c# - 用表达式树查询多个集合

Question

我有一些由 EF Core 提供的 IQueryable 集合，我想使用 System.Linq.Expressions for Entity 为它们构建动态查询以将其转换为 SQL。

IQueryable<ADP> collection1 = _context.Adps;
IQueryable<VHStr> collection2 = _context.VHStrParams;

var q = (from ADP a in collection1
        where collection2.Any(p => p.Value.Contains("I") && p.Entid == a.Id)
        select a);

var l = q.ToList();

这正常工作并且实体生成正确的 SQL。

问题是——如何使用表达式构建这样的查询？我只是不知道如何在为第一个元素构建的表达式中访问另一个集合...

编辑:万一有人找到它，解决方案是:

        IQueryable<ADP> collection1 = _context.Adps;
        IQueryable<VHStr> collection2 = _context.VHStrParams;


        var q = (from ADP a in collection1
                 where collection2.Any(p => p.Value.Contains("I") && p.Entid == a.Id)
                 select a);

        var paramExpA = Expression.Parameter(typeof(ADP), "a");
        var paramExpV = Expression.Parameter(typeof(VHStr), "v");

        var entIdExp = Expression.PropertyOrField(paramExpV, "Entid");
        var adpIdExp = Expression.PropertyOrField(paramExpA, "Id");
        var convertedAdpIdExp = Expression.Convert(adpIdExp, typeof(long?));

        var valueExp = Expression.PropertyOrField(paramExpV, "Value");
        var containsStringMethod = typeof(string).GetMethod("Contains", new[] {typeof(string)});
        var constValueExp = Expression.Constant("I", typeof(string));
        var containsExp = Expression.Call(valueExp, containsStringMethod, constValueExp);

        var equalIdsExp = Expression.Equal(entIdExp, convertedAdpIdExp);
        var andExp = Expression.AndAlso(containsExp, equalIdsExp);

        var lambda1 = Expression.Lambda<Func<VHStr, bool>>(andExp, paramExpV);
        var vhstrAnyMethod =
            typeof(Queryable)
                .GetTypeInfo()
                .GetMethods()
                .First(m => m.Name == "Any" && m.GetParameters().Count() == 2)
                .MakeGenericMethod(typeof(VHStr));

        var collection2ConstExpr = Expression.Constant(collection2);
        var anyCallExp = Expression.Call(vhstrAnyMethod, collection2ConstExpr, lambda1);

        var collection1ConstExpr = Expression.Constant(collection1);
        var lambda2 = Expression.Lambda<Func<ADP, bool>>(anyCallExp, paramExpA);

        var whereExp = Expression.Call(
            typeof(Queryable),
            "Where",
            new Type[] { typeof(ADP) },
            collection1ConstExpr,
            lambda2);

        var lambda3 = Expression.Lambda<Func<IQueryable<ADP>>>(whereExp);
        var resultFunc = lambda3.Compile();
        var resultQuerable = resultFunc();

        var resultList = resultQuerable.ToList();

Answer 1

I just can't figure out how can I access another collection in the expressions built for elements of the first one...

好吧，编译时查询使用闭包类将外部变量传递给查询。您可以在手动构建表达式树时执行相同的操作，但没有必要这样做，因为您可以简单地使用 Expression.Constant。将变量转换为表达式的方法:

var collection2expr = Expression.Constant(collection2);
var anyParam = Expression.Parameter(collection2.ElementType, "p");
var anyPredicate = Expression.Lambda(
    dynamically_built_predicate_body, // p.Value.Contains("I") && p.Entid == a.Id
    anyParam);
var anyCall = Expression.Call(
    typeof(Queryable), "Any", new Type[] { anyParam.Type },
    collection2Expr, Expression.Quote(anyPredicate));
// etc...