C# 动态lambda表达式不'；t在LINQ to EF Where函数中工作

我正在尝试在asp.net mvc中完成一个页面，以过滤sql server视图。 我的页面如下所示：

每个“过滤器”都是一个

SearchViewModel

，其类定义如下：

public class SearchViewModel
{
   //Property has a property called "SqlColumnName"
   public Property Property { get; set; }
   public Enums.SearchOperator Operator { get; set; }
   public string Value { get; set; }
}

var lambda = Helper.GetLamdaExpression(lstSearchViewModels);
var eventEntries = DataBase.ViewEvents.Where(lambda);

当我提交表格a

IList

时，它将被传递给控制器。在这之前一切都很好

在控制器类中，我将

IList

传递给助手方法：

public static Func<ViewEvents, bool> GetLamdaExpression(IEnumerable<SearchViewModel> lstSearchViewModels)
{
    Expression dynamiclambda = null;
    Expression call;

    //creating the first part of the lambda expression (select.Where(ve => ve.Name == "exp"))
    // this function returns "(ve =>" as typeof ViewEvents
    var param = Expression.Parameter(typeof(ViewEvents), "viewEvents");

    //storing functions for use with the combiner
    var containsMethod = typeof(string).GetMethod("Contains");
    var equalsMethod = typeof(string).GetMethod("Equals", new[] { typeof(string) });
    var startsWithMethod = typeof(string).GetMethod("StartsWith", new[] { typeof(string) });
    var endsWithMethod = typeof(string).GetMethod("EndsWith", new[] { typeof(string) });
    var toLowerMethod = typeof(string).GetMethod("ToLower", Type.EmptyTypes);

    foreach (var lstSearchViewModel in lstSearchViewModels.Where(svm => svm.Value != null))
    {
        //get the property info of the property from the SearchViewModel
        var property = typeof(ViewEvents).GetProperty(lstSearchViewModel.Property.SqlColumnName);
        if (property == null)
            throw new ObjectNotFoundException($"The object {typeof(ViewEvents).ToString()} does not have a property called {lstSearchViewModel.Property.SqlColumnName}");

        //create the second part of the lambda expression
        //this function returns "ve.Property"
        var propertyAccess = Expression.MakeMemberAccess(param, property);

        //add the "toLower" function to the property
        //this function returns "(ve.Property.ToLower()"
        var toLower = Expression.Call(propertyAccess, toLowerMethod);

        //adds the operator to the lambda expression
        //functions return p.ex.: ve.Property.ToLower().Contains("value")
        //                     or ve.Property.ToLower().Equals("value") != true  (NotEquals)
        switch (lstSearchViewModel.Operator)
        {
            case Enums.SearchOperator.Contains:
                call = Expression.Call(toLower, containsMethod,
                    Expression.Constant(lstSearchViewModel.Value.ToLower()));
                break;
            case Enums.SearchOperator.ContainsNot:
                call = Expression.Call(toLower, containsMethod,
                    Expression.Constant(lstSearchViewModel.Value.ToLower()));
                //adding ..Contains("value") != true ; used like .ContainsNot("value")
                call = Expression.NotEqual(call, Expression.Constant(true));
                break;
            case Enums.SearchOperator.StartsWith:
                call = Expression.Call(toLower, startsWithMethod,
                    Expression.Constant(lstSearchViewModel.Value.ToLower()));
                break;
            case Enums.SearchOperator.EndsWith:
                call = Expression.Call(toLower, endsWithMethod,
                    Expression.Constant(lstSearchViewModel.Value.ToLower()));
                break;
            case Enums.SearchOperator.Equals:
                call = Expression.Call(toLower, equalsMethod,
                    Expression.Constant(lstSearchViewModel.Value.ToLower()));
                break;
            case Enums.SearchOperator.EqualsNot:
                call = Expression.Call(toLower, equalsMethod,
                    Expression.Constant(lstSearchViewModel.Value.ToLower()));
                //adding ..Equals("value") != true ; used like .NotEquals("value")
                call = Expression.NotEqual(call, Expression.Constant(true));
                break;
            default:
                throw new ArgumentOutOfRangeException();
        }
        //Combind the filters with an and combiner
        dynamiclambda = dynamiclambda == null ? call : Expression.And(dynamiclambda, call);
    }

    if (dynamiclambda == null)
    {
        throw new InvalidOperationException("No dynamiclambda was created");
    }

    //gets the actual lambda expression like: (ve => ve.Property.ToLower().Contains("value") AND ve.Property.ToLower().Equals("value") ...
    var predicate = Expression.Lambda<Func<ViewEvents, bool>>(dynamiclambda, param);
    var compiled = predicate.Compile();

    return compiled;
}

“错误”来了。没有从数据库返回任何行。但是当我把代码改成

var lambda = Helper.GetLamdaExpression(lstSearchViewModels);
var eventEntries = DataBase.ViewEvents.Where(viewEvents => viewEvents.Level.ToLower().Contains("1"));

返回预期的行数

---

有人知道错误在哪里吗？

问题是必须使用

表达式，而不是
函数。第一个包含一个表达式树，可以通过解析将
Where
子句转换为SQL。后者不能做到这一点

此外，您的操作过于复杂：您可以直接返回lambda，而无需做太多工作。这是一个简化的示例：

public Expression<Func<ViewEvents, bool>> GetEqualIdLambda(int id)
{
    return (ViewEvents ve) => ve.Id == id;
}

公共表达式GetEqualIdLambda（int-id）
{
return（ViewEvents ve）=>ve.Id==Id；
}
问题是您必须使用
表达式，而不是
Func
。第一个包含一个表达式树，可以通过解析将
Where
子句转换为SQL。后者不能做到这一点

此外，您的操作过于复杂：您可以直接返回lambda，而无需做太多工作。这是一个简化的示例：

public Expression<Func<ViewEvents, bool>> GetEqualIdLambda(int id)
{
    return (ViewEvents ve) => ve.Id == id;
}

公共表达式GetEqualIdLambda（int-id）
{
return（ViewEvents ve）=>ve.Id==Id；
}