扩展IQueryable实现属性名称排序

基于IEnumerable<T>接口的LINQ to Object扩展函数运行开发人员提供Func<T, TResult>类型的委托实现排序，看一个简单的例子：

public class Person
{
    public Person()
        : this(Guid.NewGuid().ToString("N"), DateTime.Now) { }

    public Person(String name, DateTime birth)
    {
        this.Name = name;
        this.Birth = birth;
    }

    public String Name { get; set; }

    public DateTime Birth { get; set; }
}

 

IEnumerable<Person> persons = new Person[] { new Person(), new Person(), new Person() };

IOrderedEnumerable<Person> persons2 = persons.OrderBy(p => p.Name);
IOrderedEnumerable<Person> persons3 = persons.OrderBy(delegate(Person p) { return p.Name; });
IOrderedEnumerable<Person> persons4 = persons.OrderBy(p => p.Name, StringComparer.CurrentCultureIgnoreCase);

 

上面类型中的属性是已知并且可被推导的，如果属性只有一个名称怎么才能够依然可以排序呢？比如下面的写法：

IOrderedEnumerable<Person> persons = persons.OrderBy("Name");

 

IQueryable<T>接口可以将查询解析为表达式树，我们可以看到IQueryable<T>的OrderBy、OrderByDescending扩展函数分别相比IEnumerable<T>多了两个接受Expression<Func<T, TResult>>的函数：

public static IOrderedQueryable<TSource> OrderBy<TSource, TKey>(this IQueryable<TSource> source, Expression<Func<TSource, TKey>> keySelector);

public static IOrderedQueryable<TSource> OrderBy<TSource, TKey>(this IQueryable<TSource> source, Expression<Func<TSource, TKey>> keySelector, IComparer<TKey> comparer);

public static IOrderedQueryable<TSource> OrderByDescending<TSource, TKey>(this IQueryable<TSource> source, Expression<Func<TSource, TKey>> keySelector);

public static IOrderedQueryable<TSource> OrderByDescending<TSource, TKey>(this IQueryable<TSource> source, Expression<Func<TSource, TKey>> keySelector, IComparer<TKey> comparer);

 

也就说我们可以动态构建Lambda表达式来实现动态调用OrderBy、OrderByDescending函数。看一个例子：

Type type = typeof(T);
ParameterExpression arg = Expression.Parameter(type, "p");
PropertyInfo property = type.GetProperty(propertyName);
Expression expr = Expression.Property(arg, property);
Type delegateType = typeof(Func<,>).MakeGenericType(typeof(T), property.PropertyType);
LambdaExpression lambda = Expression.Lambda(delegateType, expr, arg);

 

arg就是传递的参数类型，也就是Func<T, TResult>的T类型。通过Expression.Property创建了函数执行体，并定义访问传递参数的某一个属性。最后创建出表达式，等同于p => p.属性。最后找到Queryable静态类型中定义的OrderBy、OrderByDescending函数动态调用就可以实现传递属性名称实现排序。

public static IOrderedQueryable<T> OrderBy<T>(this IQueryable<T> source, String propertyName)
{
    return InternalOrder<T>(source, propertyName, "OrderBy");
}

 

public static IOrderedQueryable<T> OrderByDescending<T>(this IQueryable<T> source, String propertyName)
{
    return InternalOrder<T>(source, propertyName, "OrderByDescending");
}

 

public static IOrderedQueryable<T> ThenBy<T>(this IOrderedQueryable<T> source, String propertyName)
{
    return InternalOrder<T>(source, propertyName, "ThenBy");
}
public static IOrderedQueryable<T> ThenByDescending<T>(this IOrderedQueryable<T> source, String propertyName)
{
    return InternalOrder<T>(source, propertyName, "ThenByDescending");
}

 

private static IOrderedQueryable<T> InternalOrder<T>(IQueryable<T> source, String propertyName, String methodName)
{
    Type type = typeof(T);
    ParameterExpression arg = Expression.Parameter(type, "p");
    PropertyInfo property = type.GetProperty(propertyName);
    Expression expr = Expression.Property(arg, property);
    Type delegateType = typeof(Func<,>).MakeGenericType(typeof(T), property.PropertyType);
    LambdaExpression lambda = Expression.Lambda(delegateType, expr, arg);

 

    return ((IOrderedQueryable<T>)(typeof(Queryable).GetMethods().Single(
        p => String.Equals(p.Name, methodName, StringComparison.Ordinal)
            && p.IsGenericMethodDefinition
            && p.GetGenericArguments().Length == 2
            && p.GetParameters().Length == 2)
        .MakeGenericMethod(typeof(T), property.PropertyType)
        .Invoke(null, new Object[] { source, lambda })));
}

 

IEnumerable<Person> persons = new Person[] { new Person(), new Person(), new Person() };
IOrderedQueryable<Person> persons2 = persons.AsQueryable().OrderBy("Name");
IOrderedQueryable<Person> persons3 = persons.AsQueryable().OrderByDescending("Name");