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为x=>x手动生成linq表达式。child==itemtocompare.child

expression-trees lambda linq

Handcraftsman · 技术社区 · 14 年前

我们有一个对象,我们希望基于该对象即时构建一个LINQ查询。此LINQ语句等价于我们要生成的内容:

Expression<Func<Sample, bool>> linqExpression 
            = x => x.Child == itemToCompare.Child;

我们无法找到正确的表达式来构建itemtoCompare.child部分。到目前为止,我们有:

var param = Expression.Parameter(typeof(T), "x");
var key = itemToCompare.GetType().GetProperty("Child");
var rhsConstant = Expression.Constant(item);
var innerLambda = Expression.Lambda<Func<T>>(rhsConstant, 
            new ParameterExpression[0]);
var rhsMemberAccess = Expression.MakeMemberAccess(innerLambda, key);
body = Expression.Equal(lhsPropertyAccess, rhsMemberAccess);
var lambda = Expression.Lambda<Func<T, bool>>(body, param);

我们手工构建的查询的表达式树如下所示:

Lambda Expression: x => (x.Child = value(SampleTests+Sample))
Expression Body: (x.Child = value(SampleTests+Sample))
Parameter 0: 'x', Type: Sample
NodeType: Lambda
   Left Lambda Expression: x.Child
   Left NodeType: MemberAccess
      Lambda Expression: x
      Expression Member: Sample Child
      NodeType: Parameter
         -- a parameter expression
   Right Lambda Expression: value(SampleTests+Sample)
   Right NodeType: Constant
      NodeType: Constant
      Value: SampleTests+Sample

实际lambda的表达式树如下所示:

Lambda Expression: x => (x.Child = value(SampleTests+<>c__DisplayClass15).itemToCompare.Child)
Expression Body: (x.Child = value(SampleTests+<>c__DisplayClass15).itemToCompare.Child)
Parameter 0: 'x', Type: Sample
NodeType: Lambda
   Left Lambda Expression: x.Child
   Left NodeType: MemberAccess
      Lambda Expression: x
      Expression Member: Sample Child
      NodeType: Parameter
         -- a parameter expression
   Right Lambda Expression: value(SampleTests+<>c__DisplayClass15).itemToCompare.Child
   Right NodeType: MemberAccess
      Lambda Expression: value(SampleTests+<>c__DisplayClass15).itemToCompare
      Expression Member: Sample Child
      NodeType: MemberAccess
         Lambda Expression: value(SampleTests+<>c__DisplayClass15)
         Expression Member: Sample itemToCompare
         NodeType: Constant
            NodeType: Constant
            Value: SampleTests+<>c__DisplayClass15

编辑:

我们认为这是为了

Expression<Func<Sample, bool>> linqExpression 
            = x => x.Child == (()=>itemToCompare).Child;

这就是我们试图在表达中重现的。

最终的目标是生成一个表达式,在其中使用其类型的.equals()方法比较子值。

谢谢!

1 回复 | 直到 14 年前

Joe Albahari 14 年前

var param = Expression.Parameter(typeof(Sample), "x");
var key = itemToCompare.GetType().GetProperty("Child");
var rhs = Expression.MakeMemberAccess(Expression.Constant(itemToCompare), key);
var lhs = Expression.MakeMemberAccess(param, key);
var body = Expression.Equal(lhs, rhs);
var lambda = Expression.Lambda<Func<Sample, bool>>(body, param);