#Order by Not Null使用表达式扩展linq

Question

我有一个自定义的IOrderedQueryable函数，如下所示。

public static IOrderedQueryable<T> OrderBy<T>(this IQueryable<T> srcQuery, 
string orderColumn, bool isAscending)
{
  var type = typeof(T);
  var property = type.GetProperty(orderColumn);

  if (property == null)
    throw new Exception("Column property \"" + orderColumn + "\" does not exist on the type \"" + typeof(T).FullName + "\"");

  var parameter = Expression.Parameter(type, "p");
  var propertyAccess = Expression.MakeMemberAccess(parameter, property);
  var orderByExp = Expression.Lambda(propertyAccess, parameter);
  MethodCallExpression resultExp = 
  Expression.Call(typeof(Queryable),isAscending ? "OrderBy" : 
 "OrderByDescending", new Type[] { type, property.PropertyType }, 
  srcQuery.Expression, Expression.Quote(orderByExp));

   return (IOrderedQueryable<T>)srcQuery.Provider.CreateQuery<T>(resultExp);
}

我想知道无论如何我都可以使用condition属性执行命令！= null以显示非空值，然后在上面的orderByExp表达式中显示空值

Answer 1

您可以修改属性访问权限以执行相当于query.OrderBy(x => x.property == null ? 0 : 1)

的操作

public static IOrderedQueryable<T> OrderByNull<T>(IQueryable<T> srcQuery, string orderColumn, bool isAscending)
{
    var type = typeof(T);
    var property = type.GetProperty(orderColumn);

    if (property == null)
        throw new Exception("Column property \"" + orderColumn + "\" does not exist on the type \"" + typeof(T).FullName + "\"");

    var parameter = Expression.Parameter(type, "p");
    var propertyAccess = Expression.Condition(
        Expression.Equal(Expression.MakeMemberAccess(parameter, property), Expression.Constant(null, property.PropertyType)),
        Expression.Constant(1),
        Expression.Constant(0));

    var orderByExp = Expression.Lambda(propertyAccess, parameter);
    MethodCallExpression resultExp =Expression.Call( typeof(Queryable), isAscending ? "OrderBy" : "OrderByDescending", new Type[] { type,  typeof(int) },
    srcQuery.Expression, Expression.Quote(orderByExp));

    return (IOrderedQueryable<T>)srcQuery.Provider.CreateQuery<T>(resultExp);
}

注意：如果属性不可为空（例如int?或double?），则会发生错误，

修改：

以上版本仅适用于可空属性。另一个版本是按可空列的默认值排序，并为所有其他列表进行正常排序：if (property is nullable) query.OrderBy(x => x.property == null ? default(basePropertyType) : x.property) else query.OrderBy(x => x.property)

该版本的版本如下：

static Dictionary<Type, object> DefaultTypeValues = new Dictionary<Type, object>
{
    { typeof(string), "" },
   // { typeof(DateTime?), new DateTime(1753,1,1) } // Min date for sql date 
      { typeof(DateTime?), new DateTime(9999,12,31} // Max date for sql date 
};

public static object GetDefaultValue(Type t)
{
    object defaultValue;
    if(!DefaultTypeValues.TryGetValue(t, out defaultValue))
    {
        if(t.IsGenericType && t.GetGenericTypeDefinition() == typeof(Nullable<>))
        {
            defaultValue = Activator.CreateInstance(t.GetGenericArguments().Single());
        }
        else
        {
            throw new NotSupportedException("Could not get default value for type " + t.FullName + " consider adding it in DefaultTypeValues");
        }
    }
    return defaultValue;
}
public static IOrderedQueryable<T> OrderBy<T>(IQueryable<T> srcQuery, string orderColumn, bool isAscending)
{
    var type = typeof(T);
    var property = type.GetProperty(orderColumn);

    if (property == null)
        throw new Exception("Column property \"" + orderColumn + "\" does not exist on the type \"" + typeof(T).FullName + "\"");

    var parameter = Expression.Parameter(type, "p");
    // default sort is performed by o=> o.Prop
    Expression propertyAccess = Expression.MakeMemberAccess(parameter, property);
    var propType = property.PropertyType;

    // If property is nullable we add teh null check 
    if (propType == typeof(string) || (propType.IsGenericType && propType.GetGenericTypeDefinition() == typeof(Nullable<>)))
    {
        var defaultValue = GetDefaultValue(propType);
        // If the property is nullable we sort by (o => o.Prop == null ? default(propType) : o.Prop)
        propertyAccess = Expression.Condition(
            Expression.Equal(propertyAccess, Expression.Constant(null, propType)),
            Expression.Constant(defaultValue, propType),
            propertyAccess
        );
    }

    var orderByExp = Expression.Lambda(propertyAccess, parameter);
    MethodCallExpression resultExp = Expression.Call(typeof(Queryable), isAscending ? "OrderBy" : "OrderByDescending", new Type[] { type, propType },
    srcQuery.Expression, Expression.Quote(orderByExp));

    return (IOrderedQueryable<T>)srcQuery.Provider.CreateQuery<T>(resultExp);
}

默认值可以通过DefaultTypeValues中的类型进行自定义，或者如果未指定任何值，则默认值将默认为基础可空类型（例如，int?将为0）