StringInterpolation.cs

using System;
using System.Diagnostics;
using System.Runtime.CompilerServices;


namespace System.Runtime.CompilerServices
{
#pragma warning disable 1591
	// ReSharper disable ArrangeTypeModifiers
	public static class FormattableStringFactory
	{
		[MethodImpl(MethodImplOptions.AggressiveInlining)]
#pragma warning disable CS0436 
		public static FormattableString Create(string s, params object[] args) => new FormattableString(s, args);
#pragma warning restore CS0436
	}
}

namespace System
{
	public struct FormattableString : IFormattable
	{
		private readonly string _s;
		private readonly object[] _args;

		public FormattableString(string s, object[] args)
		{
			_s = s;
			_args = args;
		}

		[MethodImpl(MethodImplOptions.AggressiveInlining)]
		public override string ToString() => string.Format(_s, _args);

		[MethodImpl(MethodImplOptions.AggressiveInlining)]
		public string ToString(string ignored, IFormatProvider formatProvider) => string.Format(formatProvider, _s, _args);
	}
}
#pragma warning restore 1591

namespace Samples
{
	class Program
	{
		static void Assert(bool x, string message)
		{
			if (!x)
				throw new InvalidOperationException(message);
		}
		static void Assert(bool x, string messageFormat, params object[] args)
		{
			if (!x)
				throw new InvalidOperationException(string.Format(messageFormat, args));
		}

#pragma warning disable CS0436
		static void Assert(bool x, FormattableString messageFormat)
#pragma warning restore CS0436
		{
			if (!x)
				throw new InvalidOperationException(messageFormat.ToString());
		}

		static void Main(string[] args)
		{
			const int Count = 10 * 1000 * 1000;

			int x = 2;

			var sw = Stopwatch.StartNew();
			for (int i = 0; i < Count; i++)
			{
				Assert(x > 0, "Value of x ({0}) should be greater than zero.", x);
			}
			var t1 = sw.Elapsed;

			sw.Restart();
			for (int i = 0; i < Count; i++)
			{
				Assert(x > 0, $"Value of x ({x}) should be greater than zero.");
			}
			var t2 = sw.Elapsed;

			sw.Restart();
			for (int i = 0; i < Count; i++)
			{
#pragma warning disable CS0436
				Assert(x > 0, (FormattableString)$"Value of x ({x}) be should be greater than zero.");
#pragma warning restore CS0436
			}
			var t3 = sw.Elapsed;

			Console.WriteLine("Format+args:           " + t1.TotalMilliseconds);
			Console.WriteLine("Interpolated:          " + t2.TotalMilliseconds);
			Console.WriteLine("Formattable(explicit): " + t3.TotalMilliseconds);

			Console.WriteLine("\r\nDone.");
			Console.ReadKey();
		}
	}
}