Functional if statements for C# that pass the calling object into the conditional function, then function, and else function.

FunctionalIf.cs

// Use like so:
//   Console.WriteLine("hello".If(s => s.length == 5, s => s + " world"));
//   var newObj = someObj.If(null, new Foo(), o => o.bar());
//   return response
//     .If(r => r?.Content == null, "", r => r.Content.ReadAsStringAsync().Response)
//     .If(string.IsNullOrWhiteSpace, "", content => $"----\n\t{content}\n----")
namespace FuncIf
{
    public static class FuncIfExtensions
    {
        public static TResult If<TSource, TResult>(this TSource source, bool condition, TResult thenObj, TResult elseObj)
        {
            return condition ? thenObj : elseObj;
        }

        public static TResult If<TSource, TResult>(this TSource source, bool condition, Func<TSource, TResult> thenFunc, TResult elseObj)
        {
            if (thenFunc == null)
            {
                throw new ArgumentNullException(nameof(thenFunc));
            }
            return condition ? thenFunc(source) : elseObj;
        }

        public static TResult If<TSource, TResult>(this TSource source, bool condition, TResult thenObj, Func<TSource, TResult> elseFunc = null)
        {
            return condition
                ? thenObj
                : elseFunc != null
                    ? elseFunc(source)
                    : default(TResult);
        }

        public static TResult If<TSource, TResult>(this TSource source, bool condition, Func<TSource, TResult> thenFunc, Func<TSource, TResult> elseFunc = null)
        {
            if (thenFunc == null)
            {
                throw new ArgumentNullException(nameof(thenFunc));
            }
            return condition
                ? thenFunc(source)
                : elseFunc != null
                    ? elseFunc(source)
                    : default(TResult);
        }

        public static TResult If<TSource, TResult>(this TSource source, Func<TSource, bool> condition, TResult thenObj, TResult elseObj)
        {
            if (condition == null)
            {
                return source == null ? thenObj : elseObj;
            }
            return condition(source) ? thenObj : elseObj;
        }

        public static TResult If<TSource, TResult>(this TSource source, Func<TSource, bool> condition, Func<TSource, TResult> thenFunc, TResult elseObj)
        {
            if (thenFunc == null)
            {
                throw new ArgumentNullException(nameof(thenFunc));
            }

            if (condition == null)
            {
                return source == null ? thenFunc(source) : elseObj;
            }
            return condition(source) ? thenFunc(source) : elseObj;
        }

        public static TResult If<TSource, TResult>(this TSource source, Func<TSource, bool> condition, TResult thenObj, Func<TSource, TResult> elseFunc = null)
        {
            if (condition == null)
            {
                return source == null
                    ? thenObj
                    : elseFunc != null
                        ? elseFunc(source)
                        : default(TResult);
            }
            return condition(source)
                ? thenObj
                : elseFunc != null
                    ? elseFunc(source)
                    : default(TResult);
        }

        public static TResult If<TSource, TResult>(this TSource source, Func<TSource, bool> condition, Func<TSource, TResult> thenFunc, Func<TSource, TResult> elseFunc = null)
        {
            if (thenFunc == null)
            {
                throw new ArgumentNullException(nameof(thenFunc));
            }

            if (condition == null)
            {
                return source == null
                    ? thenFunc(source)
                    : elseFunc != null
                        ? elseFunc(source)
                        : default(TResult);
            }
            return condition(source)
                ? thenFunc(source)
                : elseFunc != null
                    ? elseFunc(source)
                    : default(TResult);
        }

        public static bool notNull<TValue>(TValue value)
        {
            return value != null;
        }
    }
}