组合 Lambda 表达式时出错：“'Foo' 类型的变量 'foo' 从范围 '' 引用，但未定义

Question

我正在尝试 combine two lambda expressions 构建带有 OR 子句的东西，但失败并显示以下异常消息：

variable 'foo' of type 'Foo' referenced from scope '', but it is not defined.

为什么，我该如何解决？

这是一个失败的代码示例，基于 Marc Gravell's answer 上面链接的问题：

static Expression<Func<T, bool>> Or<T>(Expression<Func<T, bool>> a, Expression<Func<T, bool>> b)
    => Expression.Lambda<Func<T, bool>>(Expression.OrElse(a.Body, b.Body), b.Parameters);

static Expression<Func<Foo, bool>> BarMatches(Bar bar) => foo => foo.Bar.Value == bar.Value;
static Expression<Func<Foo, bool>> BazMatches(Baz baz) => foo => foo.Baz.Value == baz.Value;

// sample usage (see below): foos.Where(Or(MatchesBar(bar), MatchesBaz(baz)))

void Main()
{
    var foos = new[]
    {
        new Foo
        {
            Bar = new Bar
            {
                Value = "bar"
            },
            Baz = new Baz
            {
                Value = "baz"
            }
        },
        new Foo
        {
            Bar = new Bar
            {
                Value = "not matching"
            },
            Baz = new Baz
            {
                Value = "baz"
            }
        }
    }.AsQueryable();

    var bar = new Bar { Value = "bar" };
    var baz = new Baz { Value = "baz" };

    Console.WriteLine(foos.Where(Or(BarMatches(bar), BazMatches(baz))).Count());
}


// Define other methods and classes here
class Foo
{
    public Bar Bar { get; set; }
    public Baz Baz { get; set; }
}

class Bar
{
    public string Value { get; set; }
}

class Baz
{
    public string Value { get; set; }
}

Answer 1

问题是 BarMatches 和 BazMatches 的参数 (foo) 不同。因此，您需要统一参数，以便 Or-ed 表达式使用相同的参数。这可以通过表达式替换器（从 this answer 中窃取）来完成：

static TExpr ReplaceExpressions<TExpr>(TExpr expression,
                                              Expression orig,
                                              Expression replacement)
where TExpr : Expression 
{
    var replacer = new ExpressionReplacer(orig, replacement);
    return replacer.VisitAndConvert(expression, "ReplaceExpressions");
}

private class ExpressionReplacer : ExpressionVisitor
{
    private readonly Expression From;
    private readonly Expression To;

    public ExpressionReplacer(Expression from, Expression to) {
        From = from;
        To = to;
    }

    public override Expression Visit(Expression node) {
        if (node == From) {
            return To;
        }
        return base.Visit(node);
    }
}

此 class 会将一个表达式的所有实例替换为另一个。我们现在可以使用它来创建一个统一的表达式：

static Expression<Func<T, bool>> Or<T>(Expression<Func<T, bool>> a, Expression<Func<T, bool>> b) {
    // We know Parameters is exactly of length 1.
    // If you had multiple parameters you would need to invoke this for each parameter
    var replaced = ReplaceExpressions(a.Body, a.Parameters[0], b.Parameters[0]);
    return Expression.Lambda<Func<T, bool>>(Expression.OrElse(replaced, b.Body), b.Parameters);
}

虽然此代码可以满足您的需求，但我建议使用一个库来执行此操作以及更多操作：LinqKIT

组合 Lambda 表达式时出错：“'Foo' 类型的变量 'foo' 从范围 '' 引用，但未定义

Error when combining Lambda expressions: "variable 'foo' of type 'Foo' referenced from the scope '', but it is not defined

c#

linq

lambda

linq-expressions