Question

我正在尝试构建一个Entity Framework 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，而且我很熟悉这样做，就像这样一个简单的表达式：

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Northwind数据库，其中有多个连接（Customers＆lt; - CustomerCustomerDemo - ＆gt; CustomerDemographics）。我遇到的问题是在下面的表达式中存在嵌套的lambda时构建表达式树，我将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Expression树，其中包含“Any”的嵌套lambda？任何帮助都表示赞赏，即使它是如何做的伪代码。关于这个问题并不多，我很绝望，请帮忙！

Answer 1

以下是我提出的解决方案，我最大的帮助来自Calling a Method from an Expression 最重要的是创建一个通用的“Any”方法。其余的只是将这些电话联系在一起。

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;

}

为实体框架构建嵌套的lambda表达式树多对多关系？

1 个答案: