[英]How to quickly check if two data transfer objects have equal properties in C#?
我有這些數據傳輸對象:
public class Report
{
public int Id { get; set; }
public int ProjectId { get; set; }
//and so on for many, many properties.
}
我不想寫
public bool areEqual(Report a, Report b)
{
if (a.Id != b.Id) return false;
if (a.ProjectId != b.ProjectId) return false;
//Repeat ad nauseum
return true;
}
有沒有更快的方法來測試兩個只有屬性的對象是否具有相同的值(每個屬性不需要一行代碼或一個邏輯表達式?)
切換到結構不是一種選擇。
一些反射怎么樣,也許使用Expression.Compile()
來提高性能? (注意這里的靜態ctor確保我們每T
只編譯一次):
using System;
using System.Linq.Expressions;
public class Report {
public int Id { get; set; }
public int ProjectId { get; set; }
static void Main() {
Report a = new Report { Id = 1, ProjectId = 13 },
b = new Report { Id = 1, ProjectId = 13 },
c = new Report { Id = 1, ProjectId = 12 };
Console.WriteLine(PropertyCompare.Equal(a, b));
Console.WriteLine(PropertyCompare.Equal(a, c));
}
}
static class PropertyCompare {
public static bool Equal<T>(T x, T y) {
return Cache<T>.Compare(x, y);
}
static class Cache<T> {
internal static readonly Func<T, T, bool> Compare;
static Cache() {
var props = typeof(T).GetProperties();
if (props.Length == 0) {
Compare = delegate { return true; };
return;
}
var x = Expression.Parameter(typeof(T), "x");
var y = Expression.Parameter(typeof(T), "y");
Expression body = null;
for (int i = 0; i < props.Length; i++) {
var propEqual = Expression.Equal(
Expression.Property(x, props[i]),
Expression.Property(y, props[i]));
if (body == null) {
body = propEqual;
} else {
body = Expression.AndAlso(body, propEqual);
}
}
Compare = Expression.Lambda<Func<T, T, bool>>(body, x, y)
.Compile();
}
}
}
編輯:更新以處理字段:
static class MemberCompare
{
public static bool Equal<T>(T x, T y)
{
return Cache<T>.Compare(x, y);
}
static class Cache<T>
{
internal static readonly Func<T, T, bool> Compare;
static Cache()
{
var members = typeof(T).GetProperties(
BindingFlags.Instance | BindingFlags.Public)
.Cast<MemberInfo>().Concat(typeof(T).GetFields(
BindingFlags.Instance | BindingFlags.Public)
.Cast<MemberInfo>());
var x = Expression.Parameter(typeof(T), "x");
var y = Expression.Parameter(typeof(T), "y");
Expression body = null;
foreach(var member in members)
{
Expression memberEqual;
switch (member.MemberType)
{
case MemberTypes.Field:
memberEqual = Expression.Equal(
Expression.Field(x, (FieldInfo)member),
Expression.Field(y, (FieldInfo)member));
break;
case MemberTypes.Property:
memberEqual = Expression.Equal(
Expression.Property(x, (PropertyInfo)member),
Expression.Property(y, (PropertyInfo)member));
break;
default:
throw new NotSupportedException(
member.MemberType.ToString());
}
if (body == null)
{
body = memberEqual;
}
else
{
body = Expression.AndAlso(body, memberEqual);
}
}
if (body == null)
{
Compare = delegate { return true; };
}
else
{
Compare = Expression.Lambda<Func<T, T, bool>>(body, x, y)
.Compile();
}
}
}
}
我已經將Marc的代碼擴展為一個完全成熟的IEqualityComparer實現,用於我自己的用途,並認為這可能對將來的其他人有用:
/// <summary>
/// An <see cref="IEqualityComparer{T}"/> that compares the values of each public property.
