dict.FirstOrDefault(d => d.Key == 3 && (found=true));
        if (found)
        {
            Console.WriteLine("found: " + entry.Value);
        }
        else
        {
            Console.WriteLine("not found");
        }
        Console.ReadLine();
    }

Jon's answer will work with Dictionary<string, int>, as that can't have a null key value in the dictionary. It wouldn't work with Dictionary<int, string>, however, as that doesn't represent a null key value... the "failure" mode would end up with a key of 0.

Two options:

Write a TryFirstOrDefault method, like this:

public static bool TryFirstOrDefault<T>(this IEnumerable<T> source, out T value)
{
    value = default(T);
    using (var iterator = source.GetEnumerator())
    {
        if (iterator.MoveNext())
        {
            value = iterator.Current;
            return true;
        }
        return false;
    }
}

Alternatively, project to a nullable type:

var entry = dict.Where(e => e.Value == 1)
                .Select(e => (KeyValuePair<string,int>?) e)
                .FirstOrDefault();

if (entry != null)
{
    // Use entry.Value, which is the KeyValuePair<string,int>
}