如何获得Lambda表达式的值 [英] How to get the value of a lambda expression

问题描述

说我们有以下内容:

Say we have the following :

        private static string Query<T>(Expression<Func<T, bool>> pr)
        {
            return pr.ToString();
        }
...
int ii = 1000;
string str = Query((SalesInvoiceRow r) => r.Serial > ii && r.CustomerName.Contains( "23"));

当您运行上面的代码时，我们得到:

When you run the above we get:

r => ((r.Serial > value(testing.program+<>c__DisplayClass0).ii) AndAlso r.CustomerName.Contains("23"))

如何获得ii的值，而不是如下所示的丑陋语句?

How can we get the value of ii instead of the ugly statement like below?

r => ((r.Serial > 1000) AndAlso r.CustomerName.Contains("23"))

推荐答案

Hello Mehdi，

这是一些与ExpressionVisitor一起调整的实验代码:

Hello Mehdi,

This is some experimental code to tweak with the ExpressionVisitor:

static void Main(string[] args)
{
    int a = 5;
    int b = 100;
    Expression<Func<int, bool>> expr = v => v > (a + b);
    WalkVisitor w = new WalkVisitor();
    w.Visit(expr);
}
public class WalkVisitor : ExpressionVisitor
{
    protected override Expression VisitMember(MemberExpression node)
    {
        var e = base.VisitMember(node);
        var c = node.Expression as ConstantExpression;
        if (c != null)
        {
            Type t = c.Value.GetType();
            var x = t.InvokeMember(node.Member.Name, BindingFlags.GetField,
                                   null, c.Value, null);
            Console.WriteLine("{0} = {1}", node.ToString(), x);
        }
        return e;
    }
}

输出为:

The output is:

value(ExprVisitor.Program+<>c__DisplayClass0).a = 5
value(ExprVisitor.Program+<>c__DisplayClass0).b = 100

您可能需要围绕此代码段实现打印访问者...

干杯

Andi

You might need to implement a print visitor around this snippet...

Cheers

Andi

您应该能够编译该表达式，然后对其进行ToString.

You should be able to compile the expression and then ToString it.

pr.Compile().ToString();

我只是意识到您想要ii.我现在很困惑.您不是将ii设置为1000吗?您为什么现在想要该值?
也许您是想让查询解决.好吧，你应该做我随后发表的事情，Compile:)

I just realized you want ii. I am totally confused now. Aren''t you setting ii to 1000? Why do you now want the value?
Maybe you meant you want the query resolved. Well you should do what I posted then, Compile :)

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