ViIvanov

static class C
{
  static object M0<T>(IEnumerable<T> source) {
    // linq с новой строки
    var e1 = 
      from item in source
      select item.ToString();
    return e1;
  }

ViIvanov

using System.Collections.Generic;
using System.Collections.ObjectModel;

internal static class Program
{
  private static TValue GetValueOrDefault<TKey, TValue>(this IDictionary<TKey, TValue> source, TKey key) { return default(TValue); }
  private static TValue GetValueOrDefault<TKey, TValue>(this IReadOnlyDictionary<TKey, TValue> source, TKey key) { return default(TValue); }

  private static void Main() {
    var dictionary = new Dictionary<int, int>();

ViIvanov

using System;
using System.ComponentModel;
using System.Diagnostics;
using System.Globalization;

namespace Xxx.ComponentModel
{
  public abstract class CustomPropertyDescriptor : PropertyDescriptor
  {
    protected CustomPropertyDescriptor(MemberDescriptor descr) : base(descr) { }

ViIvanov

void M<T>(ICollection<T> source) {
  var a = source.Any(); // OK, statically known, that source is ICollection<T> and call is optimized
  var b = source.Count(); // Warning, source.Count is better
}

bool M1<T>(IEnumerable<T> source) {
  return source.Any(); // OK - one iteration in a worst case
}

int M1<T>(IEnumerable<T> source) {