Generic methods should provide type parameter

Cause

The parameter signature of an externally visible generic method does not contain types that correspond to all the type parameters of the method.

Rule Description

Inference is how the type argument of a generic method is determined by the type of argument passed to the method, instead of by the explicit specification of the type argument. To enable inference, the parameter signature of a generic method must include a parameter that is of the same type as the type parameter for the method. In this case, the type argument does not have to be specified. When using inference for all type parameters, the syntax for calling generic and non-generic instance methods is identical. This simplifies the usability of generic methods.

How to Fix Violations

To fix a violation of this rule change the design so that the parameter signature contains an identical type for each of the type parameters of the method.

When to Exclude Warnings

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

Example

The following example shows the syntax for calling two generic methods. The type argument for InferredTypeArgument is inferred, while the type argument for NotInferredTypeArgument must be explicitly specified.

Imports System

Namespace DesignLibrary

   Public Class Inference

      ' This method violates the rule.
      Sub NotInferredTypeArgument(Of T)()

         Console.WriteLine(GetType(T))

      End Sub
   
      ' This method satisfies the rule.
      Sub InferredTypeArgument(Of T)(sameAsTypeParameter As T)

         Console.WriteLine(sameAsTypeParameter)

      End Sub

   End Class

   Class Test
   
      Shared Sub Main()
      
         Dim infer As New Inference()
         infer.NotInferredTypeArgument(Of Integer)()
         infer.InferredTypeArgument(3)

      End Sub

   End Class

End Namespace

using System;

namespace DesignLibrary
{
   public class Inference
   {
      // This method violates the rule.
      public void NotInferredTypeArgument<T>()
      {
         Console.WriteLine(typeof(T));
      }

      // This method satisfies the rule.
      public void InferredTypeArgument<T>(T sameAsTypeParameter)
      {
         Console.WriteLine(sameAsTypeParameter);
      }
   }

   class Test
   {
      static void Main()
      {
         Inference infer = new Inference();
         infer.NotInferredTypeArgument<int>();
         infer.InferredTypeArgument(3);
      }
   }
}

Related Rules

Avoid excessive parameters on generic types

Collections should implement generic interface

Do not declare static members on generic types

Do not expose generic lists

Do not nest generic types in member signatures

Use generic event handler instances

Use generics where appropriate

See Also

Concepts

Generics (C# Programming Guide)