[英]Expression trees - parameter not mapped from lambda function argument to Expression Parameter
I am trying to wrap my head around Expression trees and I came to the below issue which I can not resolve.我试图围绕表达式树进行思考,但遇到了以下我无法解决的问题。 I am trying to generate a simple lambda function to check whether an integer value is even:
我正在尝试生成一个简单的 lambda 函数来检查整数值是否为偶数:
public static Func<int, bool> Generate_IsEven_Func()
{
var numParam = Expression.Parameter(typeof(int), "numParam");
var returnValue = Expression.Variable(typeof(bool), "returnValue");
var consoleWL = typeof(Console).GetMethod(nameof(Console.WriteLine), new[] { typeof(int) });
var body = Expression.Block(
// Scoping the variables
new[] { numParam, returnValue },
// Printing the current value for numParam
Expression.Call(
null,
consoleWL,
numParam
),
// Assign the default value to return
Expression.Assign(returnValue, Expression.Constant(false, typeof(bool))),
// If the numParam is even the returnValue becomes true
Expression.IfThen(
Expression.Equal(
Expression.Modulo(
numParam,
Expression.Constant(2, typeof(int))
),
Expression.Constant(0, typeof(int))
),
Expression.Assign(returnValue, Expression.Constant(true, typeof(bool)))
),
// value to return
returnValue
);
var lambda = Expression.Lambda<Func<int, bool>>(body, numParam).Compile();
return lambda;
}
When I call the newly created lambda function it appears that the value I pass as an argument is not 'mapped' with the respective expression parameter - numParam
.当我调用新创建的 lambda 函数时,我作为参数传递的值似乎没有与相应的表达式参数 -
numParam
“映射”。 Within the block expression I make a call to the Console.WriteLine
method to check the current value of numParam
and it is 0 every time:在块表达式中,我调用
Console.WriteLine
方法来检查numParam
的当前值,并且每次都为 0:
var isEvenMethod = Generate_IsEven_Func();
var cond = isEvenMethod(21);
Console.WriteLine(cond);
// Prints:
// 0
// True
You should not include numParam
in the array of variables you're passing as the first parameter to Expression.Block(...)
- right now you're essentially creating a new numParam
variable within the block that is unrelated to the numParam
parameter passed to the lambda.您不应该在作为第一个参数传递给
Expression.Block(...)
的变量数组中包含numParam
- 现在您实际上是在块中创建一个新的numParam
变量,该变量与传递的numParam
参数无关到拉姆达。 Since the variable is automatically initialized with the default value for int
(0), that explains the output you're seeing.由于变量使用
int
(0) 的默认值自动初始化,这就解释了您看到的输出。
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