[英]Entity Framework's navigation properties are null after sort by nullable
我使用以下代码按表达式转换顺序,因此可空列也可以按顺序进行排序。
protected virtual Expression<Func<T, object>> GetSorting(string ordering)
{
Expression<Func<T, object>> expression = default(Expression<Func<T, object>>);
IEnumerable<Order> sortObjects = string.IsNullOrEmpty(ordering) ? null : JsonConvert.DeserializeObject<IEnumerable<Order>>(ordering);
if (sortObjects != null)
{
foreach (Order sortObject in sortObjects)
{
Expression<Func<T, object>> currentExpression = this.GetExpression(sortObject.Property);
expression = this.CombineExpressions(expression, currentExpression);
}
}
return expression;
}
private Expression<Func<T, object>> GetExpression(string propertyName)
{
Type type = typeof(T);
ParameterExpression parameter = Expression.Parameter(type, "x");
MemberExpression propertyReference = Expression.Property(parameter, propertyName);
Expression conversion = Expression.Convert(propertyReference, typeof(object));
Expression<Func<T, object>> currentExpression = Expression.Lambda<Func<T, object>>(conversion, new[] { parameter });
return currentExpression;
}
private Expression<Func<T, object>> CombineExpressions(Expression<Func<T, object>> expression, Expression<Func<T, object>> currentExpression)
{
if (expression == default(Expression<Func<T, object>>))
{
expression = currentExpression;
}
else
{
// Combine the two expressions' body together
BinaryExpression body = Expression.AndAlso(expression.Body, currentExpression.Body);
ParameterExpression[] parameters = new ParameterExpression[1] { Expression.Parameter(typeof(T), expression.Parameters.First().Name) };
// Convert the BinaryExpression to the requested type
Expression<Func<T, object>> lambda = Expression.Lambda<Func<T, object>>(body, parameters);
expression = lambda;
}
return expression;
}
该代码对于所有非导航属性都非常适用,但是似乎不再需要查询导航属性了。 我使用Select表达式加载导航属性,如下所示:
protected override Expression<Func<Resource, ResourceViewModel>> Selector
{
get
{
return (x) => new ResourceViewModel()
{
ResourceId = x.ResourceId,
DisplayName = x.DisplayName,
ResourceType = x.ResourceType != null ? x.ResourceType.Name : string.Empty,
}
}
}
如果我没有要订购的东西,那么将加载导航属性。 但是,一旦有什么要订购的,导航属性为null。 如果我跳过三元操作并直接转到ResourceType.Name属性,则会收到一个异常,告诉我lambda_method引发了NullReference异常。
我知道订购导航属性无法正常工作,但这不是问题。 按“常规”属性排序会导致此问题。
有什么想法吗?
事实证明,这个问题并不像我想的那么复杂。 问题是我创建了错误的表达式树。 您可以嵌套表达式,以便导航到属性的属性。
以下解决方案应对此进行解释(这不是我实际解决此问题的方式,但应明确说明):
private Expression<Func<T, object>> GetExpression(string parentClass, string propertyName)
{
Type type = typeof(T);
ParameterExpression parameter = Expression.Parameter(type, "x");
// Get parent class expression
// Will result in (x) => x.MyNavigationPropertyWhichIsAClass
MemberExpression propertyReference1 = Expression.Property(parameter, parentclass);
// Navigate to the property of the navigation property class
// Will result in (x) => x.MyNavigationPropertyWhichIsAClass.MyPropertyIWantToSort
MemberExpression propertyReference2 = Expression.Property(propertyRefernce1, propertyName);
Expression conversion = Expression.Convert(propertyReference2, typeof(object));
Expression<Func<T, object>> currentExpression = Expression.Lambda<Func<T, object>>(conversion, new[] { parameter });
return currentExpression;
}
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