表达式树,对象比较器

Question

我需要生成表达式树而不是检查两个对象（参数）是否相等。我知道这些对象会有属性，所以我要比较它们的值，怎么做呢？ 所以我有类似obj1、obj2 和我需要检查的属性名称的字符串数组。 这是我的看法：

var leftObject = E.Parameter(typeof (object), "leftObject");
var rightObject = E.Parameter(typeof (object), "rightObject");
var properties = E.Parameter(typeof (string[]), "properties");
var i = E.Parameter(typeof(int), "i");
var equal = E.Parameter(typeof (bool), "equal");

var body = E.Block
    (
        new[] { properties, i},
        E.Assign(properties,E.Constant(props)),
        E.Assign(i,E.Constant(0)),
        E.Assign(equal,E.Constant(true)),

        E.Loop
        (
            E.Property(leftObject,props[i]) == E.Property(rightObject,props[i])
        )
    );

如何在Loop中实现对属性的一一访问？

附： E 是我的别名 Expression。

Answer 1

您可以使用反射来做到这一点。

bool eq = true;
foreach (string prop in PropertiesYouWantToCheck){
    PropertyInfo propInfo = obj1.GetType().GetProperties().Single(x => x.Name == prop);
    if(propInfo.GetValue(obj1, null) != propInfo.GetValue(obj2, null)){
        eq = false;
        break;
    }
}

如果您使用上述方法，请确保将字符串转换为字符串而不是对象。

 string[] props = null;
 var leftObject = Expression.Parameter(typeof(object), "leftObject");
 var rightObject = Expression.Parameter(typeof(object), "rightObject");
 var equal = Expression.Variable(typeof(bool), "equal");
 var lbl = Expression.Label();
 var returnTarget = Expression.Label();

 var body = Expression.Block
  (
   typeof(bool),
   equal,
   Expression.Assign(equal, Expression.Constant(true)),

   Expression.Block(
    props.Select(property =>
      Expression.IfThen(
       Expression.NotEqual(Expression.Property(leftObject, property),
            Expression.Property(rightObject, property)),
       Expression.Block(
        Expression.Assign(equal, Expression.Constant(false)),
        Expression.Goto(lbl)
       )
      )
     )
   ),

   Expression.Label(lbl),
   Expression.Return(returnTarget, equal, typeof(bool)),
   Expression.Label(returnTarget)
  );

Answer 2

您应该使用反射来发现您想要的属性，然后基本上创建一大系列 AndAlso 表达式。 例如

    public static Func<T, T, bool> BuildStructuralComparerDelegate<T>() where T:class
    {
        var left = Expression.Parameter(typeof(T), "left");
        var right = Expression.Parameter(typeof(T), "right");
        var referenceEquals = typeof(object).GetMethod("ReferenceEquals");
        Expression expression = Expression.AndAlso(
            Expression.Not(
                Expression.Call(
                    null,
                    referenceEquals,
                    left,
                    Expression.Default(typeof(T))
                )
            ),
            Expression.Not(
                null,
                Expression.Call(
                    referenceEquals,
                    right,
                    Expression.Default(typeof(T))
                )
            )
        );
        Array.ForEach(typeof(T).GetProperties(),property =>
            expression = Expression.AndAlso(
                expression,
                Expression.Equal(
                    Expression.Property(left, property),
                    Expression.Property(right, property)
                )
            )
        );
        var lambdaExp = Expression.Lambda<Func<T, T, bool>>(
            Expression.OrElse(
                Expression.Call(
                    null,
                    referenceEquals,
                    left,
                    right
                ),
                expression
            ),
            left,
            right
        );
        return lambdaExp.Compile();
    }

上面的代码只适用于类，创建一个大致的表达式

(left,right)=> object.ReferenceEquals(left,right) || (left != null && right != null && left.Property1 == right.Property1 && left.Property2 == right.Property2 && ... && left.PropertyN == right.PropertyN);

这不是一个完整的解决方案，因为它假定您要比较所有属性并且所有属性都是可读的，但它应该让您走上正轨。如果您关心这些事情，它也与 .NET 3.5 兼容。

Answer 3

可能是这样的：Hows to quick check if data transfer two objects have equal properties in C#?