Do not nest generic types in member signatures
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Do not nest generic types in member signatures

TypeName

DoNotNestGenericTypesInMemberSignatures

CheckId

CA1006

Category

Microsoft.Design

Breaking Change

Breaking

An externally visible member has a signature that contains a nested type argument.

A nested type argument is a type argument that is also a generic type. To call a member whose signature contains a nested type argument, the user must instantiate one generic type and pass this type to the constructor of a second generic type. The required procedure and syntax is complex and should be avoided.

To fix a violation of this rule, change the design to remove the nested type argument.

Do not exclude a warning from this rule. Providing generics in a syntax that is easy to understand and use reduces the time it that is required to learn and increases the adoption rate of new libraries.

The following example shows a method that violates the rule and the syntax required to call the violating method.

using System;
using System.Collections.Generic;

namespace DesignLibrary
{
   public class IntegerCollections
   {
      public void NotNestedCollection(ICollection<int> collection)
      {
         foreach(int i in collection)
         {
            Console.WriteLine(i);
         }
      }

      // This method violates the rule.
      public void NestedCollection(
         ICollection<ICollection<int>> outerCollection)
      {
         foreach(ICollection<int> innerCollection in outerCollection)
         {
            foreach(int i in innerCollection)
            {
               Console.WriteLine(i);
            }
         }
      }
   }

   class Test
   {
      static void Main()
      {
         IntegerCollections collections = new IntegerCollections();

         List<int> integerListA = new List<int>();
         integerListA.Add(1);
         integerListA.Add(2);
         integerListA.Add(3);

         collections.NotNestedCollection(integerListA);

         List<int> integerListB = new List<int>();
         integerListB.Add(4);
         integerListB.Add(5);
         integerListB.Add(6);

         List<int> integerListC = new List<int>();
         integerListC.Add(7);
         integerListC.Add(8);
         integerListC.Add(9);

         List<ICollection<int>> nestedIntegerLists = 
            new List<ICollection<int>>();
         nestedIntegerLists.Add(integerListA);
         nestedIntegerLists.Add(integerListB);
         nestedIntegerLists.Add(integerListC);

         collections.NestedCollection(nestedIntegerLists);
      }
   }
}

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