ConstantExpression不是常量

Question

给定Msdn:一个constant-expression是一个可以在编译时完全评估的表达式.

但是在下面的示例代码中,我有一个无法在编译时评估的contantExpression.

我应该错过什么,但是什么？

public class SomeClass
{
    public string Key { get; set; }
}

public static void Sample()
{
    var wantedKey = Console.ReadLine();
    Expression<Func<SomeClass, bool>> expression = c => c.Key == wantedKey;

    var maybeAConstantExpression = ((MemberExpression)((BinaryExpression)expression.Body).Right).Expression;

    //Both are true, so we have a constantExpression,righ and Value should be known
    Console.WriteLine(maybeAConstantExpression.NodeType == ExpressionType.Constant);
    Console.WriteLine(maybeAConstantExpression.GetType() == typeof(ConstantExpression));

    var constantExpression = ((ConstantExpression)maybeAConstantExpression);
    var constantValue = constantExpression.Value;

    //ConsoleApplication1.Program+<>c__DisplayClass0
    //Do not looks like a constant..this is a class...
    Console.WriteLine(constantValue);

    var fakeConstantValue = constantValue.GetType().GetField("wantedKey").GetValue(constantValue);
    //Return the value entered whith Console.ReadLine
    //This is not really known at compile time...
    Console.WriteLine(fakeConstantValue);
}

Answer 1

A const是可以在编译时进行评估的东西.a ConstantExpression仅代表固定值.这可能来自编译时,但不是必须的,而且往往不是.

常量表达式(在C#语言意义上)和ConstantExpression(运行时对象)之间存在差异.

在constantValue你的情况表示捕获上下文-吊沉默类wantedKey.基本上,该代码是(通过编译器):

class HorribleNameThatYouCannotSay {
    public string wantedKey; // yes, a public field
}
...
static void Sample()
{
    var ctx = new HorribleNameThatYouCannotSay();
    ctx.wantedKey = Console.ReadLine();
    var p = Expression.Parameter(typeof(SomeClass), "c");
    Expression<Func<SomeClass, bool>> expression =
        Expression.Lambda<Func<SomeClass, bool>>(
            Expression.Equal(
                Expression.PropertyOrField(p, "Key"),
                Expression.PropertyOrField(
                    Expression.Constant(ctx), "wantedKey")
                ), p);
    );
}

或者非常接近的东西

但它可能只是:

string wantedKey = Console.ReadLine();
var p = Expression.Parameter(typeof(SomeClass), "c");
Expression<Func<SomeClass, bool>> expression =
    Expression.Lambda<Func<SomeClass, bool>>(
        Expression.Equal(
            Expression.PropertyOrField(p, "Key"),
            Expression.Constant(wantedKey, typeof(string))
            ), p);

如果你用ExpresionAPI 手动编写它(如上所述)