Simple coalesce extension method for strings. Behaves similarly to the "??" null coalesce operator, but will properly collapse in situations of empty/whitespace strings.

coalesce.cs

public static class StringCoalesceExtension
    {
        public enum CoalesceProperty
        {
            OnNull = 1,
            OnNullOrEmpty = 2,
            OnNullOrWhiteSpace = 4
        }

        public static string Coalesce(this string originalString, params string[] strings)
        {
            return Coalesce(originalString, CoalesceProperty.OnNullOrWhiteSpace, strings);
        }

        public static string Coalesce(this string originalString, CoalesceProperty property, params string[] strings)
        {
            if (strings.Length < 1)
                throw new ArgumentException("Must contain at least one parameter.");

            var stringArray = new string[strings.Length + 1];
            stringArray[0] = originalString;
            strings.CopyTo(stringArray, 1);

            foreach (var value in stringArray)
            {
                switch (property)
                {
                    case CoalesceProperty.OnNull:
                        if (value == null) continue;
                        break;
                    case CoalesceProperty.OnNullOrEmpty:
                        if (String.IsNullOrEmpty(value)) continue;
                        break;
                    case CoalesceProperty.OnNullOrWhiteSpace:
                        if (String.IsNullOrWhiteSpace(value)) continue;
                        break;
                }
                return value;
            }

            throw new ConstraintException("All parameters are null or empty. There must be at least one non-empty parameter to coalesce to.");
        }
    }