ترجمة التعبيرات وتنفيذها

من الشجرة إلى Delegate

using System;
using System.Linq.Expressions;

Expression<Func<int, int>> tree = x => x + 1;
Func<int, int> fn = tree.Compile();
Console.WriteLine(fn(5)); // 6

تجميع شجرة منشأة يدويًا

using System;
using System.Linq.Expressions;

var x = Expression.Parameter(typeof(int), "x");
var body = Expression.Multiply(x, Expression.Constant(3));
var lambda = Expression.Lambda<Func<int, int>>(body, x);
var fn = lambda.Compile();
Console.WriteLine(fn(4)); // 12

استدعاء النتيجة

using System;
using System.Linq.Expressions;

Expression<Func<int, int, int>> addExpr = (a, b) => a + b;
var add = addExpr.Compile();
Console.WriteLine(add(2, 3)); // 5
Console.WriteLine(add(10, 20)); // 30

التجميع ليس مجانيًا

using System;
using System.Linq.Expressions;

Expression<Func<int, int>> tree = x => x * x;
var square = tree.Compile(); // compile once
for (int i = 1; i <= 3; i++)
    Console.Write(square(i) + " "); // 1 4 9
Console.WriteLine();

إنشاء predicates ديناميكيًا

using System;
using System.Linq.Expressions;

var n = Expression.Parameter(typeof(int), "n");
var isPositive = Expression.GreaterThan(n, Expression.Constant(0));
var pred = Expression.Lambda<Func<int, bool>>(isPositive, n).Compile();
Console.WriteLine(pred(5));  // True
Console.WriteLine(pred(-2)); // False

دمج الشروط

using System;
using System.Linq.Expressions;

var n = Expression.Parameter(typeof(int), "n");
var inRange = Expression.AndAlso(
    Expression.GreaterThanOrEqual(n, Expression.Constant(1)),
    Expression.LessThanOrEqual(n, Expression.Constant(10)));
var fn = Expression.Lambda<Func<int, bool>>(inRange, n).Compile();
Console.WriteLine(fn(5) + " " + fn(11)); // True False

تجميع الحسابات

using System;
using System.Linq.Expressions;

var p = Expression.Parameter(typeof(double), "p");
// price * 1.2 (add 20% tax)
var withTax = Expression.Multiply(p, Expression.Constant(1.2));
var calc = Expression.Lambda<Func<double, double>>(withTax, p).Compile();
Console.WriteLine(calc(100)); // 120

استخدام DynamicInvoke مع lambdas غير المحددة النوع

using System;
using System.Linq.Expressions;

var x = Expression.Parameter(typeof(int), "x");
var body = Expression.Add(x, Expression.Constant(10));
Delegate d = Expression.Lambda(body, x).Compile();
Console.WriteLine(d.DynamicInvoke(5)); // 15

ملاحظة حول الأداء

using System;
using System.Linq.Expressions;

Expression<Func<int, int>> tree = x => x + x;
var doubler = tree.Compile();
int total = 0;
for (int i = 0; i < 5; i++) total += doubler(i);
Console.WriteLine(total); // 0+2+4+6+8 = 20

تظهر الأخطاء أثناء التجميع

using System;
using System.Linq.Expressions;

var x = Expression.Parameter(typeof(int), "x");
var y = Expression.Parameter(typeof(int), "y");
try {
    // body references y but lambda only declares x
    var lambda = Expression.Lambda<Func<int, int>>(Expression.Add(x, y), x);
    lambda.Compile();
} catch (InvalidOperationException) {
    Console.WriteLine("unbound parameter");
}

تجميع المفاهيم

using System;
using System.Linq.Expressions;

var x = Expression.Parameter(typeof(int), "x");
var formula = Expression.Add(
    Expression.Multiply(x, Expression.Constant(2)),
    Expression.Constant(3)); // 2x + 3
var fn = Expression.Lambda<Func<int, int>>(formula, x).Compile();
Console.WriteLine(fn(4)); // 11

اختبار سريع

مراجعة