如何创建用于按字符串排序的表达式

Question

我是表达树的新手。我需要根据字符串长度对字符串进行排序。我不想为每个实体重复我的代码。任何人都知道如何用表达式树写它？

这样的事情：

public static IQueryable<T> OrderByPropertyOrField<T>(this IQueryable<T> queryable,
  string propertyOrFieldName, bool ascending = true)
{
    var elementType = typeof(T);
    var orderByMethodName = ascending ? "OrderBy" : "OrderByDescending";

    var parameterExpression = Expression.Parameter(elementType);
    var propertyOrFieldExpression = Expression.PropertyOrField(parameterExpression, propertyOrFieldName);
    var selector = Expression.Lambda(propertyOrFieldExpression, parameterExpression);

    var orderByExpression = Expression.Call(typeof(Queryable), orderByMethodName,
        new[] { elementType, propertyOrFieldExpression.Type }, queryable.Expression, selector);

    return queryable.Provider.CreateQuery<T>(orderByExpression);
}

Answer 1

如果您需要按字符串长度排序后按字符串值排序，则需要根据ThenBy值调用另一种Quryable - ThenByDescending或ascending方法。下一个代码将创建该排序：

public static class QueryableExtensions
{
    public static IQueryable<T> OrderByPropertyOrField<T>(this IQueryable<T> queryable,
      string propertyOrFieldName, bool ascending = true)
    {
        var parameter = Expression.Parameter(queryable.ElementType, "x");
        var selector = Expression.PropertyOrField(parameter, propertyOrFieldName);

        var getLength = Expression.PropertyOrField(selector, "Length");
        var orderByLength = CreateOrderExpression(parameter,
          typeof(int),
          queryable.Expression, // order source collection
          getLength,
          ascending ? "OrderBy" : "OrderByDescending");

        var orderByValue = CreateOrderExpression(parameter,
          typeof(string),
          orderByLength, // order previous collection
          selector,
          ascending ? "ThenBy" : "ThenByDescending");

        return queryable.Provider.CreateQuery<T>(orderByValue);
    }

    private static Expression CreateOrderExpression(ParameterExpression parameter, Type keyType, Expression collection, Expression selector, string methodName)
    {
        return Expression.Call(
            typeof(Queryable),
            methodName,
            new[] { parameter.Type, keyType },
            collection,
            Expression.Lambda(selector, parameter)
        );
    }
}