c# Equal函数 and 运算符'==' （原发布 csdn 2017年10月15日 20:39:26）

1、==、!=、<、>、<= 和>= 运算符为比较运算符(comparison operator)。C#语言规范5.0中文版中比较运算符的描述如下：

2、通用类型系统

3、值类型Equal函数 and 运算符'=='

3.1、常见类型 int、float、double、decimal等虽然继承自ValueType，但其结构体内部重写了Equal。

3.1.1、 int，float，double，decimal内部的Equal函数和 '=='重载符函数。

Int32
{
public override bool Equals(Object obj) {
if (!(obj is Int32)) {
return false;
}
return m_value == ((Int32)obj).m_value;
}

[System.Runtime.Versioning.NonVersionable]
public bool Equals(Int32 obj)
{
return m_value == obj;
}
}

Double
{
// True if obj is another Double with the same value as the current instance.  This is
// a method of object equality, that only returns true if obj is also a double.
public override bool Equals(Object obj) {
if (!(obj is Double)) {
return false;
}
double temp = ((Double)obj).m_value;
// This code below is written this way for performance reasons i.e the != and == check is intentional.
if (temp == m_value) {
return true;
}
return IsNaN(temp) && IsNaN(m_value);
}

public bool Equals(Double obj)
{
if (obj == m_value) {
return true;
}
return IsNaN(obj) && IsNaN(m_value);
}

[System.Runtime.Versioning.NonVersionable]
public static bool operator ==(Double left, Double right) {
return left == right;
}
}

Single
{
public override bool Equals(Object obj) {
if (!(obj is Single)) {
return false;
}
float temp = ((Single)obj).m_value;
if (temp == m_value) {
return true;
}

return IsNaN(temp) && IsNaN(m_value);
}

public bool Equals(Single obj)
{
if (obj == m_value) {
return true;
}

return IsNaN(obj) && IsNaN(m_value);
}

[System.Runtime.Versioning.NonVersionable]
public static bool operator ==(Single left, Single right) {
return left == right;
}
}

Decimal
{
// Checks if this Decimal is equal to a given object. Returns true
// if the given object is a boxed Decimal and its value is equal to the
// value of this Decimal. Returns false otherwise.
//
[System.Security.SecuritySafeCritical]  // auto-generated
public override bool Equals(Object value) {
if (value is Decimal) {
Decimal other = (Decimal)value;
return FCallCompare(ref this, ref other) == 0;
}
return false;
}

[System.Security.SecuritySafeCritical]  // auto-generated
public bool Equals(Decimal value)
{
return FCallCompare(ref this, ref value) == 0;
}

[System.Security.SecuritySafeCritical]  // auto-generated
public static bool operator ==(Decimal d1, Decimal d2) {
return FCallCompare(ref d1, ref d2) == 0;
}

//暂时不知道此函数内部代码，如有知道还望告知。
//根据测试结果，推测如果两个decimal数相等，返回0
[System.Security.SecurityCritical]  // auto-generated
[ResourceExposure(ResourceScope.None)]
[MethodImplAttribute(MethodImplOptions.InternalCall)]
[ReliabilityContract(Consistency.WillNotCorruptState, Cer.Success)]
private static extern int FCallCompare(ref Decimal d1, ref Decimal d2);

}

3.1.2、感兴趣的可去Reference Source查看全部代码。

3.1.3、测试代码：

//T is int 、float、double、decimal、byte、char
T a = 1234567890;//0.1234567890f、0.123456789、1234567890M、(byte)11、'a'
T b = 1234567890;//0.1234567890f、0.123456789、1234567890M、(byte)11、'a'

Console.WriteLine(a == b);//返回true
Console.WriteLine(a.Equals(b));//返回true
Console.WriteLine(a.Equals((object)b));//返回true

/*
Console.WriteLine((object)a == b);//编译错误：运算符‘==’无法应用与‘object’和‘T’类型操作数
Console.WriteLine(a == (object)b);//编译错误：运算符‘==’无法应用与‘object’和‘T’类型操作数
//Console.WriteLine((object)a == (object)b);//返回false，下面解释为什么是false。这个是引用类型'=='，放到下文介绍
*/

