Question

我使用LINQ表达式生成一个where条件。

我的实体如下;

public class Sample
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int AnotherId { get; set; }
    public int? RelationId { get; set; }
}

我必须根据2个键过滤数据，即AnotherId和RelationId.RelationId（可选）。因此，在我的方法中，参数relationId可能不会更新并且为0。

基于此，我需要生成一个表达式：

Expression<Func<Sample, bool>> condition = x => x.AnotherId == anotherId;
if (relationId > 0)
{
    Expression<Func<Sample, bool>> additionalCondition = x => x.RelationId == relationId;
    condition = Expression.Lambda<Func<Sample, bool>>(Expression.AndAlso(condition, additionalCondition), condition.Parameters);
}

这里我在AndAlso语句中得到以下异常：

二进制运算符AndAlso未定义类型＆＃39; System.Func``2 [Sample，System.Boolean]＆＃39;和＆＃39; System.Func`2 [Sample，System.Boolean]＆＃39;。

请帮我纠正我的问题。

Answer 1

这应该有用......

if(!(relationId>0))
{ 
  Expression<Func<Sample, bool>> condition = x => x.AnotherId == anotherId;
} else {
  Expression<Func<Sample, bool>> condition = x => x.AnotherId == anotherId && x.RelationId == relationId;
}

...或

Expression<Func<Sample, bool>> condition = (!(relationId>0))
  ? x => x.AnotherId == anotherId
  : x => x.AnotherId == anotherId && x.RelationId == relationId;

虽然大多数时候我看到有人问怎么做，但是因为他们真的想要这样做：

var query = something.Where(x=>x.AnotherId == anotherId);
if (relationId>0)
{
  query = query.Where(x=>x.RelationId == relationId);
}

Answer 2

正如其他人可能指出的那样，最简单的方法是

x => x.AnotherId == anotherId && (relationId <= 0 || x.RelationId == relationId);

但是，如果您仍想使用Expression，可能为了将来的证明，您需要定义parameter和property

此外，您需要将anotherId转换为Nullable int

示例：

ParameterExpression param = Expression.Parameter(typeof(Sample), "x");
var anotherIdParam = Expression.Property(param, "anotherId");
var condition = Expression.Equal(anotherIdParam, Expression.Constant(anotherId));

if (relationId > 0)
{
    var relationIdParam = Expression.Property(param, "relationId");
    var additionalCondition = Expression.Equal(relationIdParam, Expression.Convert(Expression.Constant(relationId), typeof(Nullable<int>)));
    condition = Expression.AndAlso(condition, additionalCondition);
}

var finalExpression = Expression.Lambda<Func<Sample, bool>>(condition, param);

LINQ表达式和Nullable类型

2 个答案: