Simple example of C# iterator (generator) decompiled. Decompiled with ILSpy; turned off decompilation for iterators to see details.

decompiled.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Threading;
internal class Test
{
	[CompilerGenerated]
	private sealed class <GetNaturals>d__0 : IEnumerable<int>, IEnumerable, IEnumerator<int>, IEnumerator, IDisposable
	{
		private int <>2__current;
		private int <>1__state;
		public int <i>5__1;
		int IEnumerator<int>.Current
		{
			[DebuggerHidden]
			get
			{
				return this.<>2__current;
			}
		}
		object IEnumerator.Current
		{
			[DebuggerHidden]
			get
			{
				return this.<>2__current;
			}
		}
		[DebuggerHidden]
		IEnumerator<int> IEnumerable<int>.GetEnumerator()
		{
			Test.<GetNaturals>d__0 result;
			if (Interlocked.CompareExchange(ref this.<>1__state, 0, -2) == -2)
			{
				result = this;
			}
			else
			{
				result = new Test.<GetNaturals>d__0(0);
			}
			return result;
		}
		[DebuggerHidden]
		IEnumerator IEnumerable.GetEnumerator()
		{
			return this.System.Collections.Generic.IEnumerable<System.Int32>.GetEnumerator();
		}
		bool IEnumerator.MoveNext()
		{
			bool result;
			switch (this.<>1__state)
			{
			case 0:
				this.<>1__state = -1;
				this.<i>5__1 = 1;
				break;
			case 1:
				this.<>1__state = -1;
				break;
			default:
				result = false;
				return result;
			}
			this.<>2__current = this.<i>5__1++;
			this.<>1__state = 1;
			result = true;
			return result;
		}
		[DebuggerHidden]
		void IEnumerator.Reset()
		{
			throw new NotSupportedException();
		}
		void IDisposable.Dispose()
		{
		}
		[DebuggerHidden]
		public <GetNaturals>d__0(int <>1__state)
		{
			this.<>1__state = <>1__state;
		}
	}
	private static void Main(string[] args)
	{
		IEnumerable<int> naturals = Test.GetNaturals();
		foreach (int current in naturals)
		{
			Console.WriteLine(current);
			if (current > 3)
			{
				break;
			}
		}
	}
	private static IEnumerable<int> GetNaturals()
	{
		return new Test.<GetNaturals>d__0(-2);
	}
}

simplified_decompiled_partial.cs

sealed class NaturalsIterator :
    IEnumerable<int>, IEnumerable,
    IEnumerator<int>, IEnumerator, IDisposable {
  int _current, _state, _i;

  public NaturalsIterator(int state) {
    _state = state;
  }

  int IEnumerator<int>.Current {
    get { return _current; }
  }

  bool IEnumerator.MoveNext() {
    switch (_state) {
    case 0:
      _i = 1;
      break;
    case 1:
      break;
    default:
      return false;
    }
    _current = _i++;
    _state = 1;
    return true;
  }
}

private static IEnumerable<int> GetNaturals() {
  return new NaturalsIterator(0);
}

t.cs

using System;
using System.Collections.Generic;

class Test {
  static void Main(string[] args) {
    IEnumerable<int> naturals = GetNaturals();
    foreach (int i in naturals) {
      Console.WriteLine(i);
      if (i > 3) break;
    }
  }

  static IEnumerable<int> GetNaturals() {
    int i = 1;
    while (true) {
      yield return i++;
    }
  }
}