3.1.4、结论：对于简单常见值类型 int、float、double、decimal等，Equal函数 and 运算符'=='，如果其值相等，返回true；否则，返回false。

3.2、 结构体struct

3.2.1、 ValueType内部的Equals函数

ValueType
{
[System.Security.SecuritySafeCritical]
public override bool Equals (Object obj) {
BCLDebug.Perf(false, "ValueType::Equals is not fast.  "+this.GetType().FullName+" should override Equals(Object)");
if (null==obj) {
return false;
}
RuntimeType thisType = (RuntimeType)this.GetType();
RuntimeType thatType = (RuntimeType)obj.GetType();

if (thatType!=thisType) {
return false;
}

Object thisObj = (Object)this;
Object thisResult, thatResult;

// if there are no GC references in this object we can avoid reflection
// and do a fast memcmp
if (CanCompareBits(this))
return FastEqualsCheck(thisObj, obj);

FieldInfo[] thisFields = thisType.GetFields(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic);

for (int i=0; i<thisFields.Length; i++) {
thisResult = ((RtFieldInfo)thisFields[i]).UnsafeGetValue(thisObj);
thatResult = ((RtFieldInfo)thisFields[i]).UnsafeGetValue(obj);

if (thisResult == null) {
if (thatResult != null)
return false;
}
else
if (!thisResult.Equals(thatResult)) {
return false;
}
}

return true;
}

[System.Security.SecuritySafeCritical]  // auto-generated
[ResourceExposure(ResourceScope.None)]
[MethodImplAttribute(MethodImplOptions.InternalCall)]
private static extern bool CanCompareBits(Object obj);

[System.Security.SecuritySafeCritical]  // auto-generated
[ResourceExposure(ResourceScope.None)]
[MethodImplAttribute(MethodImplOptions.InternalCall)]
private static extern bool FastEqualsCheck(Object a, Object b);
}

3.2.2、结构体(只有值类型，重写Equal函数 and 运算符'==')

3.2.2.1、测试代码：

struct Point
{
public double x;
public double y;
public double z;

public Point(double X, double Y, double Z)
{
this.x = X;
this.y = Y;
this.z = Z;
}

public override bool Equals(Object obj)
{
if (!(obj is Point))
{
return false;
}

if (((Point)obj).x == this.x)
{
return true;
}

return false;
}
public bool Equals(Point obj)
{
if (obj.x == this.x)
{
return true;
}

return false;
}

//运算符“Point.operator ==(Point, Point)”要求也要定义匹配的运算符“!=”
public static bool operator ==(Point left, Point right)
{
return left.x == right.x;
}

public static bool operator !=(Point left, Point right)
{
return left.x != right.x;
}
}

Point p1 = new Point(1, 2, 3);
Point p2 = p1;

p1.y = 100;
Console.WriteLine(p1 == p2);//返回true
Console.WriteLine(p1.Equals(p2)); // 返回true
Console.WriteLine(p1.Equals((object)p2)); // 返回true

3.2.2.2、结论：此时程序执行我们重写的Equal函数 and 运算符'=='。

3.2.3、结构体(只有值类型，不重写Equal函数 and 运算符'==')

3.2.3.1、测试代码：

struct Point
{
public double x;
public double y;
public double z;

public Point(double X, double Y, double Z)
{
this.x = X;
this.y = Y;
this.z = Z;
}
}

Point p1 = new Point(1, 2, 3);
Point p2 = p1;

Console.WriteLine(p1 == p2);//编译错误：运算符"=="无法应用于"Point"和"Point"类型的操作数
Console.WriteLine(p1.Equals(p2)); // 返回true
Console.WriteLine(p1.Equals((object)p2)); // 返回true
p1.y = 100;
Console.WriteLine(p1.Equals(p2)); // 返回false
Console.WriteLine(p1.Equals((object)p2)); // 返回false

3.2.3.2、程序执行时，CanCompareBits(this)返回true，代码执行return FastEqualsCheck(thisObj, obj);

