我想将谓词构建逻辑移动到基类中,但收到错误"LINQ 表达式节点类型'Invoke'在 LINQ to Entities 中不受支持"。 我希望能够连接或有条件地链接谓词中的表达式。
我希望能够传入调用者唯一的谓词部分,即使用的属性名称(GetFilterPredicate 将成为通用例程,它将对具有不同属性名称的类型进行操作保存相关值(。
protected Expression<Func<MyEntity, bool>> GetFilterPredicate(
PagingParameters pagingParameters,
Func<MyEntity, DateTime?> terminationDate,
Func<MyEntity, string> entityID,
Func<MyEntity, string> name
)
{
Expression<Func<MyEntity, bool>> validFilter = x => true;
if (pagingParameters.Active == true)
{
validFilter = x => terminationDate(x) > DateTime.Now;
}
///more conditions added here
return validFilter;
}
protected List<MyEntity> Query(IQueryable<MyEntity> source)
{
var filters = GetFilterPredicate(
new PagingParameters() { Active = true }
, i => i.TerminationDate
, i => i.EntityID
, i => i.Name
);
return source.Where(filters).AsNoTracking().ToList<MyEntity>();
}
不能使用委托构造新表达式terminationDate
需要terminationDate
更改为表达式并使用它来手动生成新表达式。
protected static Expression<Func<MyEntity, bool>> GetFilterPredicate(
PagingParameters pagingParameters,
Expression<Func<MyEntity, DateTime?>> terminationDate,
Expression<Func<MyEntity, string>> entityID,
Expression<Func<MyEntity, string>> name
)
{
Expression<Func<MyEntity, bool>> validFilter = x => true;
// We need to replace the parameter for all expressions with
// a common single parameter. I used the parameter for the default
// filter but a new Parameter expression would have worked as well.
// If you don't do this you will get an error (unbound parameter or something like that ) because the parameter
// used in the expressions (terminationDate, entityID) will be
// different then the parameter used for the new validFilter expression
var parameterReplacer = new ReplaceVisitor
{
NewParameter = validFilter.Parameters.First()
};
if (pagingParameters.Active == true)
{
validFilter = Expression.Lambda<Func<MyEntity, bool>>(
Expression.GreaterThan
(
parameterReplacer.Visit(terminationDate.Body),
Expression.Convert(Expression.Property(null, typeof(DateTime), "Now"), typeof(DateTime?))
),
parameterReplacer.NewParameter
);
}
// existing filter && x.EntityId != "A"
validFilter = Expression.Lambda<Func<MyEntity, bool>>(
Expression.And(
validFilter.Body,
Expression.NotEqual
(
parameterReplacer.Visit(entityID.Body),
Expression.Constant("A")
)
),
parameterReplacer.NewParameter
);
return validFilter;
}
/// <summary>
/// Simple Parameter Replacer, will replace the any parameter with the new
/// parameter. You will need to refine this if your expressions have nested
/// lambda, in that you will need to only replace the top most lambda
/// parameter, but for simple expressions it will work fine.
/// </summary>
class ReplaceVisitor : ExpressionVisitor
{
public ParameterExpression NewParameter { get; set; }
protected override Expression VisitParameter(ParameterExpression node)
{
return this.NewParameter;
}
}
你可以按照提香的回答来手动完成此操作,但更简单的方法是使用 LinqKit。首先安装 LinqKit nuget 包并添加using LinqKit;
,然后:
// change terminaionDate to Expression<Func<MyEntity, DateTime>>
static Expression<Func<MyEntity, bool>> GetFilterPredicate(Expression<Func<MyEntity, DateTime>> terminationDate) {
// note Invoke
Expression<Func<Error, bool>> filter = c => terminationDate.Invoke(c) <= DateTime.Now;
// note Expand
return filter.Expand();
}
您可以通过上面提供的链接阅读有关Invoke
和Expand
的信息。