C# Online Compiler | .NET Fiddle

using System;
using System.Reflection;
using System.Collections.Generic;
using System.Linq;
using System.Text;

public class Program
{
	public class Holder
	{
		public string SomeProperty
		{
			get;
			set;
		}

		public Holder()
		{
		}

		public static IEnumerable<object> SomeObjects()
		{
			var anons = new List<object>();
			var myAnonymousType = new
			{
			Number1 = 5, Number2 = 7, String1 = ""
			}

			;
			anons.Add(myAnonymousType);
			return anons;
		}

		public static void Analyse<T>(IEnumerable<T> input)
		{
			var properties = typeof(T).GetProperties();
			//var properties = input.GetType().GetGenericArguments()[0].GetProperties(); // Doesn't work either
			foreach (var item in properties)
			{
				Console.WriteLine(item.Name);
			}
			
			// By input it does work
			var inputProperties = input.First().GetType().GetProperties();
			foreach (var item in inputProperties)
			{
				//Console.WriteLine(item.Name);
			}
			
		}
	}

	public static void Main()
	{
		var holder = new Holder()
		{SomeProperty = "testing"};
		var properties = holder.GetType().GetProperties();
		
		Console.WriteLine("---Start 1---");
		foreach (var item in properties)
		{
			Console.WriteLine(item.Name);
		}
		Console.WriteLine("---End 1---");

		var myAnonymousType = new
		{
		Number1 = 5, Number2 = 7, String1 = ""
		};
		var anonProperties = myAnonymousType.GetType().GetProperties();
		
		Console.WriteLine("---Start 2---");
		foreach (var item in anonProperties)
		{
			Console.WriteLine(item.Name);
		}
		Console.WriteLine("---End 2---");

		Console.WriteLine("---Start 3---");
		var someObjects = Holder.SomeObjects();
		Holder.Analyse<object>(someObjects);
		Console.WriteLine("---End 3---");
	}
}
Cached Result
0.00500
0.005000
0.0050001
0.00500008
0.005000080