3.2.3.3、结论：程序判断struct里面所有字段的值，如果全部相等，返回true；否则，返回false。

3.2.4、复杂结构体(有值类型、引用类型，重写Equal函数 and 运算符'==')

3.2.4.1、测试代码：

public struct ValPoint
{
public double x;
public double y;
public double z;

public ValPoint(double X, double Y, double Z)
{
this.x = X;
this.y = Y;
this.z = Z;
}

public static bool operator ==(ValPoint left, ValPoint right)
{
return left.x == right.x;
}

public static bool operator !=(ValPoint left, ValPoint right)
{
return left.x != right.x;
}
}

public class RefPoint
{
public double x;
public double y;
public double z;

public RefPoint(double X, double Y, double Z)
{
this.x = X;
this.y = Y;
this.z = Z;
}
}

public struct ValLine
{
public ValPoint vPoint;       // 值类型成员

public RefPoint rPoint;       // 引用类型成员

public ValLine(ValPoint vPoint, RefPoint rPoint)
{
this.vPoint = vPoint;
this.rPoint = rPoint;
}

public override bool Equals(Object obj)
{
if (!(obj is ValLine))
{
return false;
}

if (((ValLine)obj).vPoint == this.vPoint)
{
return true;
}

return false;
}

public bool Equals(ValLine obj)
{
if (obj.vPoint == this.vPoint)
{
return true;
}

return false;
}

public static bool operator ==(ValLine left, ValLine right)
{
return left.vPoint == right.vPoint;
}

public static bool operator !=(ValLine left, ValLine right)
{
return left.vPoint != right.vPoint;
}
}

ValPoint vPoint = new ValPoint(1, 2, 3);
ValPoint vPoint2 = new ValPoint(1, 2, 3);
ValPoint vPoint3 = new ValPoint(10, 20, 30);
RefPoint rPoint = new RefPoint(4, 5, 6);
RefPoint rPoint2 = new RefPoint(7, 8, 9);

ValLine p1 = new ValLine(vPoint, rPoint);
ValLine p2 = p1;

p2.vPoint = vPoint2;
Console.WriteLine(p1 == p2); //返回true
Console.WriteLine(p1.Equals(p2)); //返回true
Console.WriteLine(p1.Equals((object)p2)); //返回true

p2 = p1;
p2.vPoint = vPoint3;
Console.WriteLine(p1 == p2); //返回true
Console.WriteLine(p1.Equals(p2)); //返回false
Console.WriteLine(p1.Equals((object)p2)); //返回false

p2 = p1;
p2.rPoint = rPoint2;
Console.WriteLine(p1 == p2); //返回true
Console.WriteLine(p1.Equals(p2)); //返回true
Console.WriteLine(p1.Equals((object)p2)); //返回true

3.2.4.2、结论：此时程序执行我们重写的Equal函数 and 运算符'=='。

3.2.5、复杂结构体(内部值类型、引用类型，不重写Equal函数 and 运算符'==')

3.2.5.1、测试代码：

public struct ValPoint
{
public double x;
public double y;
public double z;

public ValPoint(double X, double Y, double Z)
{
this.x = X;
this.y = Y;
this.z = Z;
}
}

public class RefPoint
{
public double x;
public double y;
public double z;

public RefPoint(double X, double Y, double Z)
{
this.x = X;
this.y = Y;
this.z = Z;
}
}

public struct ValLine
{
public ValPoint vPoint;       // 值类型成员

public RefPoint rPoint;       // 引用类型成员

public ValLine(ValPoint vPoint, RefPoint rPoint)
{
this.vPoint = vPoint;
this.rPoint = rPoint;
}
}

ValPoint vPoint = new ValPoint(1, 2, 3);
ValPoint vPoint2 = new ValPoint(1, 2, 3);
ValPoint vPoint3 = new ValPoint(10, 20, 30);
RefPoint rPoint = new RefPoint(4, 5, 6);
RefPoint rPoint2 = new RefPoint(7, 8, 9);

ValLine p1 = new ValLine(vPoint, rPoint);
ValLine p2 = p1;

