我在字符串中有“按属性排序”的名称。我需要使用Lambda/Linq对对象列表进行排序。
Ex:
public class Employee
{
public string FirstName {set; get;}
public string LastName {set; get;}
public DateTime DOB {set; get;}
}
public void Sort(ref List<Employee> list, string sortBy, string sortDirection)
{
//Example data:
//sortBy = "FirstName"
//sortDirection = "ASC" or "DESC"
if (sortBy == "FirstName")
{
list = list.OrderBy(x => x.FirstName).toList();
}
}
与其使用一堆if来检查字段名(sortBy)，还有一种更干净的排序方式吗 排序是否知道数据类型?
当前回答
using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Linq.Expressions;
public static class EnumerableHelper
{
static MethodInfo orderBy = typeof(Enumerable).GetMethods(BindingFlags.Static | BindingFlags.Public).Where(x => x.Name == "OrderBy" && x.GetParameters().Length == 2).First();
public static IEnumerable<TSource> OrderBy<TSource>(this IEnumerable<TSource> source, string propertyName)
{
var pi = typeof(TSource).GetProperty(propertyName, BindingFlags.Public | BindingFlags.FlattenHierarchy | BindingFlags.Instance);
var selectorParam = Expression.Parameter(typeof(TSource), "keySelector");
var sourceParam = Expression.Parameter(typeof(IEnumerable<TSource>), "source");
return
Expression.Lambda<Func<IEnumerable<TSource>, IOrderedEnumerable<TSource>>>
(
Expression.Call
(
orderBy.MakeGenericMethod(typeof(TSource), pi.PropertyType),
sourceParam,
Expression.Lambda
(
typeof(Func<,>).MakeGenericType(typeof(TSource), pi.PropertyType),
Expression.Property(selectorParam, pi),
selectorParam
)
),
sourceParam
)
.Compile()(source);
}
public static IEnumerable<TSource> OrderBy<TSource>(this IEnumerable<TSource> source, string propertyName, bool ascending)
{
return ascending ? source.OrderBy(propertyName) : source.OrderBy(propertyName).Reverse();
}
}
另一个，这次是IQueryable:
using System;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
public static class IQueryableHelper
{
static MethodInfo orderBy = typeof(Queryable).GetMethods(BindingFlags.Static | BindingFlags.Public).Where(x => x.Name == "OrderBy" && x.GetParameters().Length == 2).First();
static MethodInfo orderByDescending = typeof(Queryable).GetMethods(BindingFlags.Static | BindingFlags.Public).Where(x => x.Name == "OrderByDescending" && x.GetParameters().Length == 2).First();
public static IQueryable<TSource> OrderBy<TSource>(this IQueryable<TSource> source, params string[] sortDescriptors)
{
return sortDescriptors.Length > 0 ? source.OrderBy(sortDescriptors, 0) : source;
}
static IQueryable<TSource> OrderBy<TSource>(this IQueryable<TSource> source, string[] sortDescriptors, int index)
{
if (index < sortDescriptors.Length - 1) source = source.OrderBy(sortDescriptors, index + 1);
string[] splitted = sortDescriptors[index].Split(' ');
var pi = typeof(TSource).GetProperty(splitted[0], BindingFlags.Public | BindingFlags.FlattenHierarchy | BindingFlags.Instance | BindingFlags.IgnoreCase);
var selectorParam = Expression.Parameter(typeof(TSource), "keySelector");
return source.Provider.CreateQuery<TSource>(Expression.Call((splitted.Length > 1 && string.Compare(splitted[1], "desc", StringComparison.Ordinal) == 0 ? orderByDescending : orderBy).MakeGenericMethod(typeof(TSource), pi.PropertyType), source.Expression, Expression.Lambda(typeof(Func<,>).MakeGenericType(typeof(TSource), pi.PropertyType), Expression.Property(selectorParam, pi), selectorParam)));
