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我正在尝试构建一个实体框架 4 多对多关系过滤器，这是一个动态数据项目。我知道我需要在运行时构建一个表达式树，并且我很熟悉这样的简单 where 表达式：

private MethodCallExpression BuiltMethodCall(IQueryable _query, Type _ObjType, string _ColumnToSearch, string _SearchValue)
{
ConstantExpression value = Expression.Constant(_SearchValue);
ParameterExpression _parameter = Expression.Parameter(_ObjType, "value");
MemberExpression _property = Expression.Property(_parameter, _ColumnToSearch);
BinaryExpression comparison = Expression.Equal(_property, value);
LambdaExpression lambda = Expression.Lambda(comparison, _parameter);

//Ex: Customers.Select(c => c).Where(value => (value.City == "Seattle"))
MethodCallExpression _where = Expression.Call(typeof(Queryable), "Where", new Type[] { _query.ElementType }, new Expression[] {
    _query.Expression,
    Expression.Quote(lambda)
});
return _where;

}

为简单起见，这些示例使用了 Northwind 数据库，其中存在 (Customers <- CustomerCustomerDemo -> CustomerDemographics) 的多对多连接。我遇到的问题是当存在如下表达式中的嵌套 lambda 时构建表达式树，我在其中检索具有特定客户人口统计的所有客户。

string custDemogID = "3";

//Get customers who have a particular demog
dynamic cust = context.Customers.Where(c => c.CustomerDemographics.Any(cd => cd.CustomerTypeID == custDemogID));

如何构建具有“where”调用的表达式树，其中包括“Any”的嵌套 lambda？任何帮助表示赞赏，即使它是关于如何做到这一点的伪代码。没有太多关于这个的，我很绝望，请帮助！

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1 回答 1

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这是我想出的解决方案，我最大的帮助来自从表达式调用方法。要克服 的最大问题是创建一个通用的“任何”方法。其余的只是将调用链接在一起。

private MethodCallExpression BuiltMethodCall(IQueryable _query, string CustTypeID, Type _ParentObjType, Type _ChildObjType, string strChildObj, string strChildCol)
{

//This function will build a dynamic linq expression tree representing the ling calls of:
//Customers.Where(c => c.CustomerDemographics.Any(cd => cd.CustomerTypeID = custTypeID))

ConstantExpression value = Expression.Constant(CustTypeID);

//Build the outer part of the Where clause
ParameterExpression parameterOuter = Expression.Parameter(_ParentObjType, "c");
MemberExpression propertyOuter = Expression.Property(parameterOuter, strChildObj);

//Build the comparison inside of the Any clause
ParameterExpression parameterInner = Expression.Parameter(_ChildObjType, "cd");
MemberExpression propertyInner = Expression.Property(parameterInner, strChildCol);

BinaryExpression comparison = Expression.Equal(propertyInner, value);
LambdaExpression lambdaInner = Expression.Lambda(comparison, parameterInner);

//Create Generic Any Method
Func<MethodInfo, bool> methodLambda = m => m.Name == "Any" && m.GetParameters().Length == 2;
MethodInfo method = typeof(Enumerable).GetMethods().Where(methodLambda).Single().MakeGenericMethod(_ChildObjType);

//Create the Any Expression Tree and convert it to a Lambda
MethodCallExpression callAny = Expression.Call(method, propertyOuter, lambdaInner);
LambdaExpression lambdaAny = Expression.Lambda(callAny, parameterOuter);

//Build the final Where Call
MethodCallExpression whereCall = Expression.Call(typeof(Queryable), "Where", new Type[] { _query.ElementType }, new Expression[] {
    _query.Expression,
    Expression.Quote(lambdaAny)
});

return whereCall;

}

于 2012-10-30T22:45:53.577 回答