Public methods returning a reference needs a contract to ensure that a non-null reference is returned

Rule ND2900 .NET Framework Usage / Microsoft.Contracts

How to fix

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// <Name>Public methods returning a reference needs a contract to ensure that a non-null reference is returned</Name>
// <Id>ND2900:PublicMethodsReturningAReferenceNeedsAContractToEnsureThatANonNullReferenceIsReturned</Id>

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,
   Debt = (5+3*m.MethodsCallingMe.Count()).ToMinutes().ToDebt(),
   Severity = Severity.Medium
}

//<Expl>
// The *public {0}* returns a reference and should be tagged 
// with a contract to ensure that a non-null reference is returned
//</Expl>

//<Desc>
// **Code Contracts** decrease caller/callee ambiguity; not ensuring a
// returned reference is *non-null* leaves ambiguity. Matches methods
// returning a reference type without using **Contract.Ensure()** (from
// **Microsoft Code Contracts for .NET**), typically:
// *Contract.Ensures(Contract.Result<ReturnType>() != null,
//                   "returned reference is not null");*
// https://visualstudiogallery.msdn.microsoft.com/1ec7db13-3363-46c9-851f-1ce455f66970
//</Desc>

//<HowToFix>
// Use *Microsoft Code Contracts for .NET* on your API's public surface to
// remove *null / not null* ambiguities. For methods that might not return a
// result, prefer the **TryXXX()** pattern (like *System.Int32.TryParse()*)
// over returning *null*.
//
// Estimated Debt: 5 minutes per method possibly returning null plus 3
// minutes per caller.
//</HowToFix>

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