Console.WriteLine(p1 == p2);//编译错误：运算符"=="无法应用于"Point"和"Point"类型的操作数

p2.vPoint = vPoint2;
Console.WriteLine(p1.Equals(p2)); //返回true
Console.WriteLine(p1.Equals((object)p2)); //返回true

p2 = p1;
p2.vPoint = vPoint3;
Console.WriteLine(p1.Equals(p2)); //返回false
Console.WriteLine(p1.Equals((object)p2)); //返回false

p2 = p1;
p2.rPoint = rPoint2;
Console.WriteLine(p1.Equals(p2)); //返回false
Console.WriteLine(p1.Equals((object)p2)); //返回false

3.2.5.2、程序执行时，CanCompareBits(this)返回false，代码执行ValueType类Equal函数的下面语句

FieldInfo[] thisFields = thisType.GetFields(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic);

for (int i=0; i<thisFields.Length; i++) {
thisResult = ((RtFieldInfo)thisFields[i]).UnsafeGetValue(thisObj);
thatResult = ((RtFieldInfo)thisFields[i]).UnsafeGetValue(obj);

if (thisResult == null) {
if (thatResult != null)
return false;
}
else
if (!thisResult.Equals(thatResult)) {
return false;
}
}

return true;

3.2.5.3、结论：程序判断struct里面所有字段,值类型就判断值是否相等；引用类型就判断是否引用相等。

4、引用类型Equal函数 and 运算符'=='

4.1、字符串string

4.1.1、C#语言规范5.0中文版中的字符串相等运算符介绍

4.1.2、string的Equal函数和'=='重载运算符函数代码

String
{
// Determines whether two strings match.
[ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail)]
public override bool Equals(Object obj) {
if (this == null)                        //this is necessary to guard against reverse-pinvokes and
throw new NullReferenceException();  //other callers who do not use the callvirt instruction

String str = obj as String;
if (str == null)
return false;

if (Object.ReferenceEquals(this, obj))
return true;

if (this.Length != str.Length)
return false;

return EqualsHelper(this, str);
}

// Determines whether two strings match.
[Pure]
[ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail)]
public bool Equals(String value) {
if (this == null)                        //this is necessary to guard against reverse-pinvokes and
throw new NullReferenceException();  //other callers who do not use the callvirt instruction

if (value == null)
return false;

if (Object.ReferenceEquals(this, value))
return true;

if (this.Length != value.Length)
return false;

return EqualsHelper(this, value);
}

public static bool operator == (String a, String b) {
return String.Equals(a, b);
}

// Determines whether two Strings match.
[Pure]
public static bool Equals(String a, String b) {
if ((Object)a==(Object)b) {
return true;
}

if ((Object)a==null || (Object)b==null) {
return false;
}

if (a.Length != b.Length)
return false;

return EqualsHelper(a, b);
}

[System.Security.SecuritySafeCritical]  // auto-generated
[ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail)]
private unsafe static bool EqualsHelper(String strA, String strB)
{
Contract.Requires(strA != null);
Contract.Requires(strB != null);
Contract.Requires(strA.Length == strB.Length);

int length = strA.Length;

fixed (char* ap = &strA.m_firstChar) fixed (char* bp = &strB.m_firstChar)
{
char* a = ap;
char* b = bp;

// unroll the loop
#if AMD64
// for AMD64 bit platform we unroll by 12 and
// check 3 qword at a time. This is less code
// than the 32 bit case and is shorter
// pathlength

while (length >= 12)
{
if (*(long*)a     != *(long*)b) return false;
if (*(long*)(a+4) != *(long*)(b+4)) return false;
if (*(long*)(a+8) != *(long*)(b+8)) return false;
a += 12; b += 12; length -= 12;
}
#else
while (length >= 10)
{
if (*(int*)a != *(int*)b) return false;
if (*(int*)(a+2) != *(int*)(b+2)) return false;
if (*(int*)(a+4) != *(int*)(b+4)) return false;
if (*(int*)(a+6) != *(int*)(b+6)) return false;
if (*(int*)(a+8) != *(int*)(b+8)) return false;
a += 10; b += 10; length -= 10;
}
#endif

