c#linq结合Expression< Func< T，bool>带有其他条件的where子句中的谓词 [英] c# linq combine Expression<Func<T, bool>> predicate in where clause with another condition

问题描述

可以构造这样的where子句，其中谓词的类型为Expression< Func< T，bool>谓词:

  var resultQuery = query.Where(q =>！q.IsDeleted&&谓词).ToList(); 

我想避免像这样的double where子句:

  var resultQuery = query.Where(q =>！q.IsDeleted).Where(predicate).ToList(); 

解决方案

这是不可能的.您需要提供一个lambda表达式作为 Where 子句的参数，该子句将被编译为表达式树，然后将其转换为某些SQL查询.在您的示例中

var resultQuery = query.Where(q =>！q.IsDeleted&& predicate).ToList();

您正在组合一个lambda表达式和一个布尔检查.避免出现两个 Where 子句的唯一方法是创建一个辅助函数，该函数返回用于过滤的lambda表达式，其中包括对 IsDeleted 标志和谓词逻辑即

 私有System.Linq.Expressions.Expression< Func< T，bool>filterPredicate(int n){返回q =>！q.IsDeleted&&q.年龄n;} 

这里我们假设

q.年龄>n

是谓词函数的逻辑.然后使用这样的过滤谓词:

  var resultQuery = query.Where(filterPredicate(5)).ToList(); 

有关lambda表达式和表达式树的更多信息，您可以在这里阅读 解决方案

IT is impossible. You need to provide a lambda expression as a parameter to the Where clause which would be compiled to an expression tree and after that translated into some SQL query. In your example

var resultQuery = query.Where(q => !q.IsDeleted && predicate).ToList();

you are combining a lambda expression and a boolean check. The only way to avoid the double Where clauses is to create a helper function that returns a lambda expression for filtering which includes filtering for the IsDeleted flag and the predicate logic i.e.

 private System.Linq.Expressions.Expression<Func<T, bool>> filterPredicate(int n)
 {
     return q => !q.IsDeleted && q.Age > n; 
 }

Here we are assuming that

q.Age > n

is the logic of your predicate function. And then use the filter predicates like this:

var resultQuery = query.Where(filterPredicate(5)).ToList();

More about lambda expressions and expression trees you can read here https://docs.microsoft.com/en-us/dotnet/api/system.linq.expressions.expression-1?view=net-5.0

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