c# - “注入(inject)”一个表达式到另一个

Question

我有一个这样定义的表达式:

Expression<Func<T1, T2, bool>> firstExpression;

还有一个像这样:

Expression<Func<T1, bool>> secondExpression;

T2 是我知道的值，T1 不是。我想要的是根据参数 T2 的值将第一个表达式变成第二个表达式。如果它是常规的 Linq，它将是这样的:

var t2 = "Something I know";
secondExpression = t1 => fistExpression(t1, t2);

我如何使用 System.Linq.Expressions 执行此操作？

Answer 1

您可以使用表达式 visitor 完成此操作:

public static class EmitUtils
{
        private class ParameterReplacerVisitor : ExpressionVisitor
        {
            private ParameterExpression _source;
            private Expression _target;

            public ParameterReplacerVisitor(ParameterExpression source, Expression target)
            {
                _source = source;
                _target = target;
            }

            public override Expression Visit(Expression node) =>
                node == _source
                    ? _target
                    : base.Visit(node);
        }

        public static Expression ReplaceParameter(Expression body, ParameterExpression srcParameter, Expression dstParameter) =>
            new ParameterReplacerVisitor(srcParameter, dstParameter).Visit(body);

        public static Expression<Func<T1, T3>> BuildClosure<T1, T2, T3>(Expression<Func<T1, T2, T3>> src, T2 closureValue)
        {
            var constExpression = Expression.Constant(closureValue, typeof(T2));
            var body = ReplaceParameter(src.Body, src.Parameters[1], constExpression);
            return Expression.Lambda<Func<T1, T3>>(body, src.Parameters[0]);
        }

    }

这是示例用法:

        [Test]
        public void ClosureTest()
        {
            Expression<Func<int, string, bool>> CheckStringLength = (len, str) => str.Length < len;
            var constString = "some string";
            var result = EmitUtils.BuildClosure(CheckStringLength, constString);
            Assert.That(result.Compile().Invoke(100), Is.True);
        }