// This depends on the fact that the String objects are
// always zero terminated and that the terminating zero is not included
// in the length. For odd string sizes, the last compare will include
// the zero terminator.
while (length > 0)
{
if (*(int*)a != *(int*)b) break;
a += 2; b += 2; length -= 2;
}

return (length <= 0);
}
}

}

4.1.3、Object.ReferenceEquals(this, value)如果this、value是同一个引用，返回true；否则，返回false。

{
string a = "a1!";
string b = "a1!";
Console.WriteLine(Object.ReferenceEquals(a, b));//返回true,可以判断编译器将a与b所指向的"a1!"优化成一个地方。
}

{
string a = "Test";
string b = string.Copy(a);
Console.WriteLine(Object.ReferenceEquals(a, b));//返回false
}

{
string a = "Test";
string b = (string)a.Clone();
Console.WriteLine(Object.ReferenceEquals(a, b));//返回true
}

{
char[] ch = new char[] { 'a', 'A', '@' };
string a = "aA@";
string b = new string(ch);
Console.WriteLine(Object.ReferenceEquals(a, b));//返回false
}

4.1.4、学习EqualsHelper(String strA, String strB)函数之前，我们先看一段代码

unsafe
{
char[] firstCharA = "abc".ToCharArray();
int length = firstCharA.Length;
fixed (char* ap = firstCharA)
{
for (int i = 0; i < length; i++)
{
Console.WriteLine(*(char*)(ap + i));
}
}
}

unsafe
{
int[] firstCharA = new int[] { 1, 20, 300 };
int length = firstCharA.Length;
fixed (int* ap = firstCharA)
{
for (int i = 0; i < length; i++)
{
Console.WriteLine(*(int*)(ap + i));
}
}
}

4.1.5、修改后EqualsHelper(String strA, String strB)函数

private unsafe static bool EqualsHelper(String strA, String strB)
{
Contract.Requires(strA != null);
Contract.Requires(strB != null);
Contract.Requires(strA.Length == strB.Length);

int length = strA.Length;

char[] firstCharA = strA.ToCharArray();
char[] firstCharB = strB.ToCharArray();

fixed (char* ap = &firstCharA[0]) fixed (char* bp = &firstCharB[0])//因无法使用m_firstChar，此处是我自行修改。ps：个人认为m_firstChar是指字符串的第一字符，但是无法证明。
//fixed (char* ap = &strA.m_firstChar) fixed (char* bp = &strB.m_firstChar)
{
char* a = ap;
char* b = bp;

while (length >= 10)
{
if (*(int*)a != *(int*)b) return false;
if (*(int*)(a + 2) != *(int*)(b + 2)) return false;
if (*(int*)(a + 4) != *(int*)(b + 4)) return false;
if (*(int*)(a + 6) != *(int*)(b + 6)) return false;
if (*(int*)(a + 8) != *(int*)(b + 8)) return false;
a += 10; b += 10; length -= 10;
}

// This depends on the fact that the String objects are
// always zero terminated and that the terminating zero is not included
// in the length. For odd string sizes, the last compare will include
// the zero terminator.
while (length > 0)
{
if (*(int*)a != *(int*)b) break;
a += 2; b += 2; length -= 2;
}

return (length <= 0);
}
}

4.1.6、修改说明

1、fixed (char* ap = &strA.m_firstChar) fixed (char* bp = &strB.m_firstChar)-> fixed (char* ap = &firstCharA[0]) fixed (char* bp = &firstCharB[0])
2、(*(int*)a->获取的数据是两个char值（低位ASCII*65536+高位ASCII）[低位在前，高位在后]。 [char两个字节,范围U+0000到U+FFFF]
3、(*(char*)a->获取的数据是一个char值[见上面测试例子]

4.1.7、测试EqualsHelper(String strA, String strB)函数

{
string a = "abcd";
string b = "abcd";
Console.WriteLine(EqualsHelper(a,b));//返回true
}

{
string a = "Test";
string b = string.Copy(a);
Console.WriteLine(EqualsHelper(a, b));//返回true
}

{
string a = "Test";
string b = (string)a.Clone();
Console.WriteLine(EqualsHelper(a, b));//返回true
}

