在LINQ表达式树中有选择地从where子句中删除

与其尝试更改现有的where子句,不如将其重构为:

from a in things  
where a.Id == b.Id 
where a.Name == b.Name 
where a.Value1 == b.Value1
where a.Value2 == b.Value2
where a.Value3 == b.Value3  
select a;

然后变成:

things.Where(a => a.Id == b.Id)
      .Where(a => a.Name == b.Name)
      .Where(a => a.Value1 == b.Value1)
      .Where(a => a.Value2 == b.Value2)
      .Where(a => a.Value1 == b.Value3);

IQueryable<Whatever> query = things;
if (useId) {
    query = query.Where(a => a.Id == b.Id);
}
query = query.Where(a => a.Name == b.Name);
if (checkValue1) {
    query = query.Where(a => a.Value1 == b.Value1);
}
// etc

这也可能是另一种方法;

bool executeValue1Condition = true;
bool executeValue2Condition = true;
bool executeValue3Condition = true;

var q = from a in things  
where a.Id == b.Id &&  
a.Name == b.Name &&  
(a.Value1 == b.Value1 || executeValue1Condition) &&  
(a.Value2 == b.Value2 || executeValue2Condition) &&  
(a.Value3 == b.Value3 || executeValue3Condition) 
select a;

executeValue1Condition = false;
q = q.Select(i => i);

通过使用这种方法,您可以在执行查询后更改条件,当然也可以通过新的执行来更改。

试着把事情分开。首先将where条件与查询的其余部分隔离开来,然后对其进行处理,并使用标准的查询方法,而不是内置的语法方法。如。:

     IQueryable<MyClass> things = null;
     MyClass b = new MyClass();

     Expression<Func<MyClass, bool>> whereExp = a => a.Id == b.Id && a.Name == b.Name;
     // process where expression here. it's just an expression tree. traverse it and
     // remove nodes as desired.
     var result = things.Where(whereExp).Select(a => a);

IQueryable<MyClass> things = null;
MyClass b = new MyClass();

Expression<Func<MyClass, bool>> whereExp;
Expression<Func<MyClass, bool>> exp1 = a => a.Id == b.Id;
Expression<Func<MyClass, bool>> exp2 = a => a.Name == b.Name;
whereExp = Expression.Lambda<Func<MyClass, bool>>(Expression.And(exp1, exp2), Expression.Parameter(typeof(MyClass), "a"));

var result = things.Where(whereExp).Select(a => a);