C# 如何使用动态Linq进行左外连接？

我试图模仿左外连接，但使用动态linq扩展方法。我所拥有的：

public static IQueryable SelectMany(this IQueryable source, string selector, 
    string resultsSelector, params object[] values)
{
    if (source == null) throw new ArgumentNullException("source");
    if (selector == null) throw new ArgumentNullException("selector");

    // Parse the lambda 
    LambdaExpression lambda = DynamicExpression.ParseLambda(
        source.ElementType, null, selector, values);

    // Fix lambda by recreating to be of correct Func<> type in case  
    // the expression parsed to something other than IEnumerable<T>. 
    // For instance, a expression evaluating to List<T> would result  
    // in a lambda of type Func<T, List<T>> when we need one of type 
    // an Func<T, IEnumerable<T> in order to call SelectMany(). 
    Type inputType = source.Expression.Type.GetGenericArguments()[0];
    Type resultType = lambda.Body.Type.GetGenericArguments()[0];
    Type enumerableType = typeof(IEnumerable<>).MakeGenericType(resultType);
    Type delegateType = typeof(Func<,>).MakeGenericType(inputType, 
        enumerableType);
    lambda = Expression.Lambda(delegateType, lambda.Body, lambda.Parameters);

    ParameterExpression[] parameters = new ParameterExpression[] { 
        Expression.Parameter(source.ElementType, "outer"), 
        Expression.Parameter(resultType, "inner") 
    };

    LambdaExpression resultsSelectorLambda = DynamicExpression.ParseLambda(
        parameters, null, resultsSelector, values);

    // Create the new query 
    return source.Provider.CreateQuery(Expression.Call(typeof(Queryable), 
        "SelectMany", new Type[] { 
            source.ElementType, 
            resultType, 
            resultsSelectorLambda.Body.Type 
        }, source.Expression, Expression.Quote(lambda), 
        Expression.Quote(resultsSelectorLambda)));            
}

public static IQueryable SelectMany（此IQueryable源、字符串选择器、，
字符串结果选择器，参数对象[]值）
{
如果（source==null）抛出新的ArgumentNullException（“source”）；
如果（选择器==null）抛出新的ArgumentNullException（“选择器”）；
//解析lambda
LambdaExpression lambda=DynamicExpression.ParseLambda(
source.ElementType，null，选择器，值）；
//通过将lambda重新创建为正确的Func类型来修复
//解析为IEnumerable以外的内容的表达式。
//例如，将生成一个计算为List的表达式
//在Func类型的lambda中，当我们需要
//函数添加
void DefaultIfEmpty（）；
到
接口IEnumerableSignatures

然后使用

public static object DefaultIfEmpty(this IQueryable source)
{
    if (source == null) throw new ArgumentNullException("source");
        return source.Provider.Execute(
    Expression.Call(
        typeof(Queryable), "DefaultIfEmpty",
        new Type[] { source.ElementType },
        source.Expression));
}

那你有一个电话

var qry = Foo.GroupJoin(Bar, "outer.Id", "inner.Id", "new(outer.Id as Foo, group as Bars)").SelectMany("Bars.DefaultIfEmpty()", "new(outer.Foo as Foo, inner as Bar)");

您在这里指的是DynamicExpression的哪种实现？你们能提供更简要的信息吗？我需要使用这个：(
var qry = Foo.GroupJoin(Bar, "outer.Id", "inner.Id", "new(outer.Id as Foo, group as Bars)").SelectMany("Bars.DefaultIfEmpty()", "new(outer.Foo as Foo, inner as Bar)");