Question

（向底部回答）

我正在尝试构建一个结合

的系统

Func<T, bool>

委托ExpressionTree，它允许我传入一个值（在这种情况下为badValue）并在谓词全部返回true并考虑二进制操作时获取bool。这是我第一次使用Expression / ExpressionTrees，所以请保持温和。

我收到了这个错误：

ArgumentException：无法调用“System.Boolean”类型的表达式

在这一行：

collectAnswers = Expression.And(isEmpty.Body, Expression.Invoke(...

我已经设置了那条线，因为我需要在所有表达式中共享对value的引用（对吗？）。

我理想的情况是只有一堆

Expression<Func<blah, blah, bool>>

我可以将它们与逻辑运算符（和/或/不）一起传递到系统中，并在最后得到一个bool。希望能够动态构建值必须通过的规则。

在我要走的路上，这是否可能？如果没有，那么指示我走正确道路的几条指针将不胜感激。

string badValue = "hello!";
const int minSize = 8;
const int maxSize = 30;

Expression<Func<string, bool>> stringLengthMax = value => value.Length < maxSize;
Expression<Func<string, bool>> stringLengthMin = value => value.Length > minSize;
Expression<Func<string, bool>> isEmpty = value => !string.IsNullOrEmpty(value);

BinaryExpression collectAnswers = Expression.And(stringLengthMax.Body, Expression.Invoke(stringLengthMin, stringLengthMax.Parameters));
collectAnswers = Expression.And(isEmpty.Body, Expression.Invoke(collectAnswers, stringLengthMax.Parameters));

Func<string, bool> shouldValidate = Expression.Lambda<Func<string, bool>>(collectAnswers, stringLengthMax.Parameters).Compile();
bool result = shouldValidate(badValue);

答案我没有以正确的方式推进参数，下面是几个表达式之间共享的多个参数的示例，这些表达式被放入ExpressionTree中，并且单个布尔值来自编译的Func，isValid

const int minSize = 8;
const int maxSize = 30;

Expression<Func<string, int, bool>> stringLengthMax = (value, max) => value.Length <= max;
Expression<Func<string, int, bool>> stringLengthMin = (value, min) => value.Length >= min;
Expression<Func<string, bool>> isEmpty = value => string.IsNullOrEmpty(value);

ParameterExpression valueParameter = Expression.Parameter(typeof(string));
ParameterExpression minParameter = Expression.Parameter(typeof(int));
ParameterExpression maxParameter = Expression.Parameter(typeof(int));

Expression<Func<string, int, int, bool>> minMaxCheck =
    Expression.Lambda<Func<string, int, int, bool>>(
        Expression.And(Expression.Invoke(stringLengthMax, valueParameter, maxParameter), 
            Expression.Invoke(stringLengthMin, valueParameter, minParameter)), valueParameter, minParameter, maxParameter);

minMaxCheck = Expression.Lambda<Func<string, int, int, bool>>(
    Expression.And(Expression.Invoke(minMaxCheck, valueParameter, minParameter, maxParameter), 
        Expression.Not(Expression.Invoke(isEmpty, valueParameter))), valueParameter, minParameter, maxParameter);

Func<string, int, int, bool> isValid = minMaxCheck.Compile();
bool resultFalse1 = isValid("hello!", minSize, maxSize); // false - too short
bool resultTrue1 = isValid("hello!", "hello!".Length, maxSize); // true - adjust min
bool resultFalse2 = isValid("1234567890123456789012345678901", minSize, maxSize); // false - too long
bool resultTrue2 = isValid("1234567890123456789012345678901", minSize, "1234567890123456789012345678901".Length); // true - adjust max
bool resultFalse3 = isValid(string.Empty, minSize, maxSize); // false - empty
bool shouldBeTrue = isValid("1234567890", minSize, maxSize); // true - just right
bool resultFalse4 = isValid("1234567890", maxSize, maxSize); // false - adjust min
bool resultFalse5 = isValid("1234567890", minSize, minSize); // false - adjust max

Answer 1

如果你想用Expressions做这件事，这样的事情就可以了。这不会短路，虽然你可以建立它。你非常接近。您需要通过整个参数树线程化一个参数表达式。

string badValue = "hello!";
const int minSize = 8;
const int maxSize = 30;

Expression<Func<string, bool>> stringLengthMax = value => value.Length < maxSize;
Expression<Func<string, bool>> stringLengthMin = value => value.Length > minSize;
Expression<Func<string, bool>> isEmpty = value => !string.IsNullOrEmpty(value);

ParameterExpression pe = Expression.Parameter(typeof(string));

var x = Expression.Lambda<Func<string, bool>>(
    Expression.And(Expression.Invoke(stringLengthMax, pe), 
        Expression.And(Expression.Invoke(stringLengthMin, pe), Expression.Invoke(isEmpty, pe))), pe);

Func<string, bool> shouldValidate = x.Compile();
bool resultFalse1 = shouldValidate("hello!");
bool resultFalse2 = shouldValidate("1234567890123456789012345678901");
//bool resultFalse3 = shouldValidate(null); Throws an exception because you can't do (null).Length
bool shouldBeTrue = shouldValidate("123456789");

//LinqPad code to view results:
resultFalse1.Dump();
resultFalse2.Dump();
//resultFalse3.Dump();
shouldBeTrue.Dump();

Answer 2

在我看来，你根本不需要构建表达式树。您可以使用简单的linq组合您的func（将它们定义为Func<string, bool>，而不是Expression<Func<string, bool>>）：

Func<string, bool> shouldValidate = 
    arg => new[] {stringLengthMax, stringLengthMin, isEmpty}.All(func => func(arg));

如果您想使用的不只是and，您可以合理地组合您的功能：

Func<string, bool> shouldValidate = 
    arg => isEmpty(arg) || (stringLengthMax(arg) && stringLengthMin(arg));

无法创建复合表达式<func <string，bool =“”>＆gt;来自一组表达式</func <string，>

2 个答案: