linq to entities dynamic from lambdas build

Question

我有一套像这样的lambdas

t => t.FirstName
t => t.LastName
t => t.Profession

我想找到一种方法来构建一个表达式,该表达式可以在Linq to Entities 中的Where语句中使用,其中这些lambda与使用string.contains的值进行比较

// a filter is definded by a lambda and the string to compare it with   
var filters = new Dictionary<Expression<Func<Person, string>>, string>();
filters.Add(t => t.FirstName, "Miller");
filters.Add(t => t.Profession, "Engineer");
var filterConstraints = BuildFilterExpression(t => t, filters);
Entities.Persons.Where(filterConstraints).ToList();

public static Expression<Func<TElement, bool>> BuildFilterExpression<TElement>(Dictionary<Expression<Func<TElement, string>>, string> constraints)
{
  List<Expression> expressions = new List<Expression>();

  var stringType = typeof(string);
  var containsMethod = stringType.GetMethod("Contains", new Type[] { stringType });

  foreach (var constraint in constraints)
  {
    var equalsExpression = (Expression)Expression.Call(constraint.Key.Body, containsMethod, Expression.Constant(constraint.Value, stringType));
    expressions.Add(equalsExpression);
  }

  var body = expressions.Aggregate((accumulate, equal) => Expression.And(accumulate, equal));

  ParameterExpression p = constraints.First().Key.Parameters.First();
  return Expression.Lambda<Func<TElement, bool>>(body, p);
}

我想我在构建表达式树时做了一些非常错误的事情,因为我得到以下异常: 无效的操作异常 - 参数't'未绑定在指定的LINQ to Entities查询表达式中.

有谁知道如何解决这个问题？

Answer 1

你真的很亲密.问题是具有相同名称和类型的参数对象在技术上并不"相等".

var b = Expression.Parameter(typeof(string), "p") == 
    Expression.Parameter(typeof(string), "p");
//b is false

因此,您创建的lambda的参数是您作为输入使用的第一个表达式的参数.所有其他表达式的主体中使用的参数是不同的参数,并且它们不作为lambda的参数给出,因此错误就是这样.

解决方案实际上非常简单.您只需要将所有其他参数的所有实例替换为您要使用的实际参数.

这是一个辅助方法(使用辅助类),它在一些表达式中获取一个表达式的所有实例,并将其替换为另一个表达式:

public class ReplaceVisitor : ExpressionVisitor
{
    private readonly Expression from, to;
    public ReplaceVisitor(Expression from, Expression to)
    {
        this.from = from;
        this.to = to;
    }
    public override Expression Visit(Expression node)
    {
        return node == from ? to : base.Visit(node);
    }
}

public static Expression Replace(this Expression expression,
    Expression searchEx, Expression replaceEx)
{
    return new ReplaceVisitor(searchEx, replaceEx).Visit(expression);
}

现在我们只需在每个主体上调用一次,替换为一个通用参数:

public static Expression<Func<TElement, bool>> BuildFilterExpression<TElement>(
    Dictionary<Expression<Func<TElement, string>>, string> constraints)
{
    List<Expression> expressions = new List<Expression>();

    var stringType = typeof(string);
    var containsMethod = stringType.GetMethod("Contains", new Type[] { stringType });

    var parameter = Expression.Parameter(typeof(TElement));

    foreach (var constraint in constraints)
    {
        var equalsExpression = (Expression)Expression.Call(
            constraint.Key.Body.Replace(constraint.Key.Parameters[0], parameter),
            containsMethod, Expression.Constant(constraint.Value, stringType));
        expressions.Add(equalsExpression);
    }

    var body = expressions.Aggregate((accumulate, equal) =>
        Expression.And(accumulate, equal));

    return Expression.Lambda<Func<TElement, bool>>(body, parameter);
}