/// </summary>
/// <typeparam name="T"> The type to compare. </typeparam>
public class PropertyEqualityComparer<T> : IEqualityComparer<T>
{
// http://stackoverflow.com/questions/986572/hows-to-quick-check-if-data-transfer-two-objects-have-equal-properties-in-c/986617#986617
static class EqualityCache
{
internal static readonly Func<T, T, bool> Compare;
static EqualityCache()
{
var props = typeof(T).GetProperties();
if (props.Length == 0)
{
Compare = delegate { return true; };
return;
}
var x = Expression.Parameter(typeof(T), "x");
var y = Expression.Parameter(typeof(T), "y");
Expression body = null;
for (int i = 0; i < props.Length; i++)
{
var propEqual = Expression.Equal(
Expression.Property(x, props[i]),
Expression.Property(y, props[i]));
if (body == null)
{
body = propEqual;
}
else
{
body = Expression.AndAlso(body, propEqual);
}
}
Compare = Expression.Lambda<Func<T, T, bool>>(body, x, y).Compile();
}
}
/// <inheritdoc/>
public bool Equals(T x, T y)
{
return EqualityCache.Compare(x, y);
}
static class HashCodeCache
{
internal static readonly Func<T, int> Hasher;
static HashCodeCache()
{
var props = typeof(T).GetProperties();
if (props.Length == 0)
{
Hasher = delegate { return 0; };
return;
}
var x = Expression.Parameter(typeof(T), "x");
Expression body = null;
for (int i = 0; i < props.Length; i++)
{
var prop = Expression.Property(x, props[i]);
var type = props[i].PropertyType;
var isNull = type.IsValueType ? (Expression)Expression.Constant(false, typeof(bool)) : Expression.Equal(prop, Expression.Constant(null, type));
var hashCodeFunc = type.GetMethod("GetHashCode", BindingFlags.Instance | BindingFlags.Public);
var getHashCode = Expression.Call(prop, hashCodeFunc);
var hashCode = Expression.Condition(isNull, Expression.Constant(0, typeof(int)), getHashCode);
if (body == null)
{
body = hashCode;
}
else
{
body = Expression.ExclusiveOr(Expression.Multiply(body, Expression.Constant(typeof(T).AssemblyQualifiedName.GetHashCode(), typeof(int))), hashCode);
}
}
Hasher = Expression.Lambda<Func<T, int>>(body, x).Compile();
}
}
/// <inheritdoc/>
public int GetHashCode(T obj)
{
return HashCodeCache.Hasher(obj);
}
}
最初回答( 問題1831747 )
查看我的MemberwiseEqualityComparer ,看看它是否符合您的需求。
它非常易於使用且效率也很高。 它使用IL-emit在第一次運行時生成整個Equals和GetHashCode函數(對於每種類型使用一次)。 它將使用該類型的默認相等比較器(EqualityComparer.Default)比較給定對象的每個字段(私有或公共)。 我們已經在生產中使用了一段時間,看起來很穩定,但我不會保證=)
它會處理你在滾動自己的equals方法時很少想到的所有那些pescy edge-cases(也就是說,你不能將你自己的對象與null進行比較,除非你先把它裝在一個對象中並且很多時候關閉它更多與空相關的問題)。
我一直想寫一篇關於它的博客文章,但還沒有找到它。 代碼有點沒有文檔但如果你喜歡它我可以清理一下。
public override int GetHashCode()
{
return MemberwiseEqualityComparer<Foo>.Default.GetHashCode(this);
}
public override bool Equals(object obj)
{
if (obj == null)
return false;
return Equals(obj as Foo);
}
public override bool Equals(Foo other)
{
return MemberwiseEqualityComparer<Foo>.Default.Equals(this, other);
}
MemberwiseEqualityComparer是根據MIT許可證發布的,你可以隨意使用它,包括在專有解決方案中使用它,而不會改變你的許可。
不幸的是,您將不得不編寫比較字段值的方法。 System.ValueType
是建立在使用反射和比較的字段值struct
,但即使這是不妥當的,由於性能下降。 最好的做法是覆蓋Equals
方法,並為強類型Equals
重載實現IEquatable<T>
接口。
當你在這里時,你也可以提供一個好的GetHashCode
覆蓋以補充Equals
實現。 所有這些步驟都被認為是良好的做法。
您需要使用反射來執行此操作,請點擊此鏈接 - > 在c#中比較對象屬性
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