可以使用 CQL 查找返回“null”的方法吗？答案

【问题标题】：Can CQL be used to find methods that return `null`?可以使用 CQL 查找返回“null”的方法吗？
【发布时间】：2015-10-07 08:23:36
【问题描述】：

我想找出所有可以显式返回 null 的方法。

在 NDepend 中是否可以使用 CQL？

【问题讨论】：

标签： cql ndepend

【解决方案1】：

目前不知道，CQL 到目前为止还不知道变量的值、字段和返回的值。

但是，建议使用以下默认规则。这个想法是，如果一个方法返回一个引用，它永远不应该为空，并且应该添加一个合约来断言它。如果您希望这样的方法返回 null，请改用 Try... 模式，例如 TryParse(string s, out T val):bool。

// <Name>Public methods returning a reference needs a contract to ensure that a non-null reference is returned</Name>
warnif count > 0
let ensureMethods = Application.Methods.WithFullName(
   "System.Diagnostics.Contracts.__ContractsRuntime.Ensures(Boolean,String,String)")

from ensureMethod in ensureMethods
from m in ensureMethod.ParentAssembly.ChildMethods where 
  m.IsPubliclyVisible &&
 !m.IsAbstract &&
  m.ReturnType != null &&
  // Identify that the return type is a reference type
  (m.ReturnType.IsClass || m.ReturnType.IsInterface) &&
 !m.IsUsing(ensureMethod) &&

  // Don't match method not implemented yet!
 !m.CreateA("System.NotImplementedException".AllowNoMatch())

select new { 
   m, 
   ReturnTypeReference = m.ReturnType 
}

//<Description>
// **Code Contracts** are useful to decrease ambiguity between callers and callees.
// Not ensuring that a reference returned by a method is *non-null* leaves ambiguity 
// for the caller. This rule matches methods returning an instance of a reference type 
// (class or interface) that don't use a **Contract.Ensure()** method.
//
// *Contract.Ensure()* is defined in the **Microsoft Code Contracts for .NET** 
// library, and is typically used to write a code contract on returned reference:
// *Contract.Ensures(Contract.Result<ReturnType>() != null, "returned reference is not null");*
// https://visualstudiogallery.msdn.microsoft.com/1ec7db13-3363-46c9-851f-1ce455f66970
//</Description>

//<HowToFix>
// Use *Microsoft Code Contracts for .NET* on the public surface of your API,
// to remove most ambiguity presented to your client. Most of such ambiguities
// are about *null* or *not null* references.
//
// Don't use *null* reference if you need to express that a method might not 
// return a result. Use instead the **TryXXX()** pattern exposed for example 
// in the *System.Int32.TryParse()* method.
//</HowToFix>

【讨论】：