构建一个Expression来检查对象的两个属性是否相等？

Posted 2021-05-05

我试图理解Expressions是如何工作的，所以我想象一个方法，它接受一个具有两个int属性的对象，并返回布尔值表示它们是否相等，如下所示：

bool AreEqual(Foo foo)
{
    return foo.Value1 == foo.Value2;
}

这是我建立的表达：

//build the parameter expression of the object
ParameterExpression parameter = Expression.Parameter(typeof(Foo), "x");
//the left member
MemberExpression leftMember = Expression.Property(parameter, "Value1");
//the right member
MemberExpression rightMember = Expression.Property(parameter, "Value2");
//the left lambda
LambdaExpression leftLmbda = Expression.Lambda(leftMember, parameter);
//the right lambda
LambdaExpression rightLambda = Expression.Lambda(rightMember, parameter);
//and here I evaluate the boolean expression:
Expression equalExpression = Expression.Equal(rightLambda, leftLmbda);
//the lambda of the equal expression
LambdaExpression lambda = Expression.Lambda(equalExpression, parameter);

//the object:
Foo foo = new Foo { Value1= 5, Value2=5 };

Delegate expression = lambda.Compile();
var eq = expression.DynamicInvoke(foo);

但它总是评估为false。

我的猜测是我只构建一个lambda，但不知道如何在一个lambda中处理这两个属性

答案

Expression.Equal语句应该比较两个成员表达式。

//build the parameter expression of the object
ParameterExpression parameter = Expression.Parameter(typeof(Foo), "x");
//the left member
MemberExpression leftMember = Expression.Property(parameter, "Value1");
//the right member
MemberExpression rightMember = Expression.Property(parameter, "Value2");
//and here I evaluate the boolean expression:
Expression equalExpression = Expression.Equal(leftMember, rightMember);
//the lambda of the equal expression
LambdaExpression lambda = Expression.Lambda(equalExpression, parameter);

//the object:
Foo foo = new Foo { Value1 = 5, Value2 = 5 };

Delegate expression = lambda.Compile();
var eq = expression.DynamicInvoke(foo);