我有一个包含大约 20 个属性的组件的数据库。要了解是否需要更新,我想检查两个对象的所有属性(日期创建和 Id 除外)是否匹配。如果全部匹配没有更新,如果没有,则更新数据库。
Component comp_InApp = new Component()
{
Id = null,
Description = "Commponent",
Price = 100,
DateCreated = "2019-01-30",
// Twenty more prop
};
Component comp_InDb = new Component()
{
Id = 1,
Description = "Component",
Price = 100,
DateCreated = "2019-01-01",
// Twenty more prop
};
// Check if all properties match, except DateCreated and Id.
if (comp_InApp.Description == comp_InDb.Description &&
comp_InApp.Price == comp_InDb.Price
// Twenty more prop
)
{
// Everything up to date.
}
else
{
// Update db.
}
这有效,但对于 20 个属性,这不是一种非常干净的方式。有没有更好的方法以更干净的方式获得相同的结果?
一种方法是创建一个实现IEqualityComparer<Component>
的类来封装此逻辑并避免修改类Comparer
本身(如果您不希望此Equals
逻辑一直存在)。然后,您可以将其用于两个Component
实例的简单Equals
,甚至可以用于接受它作为附加参数的所有 LINQ 方法。
class ComponentComparer : IEqualityComparer<Component>
{
public bool Equals(Component x, Component y)
{
if (object.ReferenceEquals(x, y)) return true;
if (x == null || y == null) return false;
return x.Price == y.Price && x.Description == y.Description;
}
public int GetHashCode(Component obj)
{
unchecked
{
int hash = 17;
hash = hash * 23 + obj.Price.GetHashCode();
hash = hash * 23 + obj.Description?.GetHashCode() ?? 0;
// ...
return hash;
}
}
}
您的简单用例:
var comparer = new ComponentComparer();
bool equal = comparer.Equals(comp_InApp, comp_InDb);
如果您有两个集合并想知道差异,它也可以工作,例如:
IEnumerable<Component> missingInDb = inAppList.Except( inDbList, comparer );
/没有时间给自己写Equals
和GetHashCode
方法时,我正在使用DeepEqual。
你可以简单地从 NuGet 安装它:
Install-Package DeepEqual
并像这样使用它:
if (comp_InApp.IsDeepEqual(comp_InDb))
{
// Everything up to date.
}
else
{
// Update db.
}
但请记住,这仅适用于您想要显式比较对象的情况,而不适用于要从List
中删除对象的情况或类似情况,当调用Equals
和GetHashCode
时。
以下是反射的解决方案:
static bool AreTwoEqual(Component inApp, Component inDb)
{
string[] propertiesToExclude = new string[] { "DateCreated", "Id" };
PropertyInfo[] propertyInfos = typeof(Component).GetProperties()
.Where(x => !propertiesToExclude.Contains(x.Name))
.ToArray();
foreach (PropertyInfo propertyInfo in propertyInfos)
{
bool areSame = inApp.GetType().GetProperty(propertyInfo.Name).GetValue(inApp, null).Equals(inDb.GetType().GetProperty(propertyInfo.Name).GetValue(inDb, null));
if (!areSame)
{
return false;
}
}
return true;
}
您可以使用反射,但它可能会降低应用程序的速度。创建该比较器的另一种方法是使用 Linq 表达式生成它。试试这个代码:
public static Expression<Func<T, T, bool>> CreateAreEqualExpression<T>(params string[] toExclude)
{
var type = typeof(T);
var props = type.GetProperties(BindingFlags.Public | BindingFlags.Instance)
.Where(p => !toExclude.Contains(p.Name))
.ToArray();
var p1 = Expression.Parameter(type, "p1");
var p2 = Expression.Parameter(type, "p2");
Expression body = null;
foreach (var property in props)
{
var pare = Expression.Equal(
Expression.PropertyOrField(p1, property.Name),
Expression.PropertyOrField(p2, property.Name)
);
body = body == null ? pare : Expression.AndAlso(body, pare);
}
if (body == null) // all properties are excluded
body = Expression.Constant(true);
var lambda = Expression.Lambda<Func<T, T, bool>>(body, p1, p2);
return lambda;
}
它将生成一个看起来像
(Component p1, Component p2) => ((p1.Description == p2.Description) && (p1.Price == p2.Price))
用法简单
var comporator = CreateAreEqualExpression<Component>("Id", "DateCreated")
.Compile(); // save compiled comparator somewhere to use it again later
var areEqual = comporator(comp_InApp, comp_InDb);
编辑:为了使其更加类型安全,您可以使用lambda排除属性
public static Expression<Func<T, T, bool>> CreateAreEqualExpression<T>(
params Expression<Func<T, object>>[] toExclude)
{
var exclude = toExclude
.Select(e =>
{
// for properties that is value types (int, DateTime and so on)
var name = ((e.Body as UnaryExpression)?.Operand as MemberExpression)?.Member.Name;
if (name != null)
return name;
// for properties that is reference type
return (e.Body as MemberExpression)?.Member.Name;
})
.Where(n => n != null)
.Distinct()
.ToArray();
var type = typeof(T);
var props = type.GetProperties(BindingFlags.Public | BindingFlags.Instance)
.Where(p => !exclude.Contains(p.Name))
.ToArray();
/* rest of code is unchanged */
}
现在,在使用它时,我们有一个智能感知支持:
var comparator = CreateAreEqualExpression<Component>(
c => c.Id,
c => c.DateCreated)
.Compile();