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) {

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

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

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

void Method(Entity entity) {
  if(entity == null) {
    throw new ArgumentNullException(nameof(entity));
  } else if(entity.Some.Property > 0) {
    throw new ArgumentException(stringanize(entity.Some.Property > 0), nameof(entity));
  }//if
  
  // "stringanize" makes a string from an expression
  // and when we are renamed "Some" or "Property"
  // string is updated!

ViIvanov

using System;

class Program
{
  static void Main() {
    var x = DateTime.Now.Day > 10 ? 1 : new Program() ?? default(object);
  }
}

ViIvanov

// Overload of Debug.Assert with detailMessageFormat does not supported

// Debug.Assert has an overload with detailMessageFormat parameter: https://msdn.microsoft.com/en-us/library/cc190597(v=vs.110).aspx.

// This overload does not supported by R#:
// * No syntax highlighting and no validation of format arguments
// * Code analysises does not recognize this calls (null check not works)

class Program
{

ViIvanov

using System;
using System.Collections.Generic;

static class Program
{
  static void Add(IReadOnlyCollection<object> items) { }
  static void Add(IEnumerable<IReadOnlyCollection<object>> items) { }

  static void Main() {
    var list = new List<object[]>();

ViIvanov

using System;
using System.Linq;

static class Program
{
  static void Main(string[] args) {
    var lengths = args.Select(item => item.Length);
    // "lengths" can be enumerated a few times, depending on a size of "args".
    var xxx = args.Select(item => lengths.Select(x => x));
  }

ViIvanov

using System.Collections.Generic;

// ReSharper disable once CheckNamespace
class MultiDictionary<TKey, TValue> : Dictionary<TKey, IList<TValue>>
{
  public void Add(TKey key, params TValue[] values) { }
}

static class Program
{