}
}
你可以传递多个排序条件，像这样:
var q = dc.Felhasznalos.OrderBy(new string[] { "Email", "FelhasznaloID desc" });
其他回答
你可以做的一件事是改变Sort，以便更好地使用lambda。
public enum SortDirection { Ascending, Descending }
public void Sort<TKey>(ref List<Employee> list,
Func<Employee, TKey> sorter, SortDirection direction)
{
if (direction == SortDirection.Ascending)
list = list.OrderBy(sorter);
else
list = list.OrderByDescending(sorter);
}
现在，您可以在调用sort方法时指定要排序的字段。
Sort(ref employees, e => e.DOB, SortDirection.Descending);
Sort使用IComparable接口，如果该类型实现了该接口。 你可以通过实现一个自定义IComparer来避免if:
class EmpComp : IComparer<Employee>
{
string fieldName;
public EmpComp(string fieldName)
{
this.fieldName = fieldName;
}
public int Compare(Employee x, Employee y)
{
// compare x.fieldName and y.fieldName
}
}
然后
list.Sort(new EmpComp(sortBy));
您可以使用Reflection来获取属性的值。
list = list.OrderBy( x => TypeHelper.GetPropertyValue( x, sortBy ) )
.ToList();
TypeHelper有一个静态方法，比如:
public static class TypeHelper
{
public static object GetPropertyValue( object obj, string name )
{
return obj == null ? null : obj.GetType()
.GetProperty( name )
.GetValue( obj, null );
}
}
你可能还想看看VS2008 Samples库中的Dynamic LINQ。您可以使用IEnumerable扩展将List转换为IQueryable，然后使用Dynamic link OrderBy扩展。
list = list.AsQueryable().OrderBy( sortBy + " " + sortDirection );
不幸的是，Rashack提供的解决方案不适用于值类型(int、enum等)。
对于任何类型的属性，这是我发现的解决方案:
public static Expression<Func<T, object>> GetLambdaExpressionFor<T>(this string sortColumn)
{
var type = typeof(T);
var parameterExpression = Expression.Parameter(type, "x");
var body = Expression.PropertyOrField(parameterExpression, sortColumn);
var convertedBody = Expression.MakeUnary(ExpressionType.Convert, body, typeof(object));
var expression = Expression.Lambda<Func<T, object>>(convertedBody, new[] { parameterExpression });
return expression;
}
如果你得到排序列名和排序方向为字符串，并且不想使用switch或If \else语法来确定列，那么这个例子可能对你很有趣:
private readonly Dictionary<string, Expression<Func<IuInternetUsers, object>>> _sortColumns =
new Dictionary<string, Expression<Func<IuInternetUsers, object>>>()
{
{ nameof(ContactSearchItem.Id), c => c.Id },
{ nameof(ContactSearchItem.FirstName), c => c.FirstName },
{ nameof(ContactSearchItem.LastName), c => c.LastName },
{ nameof(ContactSearchItem.Organization), c => c.Company.Company },
{ nameof(ContactSearchItem.CustomerCode), c => c.Company.Code },
{ nameof(ContactSearchItem.Country), c => c.CountryNavigation.Code },
{ nameof(ContactSearchItem.City), c => c.City },
{ nameof(ContactSearchItem.ModifiedDate), c => c.ModifiedDate },
};
private IQueryable<IuInternetUsers> SetUpSort(IQueryable<IuInternetUsers> contacts, string sort, string sortDir)
{
if (string.IsNullOrEmpty(sort))
{
sort = nameof(ContactSearchItem.Id);
}
_sortColumns.TryGetValue(sort, out var sortColumn);
if (sortColumn == null)
{
sortColumn = c => c.Id;
}
if (string.IsNullOrEmpty(sortDir) || sortDir == SortDirections.AscendingSort)
{
contacts = contacts.OrderBy(sortColumn);
}
else
{
contacts = contacts.OrderByDescending(sortColumn);
}
return contacts;
}
基于使用Dictionary的解决方案，通过表达式>和它的键字符串连接排序列所需的字典。
