Question

我正在尝试在LINQ中动态创建联接。 This answer让我对从哪里开始有了很好的了解。但是，当“foreing key”是int?时，我特别遇到问题（int有效）。当它是int?时，它将触发异常。

我已经制作了一个简单的LINQ to Objects示例来显示问题。为了运行我的示例，您需要install the dynamic linq Nuget package，因为我将其用于DynamicExpression.ParseLambda方法。

这是触发异常的代码：

public class Contact
{
   public int Id { get; set; }

   public string Name { get; set; }
}

public class Address
{
   public int Id { get; set; }
   public int? ContactId { get; set; }

   public string Zip { get; set; }
}

class Program
{
   static void Main(string[] args)
   {
       var contacts = new List<Contact>();
       var addresses = new List<Address>();

       LambdaExpression outerSelectorLambda = DynamicExpression.ParseLambda(typeof(Contact), null, "Id", new object[0]);
       LambdaExpression innerSelectorLambda = DynamicExpression.ParseLambda(typeof(Address), null, "ContactId", new object[0]);
       ParameterExpression[] parameters = new ParameterExpression[] { Expression.Parameter(typeof(Contact), "outer"), Expression.Parameter(typeof(Address), "inner") };
       LambdaExpression resultsSelectorLambda = DynamicExpression.ParseLambda(parameters, null, "outer.Id", new object[0]);

       // BLOWS UP HERE
       var queryExpression = Expression.Call(
           typeof(Queryable), "Join",
           new Type[] { typeof(Contact), typeof(Address), typeof(int?), typeof(int) },
           contacts.AsQueryable().Expression, addresses.AsQueryable().Expression,
           Expression.Quote(outerSelectorLambda), Expression.Quote(innerSelectorLambda), Expression.Quote(resultsSelectorLambda));

       // it will not reach the following line
       var queryable = contacts.AsQueryable().Provider.CreateQuery(queryExpression);
   }
}

要使上述代码生效，只需将int?的{{1}}替换为int。一切都会好起来的。问题在于Address.ContactId是int?。

问题是：为什么会发生这种情况？我该如何解决？。

异常信息（葡萄牙语）：

System.InvalidOperationException was unhandled
 HResult=-2146233079
 Message=Nenhum método genérico 'Join' no tipo 'System.Linq.Queryable' é compatível com os argumentos e os argumentos de tipo fornecidos. Nenhum argumento de tipo deve ser fornecido se o método for não genérico. 
 Source=System.Core
 StackTrace:
      em System.Linq.Expressions.Expression.FindMethod(Type type, String methodName, Type[] typeArgs, Expression[] args, BindingFlags flags)
      em System.Linq.Expressions.Expression.Call(Type type, String methodName, Type[] typeArguments, Expression[] arguments)
      em problems.Program.Main(String[] args) na d:\POCs\problems\problems\Program.cs:linha 38
      em System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
      em System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
      em Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
      em System.Threading.ThreadHelper.ThreadStart_Context(Object state)
      em System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
      em System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
      em System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
      em System.Threading.ThreadHelper.ThreadStart()
 InnerException:

Answer 1

问题是您的密钥类型为int?，但您用于创建密钥选择器的ParseLambda方法是为您创建类型为Expression<Func<Contact, int>>的表达式外键选择器。它没有将结果映射到可空的int。该函数与Join方法的签名不兼容，因此抛出异常。

如果为了您的示例，您为选择器使用以下表达式，它将正常工作：

Expression<Func<Contact, int?>> outerSelectorLambda = c => c.Id;

查看ParseLambda的API，您应该使用第二个参数来指定lambda的返回类型，所以不要将它留下null：

DynamicExpression.ParseLambda(typeof(Contact), typeof(int?),
    "Id", new object[0]);

如何在LINQ中动态创建联接？

1 个答案: