更新记录
转载请注明出处：
2022年9月25日 发布。
2022年9月10日 从笔记迁移到博客。
== 和 Equals()区别
如果是引用类型，==运算符比较的是引用的对象是否指向同一个内存区域。 而Equals() 方法检查的是值是否相等。
Convert.ToString()和ToString()区别
Convert.ToString()和ToString()的区别是前者可以处理Null值，后者不可以。
获取控制台应用命令行参数
方法一：使用Main方法的参数
static void Main(string[] args)
{
foreach (string arg in args)
{
Console.WriteLine(arg);
}
Console.ReadKey();
}
注意：这种方法获得的参数不包含程序名
方法二：使用Environment对象
static void Main(string[] args)
{
string[] userInputArgs = Environment.GetCommandLineArgs();
foreach (string arg in userInputArgs)
{
Console.WriteLine(arg);
}
Console.ReadKey();
}
获取控制台输入字符
static void Main(string[] args)
{
string userInputLine = Console.ReadLine();
Console.WriteLine(userInputLine);
Console.ReadKey();
}
如何减少unbox和boxing
方法一：定义类型的时候，使用泛型方法、或者泛型类型
方法二：定义重载方法
参考：
https://msdn.microsoft.com/en-us/library/system.object.aspx
中的Remark
标准相等处理（Standard Equality Protocols）
说明
默认情况下
值类型使用对值进行比较来确定相等
引用类型使用对引用进行比较来确定相等
可以Override默认的相等来实现自定义的相等比较
自定义相等比较的方法：
The == and != operators
The virtual Equals method in object
The IEquatable
标准Override相等的方法：
Override GetHashCode() and Equals()
(Optionally) overload != and ==
(Optionally) implement IEquatable
实例：结构标准Override
public struct Area : IEquatable <Area>
{
public readonly int Measure1;
public readonly int Measure2;
public Area (int m1, int m2)
{
Measure1 = Math.Min (m1, m2);
Measure2 = Math.Max (m1, m2);
}
public override bool Equals (object other)
{
if (!(other is Area)) return false;
return Equals ((Area) other); // Calls method below
}
public bool Equals (Area other) // Implements IEquatable<Area>
=> Measure1 == other.Measure1 && Measure2 == other.Measure2;
public override int GetHashCode()
=> HashCode.Combine (Measure1, Measure2);
public static bool operator == (Area a1, Area a2) => a1.Equals (a2);
public static bool operator != (Area a1, Area a2) => !a1.Equals (a2);
}
相等重写最佳实践
重写equality operators (Equals(), ==, and !=)
重写GetHashCode() on value types if equality is meaningful.
实现IEquatable
Equals比较
使用Equals比较如果参数为Null，会抛出NullReferenceException异常
可以使用下面这种Override的方式进行重写：
public static bool AreEqual (object obj1, object obj2)
{
if (obj1 == null) return obj2 == null;
return obj1.Equals (obj2);
}
实例：注意==和Equal的差异
double x = double.NaN;
Console.WriteLine (x == x); // False
Console.WriteLine (x.Equals (x)); // True
实例：注意==和Equal的差异
var sb1 = new StringBuilder ("foo");
var sb2 = new StringBuilder ("foo");
Console.WriteLine (sb1 == sb2); // False (referential equality)
Console.WriteLine (sb1.Equals (sb2)); // True (value equality)
object.Equals静态方法
public static bool Equals (object objA, object objB)
实例：
object x = 3, y = 3;
Console.WriteLine (object.Equals (x, y)); // True
x = null;
Console.WriteLine (object.Equals (x, y)); // False
y = null;
Console.WriteLine (object.Equals (x, y)); // True
object.ReferenceEquals静态方法
实例：
Widget w1 = new Widget();
Widget w2 = new Widget();
Console.WriteLine (object.ReferenceEquals (w1, w2)); // False
IEquatable接口（IEquatable interface）
public interface IEquatable<T>
{
bool Equals (T other);
}
实例：
class Test<T> where T : IEquatable<T>
{
public bool IsEqual (T a, T b)
{
return a.Equals (b); // No boxing with generic T
}
}
标准比较处理（Standard Order Comparison）
标准比较重写
使用IComparable interfaces (IComparable and IComparable
重写 > and < operators
IComparable接口
大部分的基础类型都实现了IComparable接口
IComparable通常用于排序比较、集合中
具体的接口定义如下：
public interface IComparable
{
int CompareTo(object other);
}
public interface IComparable<in T>
{
int CompareTo(T other);
}
实例：string类型已经预定义了IComparable接口
string[] colors = { "Green", "Red", "Blue" };
Array.Sort (colors);
foreach (string c in colors) Console.Write (c + " "); // Blue Green Red
实例：结构重写IComparable接口和IEqualable接口
public struct Note : IComparable<Note>, IEquatable<Note>, IComparable
{
int _semitonesFromA;
public int SemitonesFromA { get { return _semitonesFromA; } }
public Note (int semitonesFromA)
{
_semitonesFromA = semitonesFromA;
}
public int CompareTo (Note other) // Generic IComparable<T>
{
if (Equals (other)) return 0; // Fail-safe check
return _semitonesFromA.CompareTo (other._semitonesFromA);
}
int IComparable.CompareTo (object other) // Nongeneric IComparable
{
if (!(other is Note))
throw new InvalidOperationException ("CompareTo: Not a note");
return CompareTo ((Note) other);
}
public static bool operator < (Note n1, Note n2)
=> n1.CompareTo (n2) < 0;
public static bool operator > (Note n1, Note n2)
=> n1.CompareTo (n2) > 0;
public bool Equals (Note other) // for IEquatable<Note>
=> _semitonesFromA == other._semitonesFromA;
public override bool Equals (object other)
{
if (!(other is Note)) return false;
return Equals ((Note) other);
}
public override int GetHashCode() => _semitonesFromA.GetHashCode();
public static bool operator == (Note n1, Note n2) => n1.Equals (n2);
public static bool operator != (Note n1, Note n2) => !(n1 == n2);
}
重写 > and < operators
大部分基础类型都重写了>和<运算符
实例：
bool after2010 = DateTime.Now > new DateTime (2010, 1, 1);
类型不同但成员相同的比较
实例：
object[] a1 = { "string", 123, true };
object[] a2 = { "string", 123, true };
Console.WriteLine (a1 == a2); // False
Console.WriteLine (a1.Equals (a2)); // False
IStructuralEquatable se1 = a1;
Console.WriteLine (se1.Equals (a2,
StructuralComparisons.StructuralEqualityComparer)); // True