{
char[] ch = new char[] { 'a', 'A', '@' };
string a = "aA@";
string b = new string(ch);
Console.WriteLine(EqualsHelper(a, b));//返回true
}

4.1.8、结论：string类型 a == b、string.Equals(a, b)、a.Equals(b)、a.Equals((object)b)，如果 a 的值与 b 的值相同，则为 true；否则为 false。

4.2、类class

4.2.1、C#语言规范5.0中文版中的引用类型相等运算符介绍

4.2.2、Object内部的Equals函数

Object
{
public virtual bool Equals(Object obj)
{
return RuntimeHelpers.Equals(this, obj);//无法查到详细代码
}

public static bool Equals(Object objA, Object objB)
{
if (objA==objB) {
return true;
}
if (objA==null || objB==null) {
return false;
}
return objA.Equals(objB);
}

[ReliabilityContract(Consistency.WillNotCorruptState, Cer.Success)]
[System.Runtime.Versioning.NonVersionable]
public static bool ReferenceEquals (Object objA, Object objB) {
return objA == objB;
}
}

4.2.3、类(不重写Equal函数 and 运算符'==')

public class RefPoint
{
public double x;
public double y;
public double z;

public RefPoint(double X, double Y, double Z)
{
this.x = X;
this.y = Y;
this.z = Z;
}
}

RefPoint p1 = new RefPoint(4, 5, 6);
RefPoint p2 = p1;
Console.WriteLine(p1.Equals(p2));//返回true
Console.WriteLine(object.Equals(p1, p2));//返回true
Console.WriteLine(object.ReferenceEquals(p1, p2));//返回true
Console.WriteLine(p1 == p2);//返回true

p2 = new RefPoint(4, 5, 6);//虽然值一样，但是引用对象不一样
Console.WriteLine(p1.Equals(p2));//返回false
Console.WriteLine(object.Equals(p1, p2));//返回false
Console.WriteLine(object.ReferenceEquals(p1, p2));//返回false
Console.WriteLine(p1 == p2);//返回false

4.2.4、类(重写Equal函数 and 运算符'==')

public class RefPoint
{
public double x;
public double y;
public double z;

public RefPoint(double X, double Y, double Z)
{
this.x = X;
this.y = Y;
this.z = Z;
}

public override bool Equals(Object obj)
{
if (!(obj is RefPoint))
{
return false;
}

if (((RefPoint)obj).x == this.x)
{
return true;
}

return false;
}

public bool Equals(RefPoint obj)
{
if (obj.x == this.x)
{
return true;
}

return false;
}

public static bool operator ==(RefPoint left, RefPoint right)
{
return left.x == right.x;
}

public static bool operator !=(RefPoint left, RefPoint right)
{
return left.x != right.x;
}
}

RefPoint p1 = new RefPoint(4, 5, 6);
RefPoint p2 = p1;
Console.WriteLine(p1.Equals(p2));//返回true
Console.WriteLine(object.Equals(p1, p2));//返回true
Console.WriteLine(object.ReferenceEquals(p1, p2));//返回true
Console.WriteLine(p1 == p2);//返回true

p2 = new RefPoint(4, 50, 60);
Console.WriteLine(p1.Equals(p2));//返回true
Console.WriteLine(object.Equals(p1, p2));//返回true
Console.WriteLine(object.ReferenceEquals(p1, p2));//返回false
Console.WriteLine(p1 == p2);//返回true

4.2.5、ReferenceEquals (Object objA, Object objB)返回objA == objB。如果objA、 objB引用同一个对象(只判断是否引用同一个对象，即使我们自行重载了'=='运算符，也没用)，返回true；否则，返回false。

5、总结

先介绍简单值类型，再到结构体，字符串，类。把每个类型Equal和'=='用法做个总结，加深自己记忆的同时，也希望能帮助到你。另：本文只代表本人观点，如果有误，还望告知。

6、参考

6.1、C#类型基础

6.2、 C#语言规范5.0中文版

6.3、Reference Source

转载于:https://www.cnblogs.com/njit-77/p/11468934.html

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