.net 实体框架 无法将 lambda 表达式转换为类型字符串，因为它不是委托类型

我在基于C#的代码中使用实体框架.我遇到了意想不到的怪事，正在寻找建议.

case 1，2，3，4...

Samples (all of these work):
RivWorks.Alpha.dll:

public static bool EndNegotitation(long ProductID)
{
    var product = (from a in _dbFeed.AutoWithImage 
                   where a.AutoID == ProductID select a).FirstOrDefault();
...
}

RivWorks.Service.dll个

public static RivWorks.Model.NegotiationAutos.AutoWithImage 
    GetProductById(long productId)
{
    var myProduct = from a in _dbFeed.AutoWithImage 
                    where a.AutoID == productId select a;

    return myProduct.FirstOrDefault();
}
public static List<RivWorks.Model.NegotiationAutos.AutoWithImage> 
    GetProductByCompany(Guid companyId)
{
    var myProduct = from a in _dbFeed.AutoWithImage 
                    where a.CompanyID == companyId select a;

    return myProduct.ToList();
}

等

case "古怪":

public NegotiateSetup GetSetup(string method, string jsonInput)
{
    ...
    long.TryParse(ProductID, out result);
    var product = (from a in _dbFeed.AutoWithImage 
                   where a.AutoID == result select a).FirstOrDefault();
    ...
}

I am getting this compile time error (the word "where" is highlighted in my editor):
Cannot convert lambda expression to type 'string' because it is not a delegate type

你知道这是什么原因吗？