c# - 带有子查询的表达式树

Question

我的目标是为动态全文搜索创建一个子查询表达式树。在 SQL 中，它相当于

SELECT * 
FROM MV 
WHERE MV.ID IN (SELECT ID  
                FROM MVF 
                WHERE title = "foo" OR Description = "foo")

所以基本思想是创建 FTS 子查询，从中获取 ID 并将它们用于 In 谓词。我的问题是第二部分。

// Get subquery for FTS tables
ParameterExpression ftsParam = Expression.Parameter(typeof(MVF), "mvfts");
var wphrase = Expression.Constant("foo");
var methodInfo = typeof(string).GetMethod("Equals", new Type[] { typeof(string) });

var ftsID = Expression.Property(ftsParam, "ID");
var ftsTitle = Expression.Property(ftsParam, "Title");
var ftsDescrip = Expression.Property(ftsParam, "Description");

var texp = Expression.Call(ftsTitle, methodInfo, wphrase);
var dexp = Expression.Call(ftsDescrip, methodInfo, wphrase);
var ftsExp = Expression.Or(texp, dexp);


// Now get ids from the above fts resultset
// THE ASSIGNMENT BELOW THROWS
var selectExp = Expression.Call(typeof(IEnumerable<MVF>), "Select", new Type[]
        {
           typeof(long)
        },
        ftsExp,
        Expression.Lambda<Func<MFV, long>>(
            ftsID,
            ftsParam
        )
);

// Now set up MV table reference
ParameterExpression vParam = Expression.Parameter(typeof(MV), "mv");
var mvID = Expression.Property(vParam, "MVID");
var containsInfo = typeof(IEnumerable<long>).GetMethod("Contains", new Type[] { typeof(long) });

// Now combine expression to get those mvs with ids in the result set of fts query
var containsExp = Expression.Call(selectExp, containsInfo, mvID);
return Expression.Lambda<Func<MV, bool>>(containsExp, vParam);

异常(exception)是:

No generic method 'Select' on type 'System.Collections.Generic.IEnumerable`1[MVF]' is compatible with the supplied type arguments and arguments. No type arguments should be provided if the method is non-generic.

Answer 1

所讨论的表达式所需的两种方法都是 Enumerable 的静态泛型扩展方法(最重要的是 static 和 generic)类:

Enumerable.Select

public static IEnumerable<TResult> Select<TSource, TResult>(
    this IEnumerable<TSource> source,
    Func<TSource, TResult> selector
)

Enumerable.Contains

public static bool Contains<TSource>(
    this IEnumerable<TSource> source,
    TSource value
)

“调用”此类方法最方便的方式如下 Expression.Call方法重载:

public static MethodCallExpression Call(
    Type type,
    string methodName,
    Type[] typeArguments,
    params Expression[] arguments
)

Type type 参数是定义被调用方法的类的类型(在本例中为 typeof(Enumerable))和 Type[] typeArguments 是具有泛型类型参数类型的数组(对于非泛型方法为空，对于 Select 应该是 { typeof(TSource), typeof(TResult) } > 和 { typeof(TSource) } 用于包含)。

将其应用到您的场景中:

var selectExp = Expression.Call(
    typeof(Enumerable), 
    "Select",
    new { typeof(MFV), typeof(long) },
    ftsExp,
    Expression.Lambda<Func<MFV, long>>(ftsID, ftsParam)
);

和

var containsExp = Expression.Call(
    typeof(Enumerable),
    "Contains",
    new [] { typeof(long) },
    selectExp,
    mvID
);